Difference between revisions of "Plate tectonics" - New World Encyclopedia

From New World Encyclopedia
m
(35 intermediate revisions by 3 users not shown)
Line 1: Line 1:
 +
{{Ready}}{{Images OK}}{{Approved}}{{Copyedited}}
 
[[Image:Plates tect2 en.svg|thumb|350px|The tectonic plates of the world were mapped in the second half of the twentieth century.]]
 
[[Image:Plates tect2 en.svg|thumb|350px|The tectonic plates of the world were mapped in the second half of the twentieth century.]]
  
'''Plate tectonics''' (from [[Greek language|Greek]] τέκτων, ''tektōn'' "builder" or "mason") describes the large scale motions of [[Earth]]'s [[lithosphere]]. The theory encompasses the older concepts of [[continental drift]], developed during the first half of the 20th century, and [[seafloor spreading]], understood during the 1960s.
+
'''Plate tectonics''' (from [[Greek language|Greek]] τέκτων, ''tektōn'' "builder" or "mason") describes the large scale motions of [[Earth]]'s [[lithosphere]]. The theory encompasses the older concepts of [[continental drift]], developed during the first half of the twentieth century, and [[seafloor spreading]], understood during the 1960s.
  
The outermost part of the Earth's interior is made up of two layers: above is the [[lithosphere]], comprising the [[Crust (geology)|crust]] and the rigid uppermost part of the [[Earth's mantle|mantle]].
+
The outermost part of the Earth's interior is made up of two layers: above is the [[lithosphere]], comprising the [[Crust (geology)|crust]] and the rigid uppermost part of the [[Earth's mantle|mantle]]. Below the lithosphere lies the [[asthenosphere]]. Although solid, the asthenosphere has relatively low [[viscosity]] and [[shear strength (soil)|shear strength]] and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure.
Below the lithosphere lies the [[asthenosphere]]. Although solid, the asthenosphere has relatively low [[viscosity]] and [[shear strength (soil)|shear strength]] and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure.
+
{{toc}}
 
+
The lithosphere is broken up into what are called ''tectonic plates''&nbsp;—in the case of Earth, there are seven major and many minor plates ([[#Major plates|see list below]]). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]] or collision boundaries, [[Divergent boundary|divergent]] or spreading boundaries, and [[Transform fault|transform]] boundaries. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[oceanic trench]] formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50—100 mm/a.<ref>H.H. Read, and Janet Watson. 1975. ''Introduction to Geology.'' (London, UK: Macmillan; New York, NY: St. Martin's Press), 13-15.</ref>
The lithosphere is broken up into what are called ''tectonic plates''&nbsp;— in the case of Earth, there are seven major and many minor plates ([[#Major plates|see list below]]). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: [[Convergent boundary|convergent]] or collision boundaries, [[Divergent boundary|divergent]] or spreading boundaries, and [[Transform fault|transform]] boundaries. [[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[oceanic trench]] formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50—100 mm/a.<ref>{{cite book |author=Read HH, Watson Janet |title=Introduction to Geology |place=New York |publisher=Halsted |year=1975 |pages=13-15}}</ref>
 
  
 
==Synopsis of the development of the theory==
 
==Synopsis of the development of the theory==
 
[[Image:Tectonic plates boundaries detailed-en.svg|thumb|375px|right|Detailed map showing the tectonic plates with their movement vectors.]]
 
[[Image:Tectonic plates boundaries detailed-en.svg|thumb|375px|right|Detailed map showing the tectonic plates with their movement vectors.]]
  
In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by [[geosyncline|geosynclinal theory]]. It was observed as early as 1596 that the opposite [[coasts]] of the [[Atlantic Ocean]]&nbsp;— or, more precisely, the edges of the [[continental shelves]]&nbsp;— have similar shapes and seem once to have fitted together.<ref>
+
In the late nineteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by [[geosyncline|geosynclinal theory]]. It was observed as early as 1596 that the opposite [[coasts]] of the [[Atlantic Ocean]]&nbsp;—or, more precisely, the edges of the [[continental shelves]]&nbsp;—have similar shapes and seem once to have fitted together.<ref>W.J. Kious, and R.I. Tilling. 1996. [http://pubs.usgs.gov/gip/dynamic/historical.html "Historical perspective".] [http://pubs.usgs.gov/gip/dynamic/dynamic.html ''This Dynamic Earth: the Story of Plate Tectonics'']. Retrieved October 29, 2008. (Washington, DC: U.S. Geological Survey. ISBN 0160482208), Retrieved October 18, 2008. “Abraham Ortelius in his work ''Thesaurus Geographicus'' … suggested that the Americas were "torn away from Europe and Africa by earthquakes and floods . The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents].</ref> Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain.<ref name="Frankel" />
{{cite book |author=Kious WJ, Tilling RI |title=This Dynamic Earth: the Story of Plate Tectonics |origyear=1996 |origmonth=February |url=http://pubs.usgs.gov/gip/dynamic/dynamic.html |accessdate=2008-01-29 |edition=Online edition |publisher=U.S. Geological Survey |isbn=0160482208 |chapter=Historical perspective |chapterurl=http://pubs.usgs.gov/gip/dynamic/historical.html |quote=[[Abraham Ortelius]] in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods ... The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."
 
}}</ref> Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain.<ref name="Frankel" />
 
  
The discovery of [[radium]] and its associated [[exothermic|heating]] properties in 1896 prompted a re-examination of the apparent [[age of the Earth]],<ref>
+
The discovery of [[radium]] and its associated [[exothermic|heating]] properties in 1896 prompted a re-examination of the apparent [[age of the Earth]],<ref>J. Joly, 1909. ''Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History.'' Archibald Constable.</ref> Those calculations implied that, even if it started at [[Thermal radiation|red heat]], the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.
{{cite book |author=Joly J |year=1909 |title=Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History |place=London|publisher=Archibald Constable |page=36 |isbn=1402135777}}</ref> since this had been estimated by its cooling rate and assumption the Earth's  surface radiated like a [[black body]].<ref>{{cite journal|author=Thomson W |year=1863 |title=On the secular cooling of the earth |journal=Philosophical Magazine |volume=4 |issue=25 |pages=1–14|doi=10.1080/14786435908238225
 
}}</ref> Those calculations implied that, even if it started at [[Thermal radiation|red heat]], the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.
 
  
Plate tectonic theory arose out of the hypothesis of [[continental drift]] proposed by [[Alfred Wegener]] in 1912<ref>
+
Plate tectonic theory arose out of the hypothesis of [[continental drift]] proposed by [[Alfred Wegener]] in 1912<ref>Patrick Hughes, [http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_2.html Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle]. ''Earth Observatory, NASA''. Retrieved October 18, 2008. "… on January 6, 1912, Wegener proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography."</ref> and expanded in his 1915 book ''The Origin of Continents and Oceans''. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density [[granite]] floating on a sea of more dense [[basalt]].<ref>Alfred Wegener. (1915 original) 1966. ''The Origin of Continents and Oceans,'' translated by John Biram. reprint ed. (Mineola, NY: Courier Dover. ISBN 0486617084), 246.</ref><ref>Patrick Hughes, [http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_4.html Alfred Wegener (1880-1930): The Origin of Continents and Oceans.] ''Earth Observatory, NASA''. Retrieved October 18, 2008. "By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it [[Pangaea]] (all lands), …"</ref> But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science proved theories proposed by English geologist [[Arthur Holmes]] in 1920 that their junctions might actually lie beneath the [[sea]] and Holmes' 1928 suggestion of convection currents within the mantle as the driving force.<ref>Arthur Holmes, 1928. Radioactivity and Earth Movements. ''Transactions of the Geological Society of Glasgow'' 18:559–606.</ref><ref>Arthur Holmes. 1978. ''Principles of Physical Geology,'' 3rd. (Hoboken, NJ: Wiley. ISBN 0471072516), 640-641.</ref><ref name="Frankel">Henry Frankel, 1978-07. Arthur Holmes and Continental Drift. ''The British Journal for the History of Science.'' 11(2):130–150.</ref>
{{cite web |url=http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_2.html |title=Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle |author=Hughes Patrick |accessdate=2007-12-26 |work=On the Shoulders of Giants |publisher=Earth Observatory, NASA |quote=... on January 6, 1912, Wegener ... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography.}}</ref> and expanded in his 1915 book ''The Origin of Continents and Oceans''. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density [[granite]] floating on a sea of more dense [[basalt]].<ref>{{cite book |author=Alfred Wegener |title=The Origin of Continents and Oceans |translator=Biram John |year=1966 |publisher=Courier Dover |pages=246 |isbn=0486617084
 
}}</ref><ref>
 
{{cite web |url=http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_4.html |title=Alfred Wegener (1880-1930): The Origin of Continents and Oceans |author=Hughes Patrick |accessdate=2007-12-26 |work=On the Shoulders of Giants |publisher=Earth Observatory, NASA |quote=By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands), ...
 
}}</ref> But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science proved theories proposed by English geologist [[Arthur Holmes]] in 1920 that their junctions might actually lie beneath the [[sea]] and Holmes' 1928 suggestion of convection currents within the mantle as the driving force.<ref>
 
{{cite journal |author=Holmes Arthur |year=1928 |title=Radioactivity and Earth Movements |journal=Transactions of the Geological Society of Glasgow |volume=18 |pages=559–606  
 
}}</ref><ref>
 
{{cite book|author=Holmes Arthur |year=1978 |title=Principles of Physical Geology |edition=3rd |publisher=Wiley |pages=640-641 |isbn=0471072516
 
}}</ref><ref name="Frankel">{{cite journal |title=Arthur Holmes and Continental Drift |author=Frankel Henry |journal=The British Journal for the History of Science |volume=11 |issue=2 |date=1978-07 |pages=130–150 |url=http://www.jstor.org/pss/4025726
 
}}</ref>
 
  
The first evidence that crust plates did move around came with the discovery of variable [[magnetic field]] direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. Initially theorized as an [[Expanding Earth theory|expansion of the global crust]],<ref>1958: The tectonic approach to continental drift. In: S. W. Carey (ed.): Continental Drift&nbsp;– A Symposium. University of Tasmania, Hobart, 177-363 (expanding Earth from p. 311 to p. 349)</ref> later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of [[seafloor spreading]] and [[Geomagnetic reversal|magnetic field reversals]] by [[Harry Hammond Hess|Harry Hess]] and [[Ron G. Mason]]<ref>
+
The first evidence that crust plates did move around came with the discovery of variable [[magnetic field]] direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. Initially theorized as an [[Expanding Earth theory|expansion of the global crust]],<ref>S.W. Carey, 1958. ''Continental drift, a symposium … held in the Geology Department of the University of Tasmania, in March, 1956.'' (Hobart, AU: Geology Dept., University of Tasmania), 177-363.</ref> later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of [[seafloor spreading]] and [[Geomagnetic reversal|magnetic field reversals]] by [[Harry Hammond Hess|Harry Hess]] and [[Ron G. Mason]]<ref>Ben J. Korgen, 1995. [http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/8_1/8.1_korgen.pdf A Voice From the Past: John Lyman and the Plate Tectonics Story.] ''Oceanography'' 8(1):19–20. Retrieved October 18, 2008.</ref><ref>Fred Spiess, and William Kuperman. 2003. [http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/16_3/16.3_spiess.pdf The Marine Physical Laboratory at Scripps.] ''Oceanography'' 16(3):45–54. Retrieved October 18, 2008.
{{cite journal |author=Korgen Ben J |year=1995 |title=A Voice From the Past: John Lyman and the Plate Tectonics Story |url=http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/8_1/8.1_korgen.pdf |format=PDF |journal=Oceanography |volume=8 |issue=1 |pages=19–20
+
</ref><ref>R.G. Mason A.D. Raff. 1961. Magnetic survey off the west coast of the United States between 32°N latitude and 42°N latitude. ''Bulletin of the Geological Society of America.'' 72:1259–1266.</ref><ref>A.D. Raff, R.G. Mason. 1961. Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude. ''Bulletin of the Geological Society of America'' 72:1267–1270.</ref> pinpointed the precise mechanism which accounted for new rock upwelling.
}}</ref><ref>
 
{{cite journal|author=Spiess Fred, Kuperman William |year=2003 |url=http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/16_3/16.3_spiess.pdf |format=PDF |title=The Marine Physical Laboratory at Scripps |journal=Oceanography |volume=16|issue=3 |pages=45–54}}</ref><ref>{{cite journal|author=Mason RG, Raff AD |year=1961 |title=Magnetic survey off the west coast of the United States between 32°N latitude and 42°N latitude |journal=Bulletin of the Geological Society of America |volume=72 |pages=1259–1266|doi=10.1130/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2
 
}}</ref><ref>
 
{{cite journal|author=Raff AD, Mason RG |year=1961 |title=Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude |journal=Bulletin of the Geological Society of America |volume=72 |pages=1267–1270|doi=10.1130/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2
 
}}</ref> pinpointed the precise mechanism which accounted for new rock upwelling.
 
  
 
Following the recognition of [[Geomagnetic reversal|magnetic anomalies]] defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a [[mid-ocean ridge]], plate tectonics quickly became broadly accepted. Simultaneous advances in early [[seismic]] imaging techniques in and around [[Wadati-Benioff zone]]s collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power.
 
Following the recognition of [[Geomagnetic reversal|magnetic anomalies]] defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a [[mid-ocean ridge]], plate tectonics quickly became broadly accepted. Simultaneous advances in early [[seismic]] imaging techniques in and around [[Wadati-Benioff zone]]s collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power.
Line 42: Line 24:
 
Study of the deep [[ocean]] floor was critical to development of the theory; the field of deep sea [[marine geology]] accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as [[paleogeography]] and [[paleobiology]].
 
Study of the deep [[ocean]] floor was critical to development of the theory; the field of deep sea [[marine geology]] accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as [[paleogeography]] and [[paleobiology]].
  
==Key principles==
+
==Key Principles==
The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on [[mechanics|mechanical]] differences and in the ways that heat is transferred. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by [[Heat conduction|conduction]] whereas the asthenosphere also transfers heat by [[convection]] and has a nearly [[adiabatic]] temperature gradient. This division should not be confused with the ''chemical'' subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The lithosphere contains both crust and some mantle. A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength. The key principle of plate tectonics is that the lithosphere exists as separate and distinct ''[[List of tectonic plates|tectonic plate]]s'', which ride on the fluid-like ([[visco-elastic]] solid) asthenosphere. Plate motions range up to a typical 10-40 mm/a ([[Mid-Atlantic Ridge]]; about as fast as [[fingernail]]s grow), to about 160 mm/a ([[Nazca Plate]]; about as fast as [[hair]] grows).<ref>
+
The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on [[mechanics|mechanical]] differences and in the ways that heat is transferred. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by [[Heat conduction|conduction]] whereas the asthenosphere also transfers heat by [[convection]] and has a nearly [[adiabatic]] temperature gradient. This division should not be confused with the ''chemical'' subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The lithosphere contains both crust and some mantle. A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength. The key principle of plate tectonics is that the lithosphere exists as separate and distinct ''[[List of tectonic plates|tectonic plate]]s,'' which ride on the fluid-like ([[visco-elastic]] solid) asthenosphere. Plate motions range up to a typical 10-40 mm/a ([[Mid-Atlantic Ridge]]; about as fast as [[fingernail]]s grow), to about 160 mm/a ([[Nazca Plate]]; about as fast as [[hair]] grows).<ref>Glenn Elert, (ed.) 1997. [http://hypertextbook.com/facts/ZhenHuang.shtml Speed of the Continental Plates.] ''The Physics Factbook''.</ref><ref>Paul L. Hancock, and Brian J Skinner. 2000. ''The Oxford Companion to The Earth.'' (Oxford, UK; New York, NY: Oxford University Press. ISBN 0198540396.)</ref>
{{
 
cite web|url=http://hypertextbook.com/facts/ZhenHuang.shtml|title=Speed of the Continental Plates |author=Huang Zhen Shao |year=1997 |work=The Physics Factbook
 
}}
 
</ref><ref>
 
{{
 
cite book |last1=Hancock |first1=Paul L |last2=Skinner |first2=Brian J |last3=Dineley |first3=David L |title=The Oxford Companion to The Earth |publisher=[[Oxford University Press]] |year=2000 |isbn=0198540396
 
}}
 
</ref>
 
  
The plates are around 100 km (60 miles) thick and consist of lithospheric mantle overlain by either of two types of crustal material: [[oceanic crust]] (in older texts called ''[[Sima (geology)|sima]]'' from [[silicon]] and [[magnesium]]) and continental crust (''[[sial]]'' from silicon and [[aluminium]]). The two types of crust differ in thickness, with continental crust considerably thicker than oceanic (50 km vs 5 km).
+
The plates are around 100 km (60 miles) thick and consist of lithospheric mantle overlain by either of two types of crustal material: [[oceanic crust]] (in older texts called ''[[Sima (geology)|sima]]'' from [[silicon]] and [[magnesium]]) and continental crust (''[[sial]]'' from silicon and [[aluminum]]). The two types of crust differ in thickness, with continental crust considerably thicker than oceanic (50 km vs. 5 km).
  
One plate meets another along a ''plate boundary'', and plate boundaries are commonly associated with geological events such as [[earthquake]]s and the creation of topographic features like [[mountain]]s, [[volcano]]es and [[oceanic trench]]es. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's [[Pacific Ring of Fire|Ring of Fire]] being most active and most widely known. These boundaries are discussed in further detail below.
+
One plate meets another along a ''plate boundary,'' and plate boundaries are commonly associated with geological events such as [[earthquake]]s and the creation of topographic features like [[mountain]]s, [[volcano]]es and [[oceanic trench]]es. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's [[Pacific Ring of Fire|Ring of Fire]] being most active and most widely known. These boundaries are discussed in further detail below.
  
Tectonic plates can include continental crust or oceanic crust, and a single plate typically carries both. For example, the [[African Plate]] includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("[[mafic]]") than continental crust ("[[felsic]]").<ref name="newgeo">
+
Tectonic plates can include continental crust or oceanic crust, and a single plate typically carries both. For example, the [[African Plate]] includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("[[mafic]]") than continental crust ("[[felsic]]").<ref name="newgeo">Victor A. Schmidt, and William Harbert. [http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html
{{
+
"The Living Machine: Plate Tectonics"], [http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/other/contents.html ''Planet Earth and the New Geosciences''], third ed. (Dubuque, IA: Kendall/Hunt Pub. Co. ISBN 0787242969.) Retrieved October 18, 2008.</ref> As a result, oceanic crust generally lies below sea level (for example most of the [[Pacific Plate]]), while the continental crust projects above sea level (see [[isostasy]] for explanation of this principle).
cite book |author=Schmidt Victor A, Harbert William |title=Planet Earth and the New Geosciences |origdate= |origyear= |origmonth= |url=http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/other/contents.html |accessdate=2008-01-28 |edition=third |isbn=0787242969 |chapter=The Living Machine: Plate Tectonics |chapterurl=http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html
 
}}
 
</ref> As a result, oceanic crust generally lies below sea level (for example most of the [[Pacific Plate]]), while the continental crust projects above sea level (see [[isostasy]] for explanation of this principle).
 
  
 
==Types of plate boundaries==
 
==Types of plate boundaries==
Line 68: Line 39:
 
# '''[[#Transform (conservative) boundaries|Transform boundaries]]''' occur where plates slide or, perhaps more accurately, grind past each other along [[transform fault]]s. The relative motion of the two plates is either [[sinistral]] (left side toward the observer) or [[dextral]] (right side toward the observer). The [[San Andreas Fault]] in California is one example.
 
# '''[[#Transform (conservative) boundaries|Transform boundaries]]''' occur where plates slide or, perhaps more accurately, grind past each other along [[transform fault]]s. The relative motion of the two plates is either [[sinistral]] (left side toward the observer) or [[dextral]] (right side toward the observer). The [[San Andreas Fault]] in California is one example.
 
# '''[[#Divergent (constructive) boundaries|Divergent boundaries]]''' occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's [[Great Rift Valley]]) are both examples of divergent boundaries.
 
# '''[[#Divergent (constructive) boundaries|Divergent boundaries]]''' occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's [[Great Rift Valley]]) are both examples of divergent boundaries.
# '''[[#Convergent (destructive) boundaries|Convergent boundaries]]''' (or ''active margins'') occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a [[orogeny|continental collision]] (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many [[hydrous]] minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the [[Andes]] mountain range in South America and the [[Japan]]ese [[island arc]].
+
# '''[[#Convergent (destructive) boundaries|Convergent boundaries]]''' (or ''active margins'') occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a [[orogeny|continental collision]] (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many [[hydrous]] minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the [[Andes]] mountain range in South America and the [[Japan]]ese [[island arc]].
  
 
===Transform (conservative) boundaries===
 
===Transform (conservative) boundaries===
Line 74: Line 45:
 
[[John Tuzo Wilson]] recognized that because of [[friction]], the plates cannot simply glide past each other. Rather, [[stress (physics)|stress]] builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated [[potential energy]] is released as [[Strain (materials science)|strain]]. Strain is both accumulative and/or instantaneous depending on the [[rheology]] of the rock; the ductile lower crust and mantle accumulates deformation gradually via [[Shear (geology)|shearing]] whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of [[earthquake]]s, a common phenomenon along transform boundaries.
 
[[John Tuzo Wilson]] recognized that because of [[friction]], the plates cannot simply glide past each other. Rather, [[stress (physics)|stress]] builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated [[potential energy]] is released as [[Strain (materials science)|strain]]. Strain is both accumulative and/or instantaneous depending on the [[rheology]] of the rock; the ductile lower crust and mantle accumulates deformation gradually via [[Shear (geology)|shearing]] whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of [[earthquake]]s, a common phenomenon along transform boundaries.
  
A good example of this type of plate boundary is the [[San Andreas Fault]] which is found in the western coast of [[North America]] and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the [[Alpine Fault]] in [[New Zealand]] and the [[North Anatolian Fault]] in [[Turkey]]. Transform faults are also found offsetting the crests of [[mid-ocean ridge]]s (for example, the [[Mendocino Fracture Zone]] offshore northern California).
+
A good example of this type of plate boundary is the [[San Andreas Fault]] which is found in the western coast of [[North America]] and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the [[Alpine Fault]] in [[New Zealand]] and the [[North Anatolian Fault]] in [[Turkey]]. Transform faults are also found offsetting the crests of [[mid-ocean ridge]]s (for example, the [[Mendocino Fracture Zone]] offshore northern California).
  
 
===Divergent (constructive) boundaries===
 
===Divergent (constructive) boundaries===
Line 81: Line 52:
 
At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten [[magma]] that forms below. The origin of new divergent boundaries at [[triple junction]]s is sometimes thought to be associated with the phenomenon known as [[hotspot (geology)|hotspots]]. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the [[kinetic energy]] is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.
 
At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten [[magma]] that forms below. The origin of new divergent boundaries at [[triple junction]]s is sometimes thought to be associated with the phenomenon known as [[hotspot (geology)|hotspots]]. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the [[kinetic energy]] is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.
  
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the [[Mid-Atlantic Ridge]] and the [[East Pacific Rise]], and in the continental lithosphere by rift valleys such as the famous East African [[Great Rift Valley]]. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the [[fracture zone]]s, many bearing names, that are a major source of [[submarine]] earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by [http://pubs.usgs.gov/publications/text/baseball.html linear features] perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and [[subsidence]]).{{Fact|date=January 2008}}
+
Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the [[Mid-Atlantic Ridge]] and the [[East Pacific Rise]], and in the continental lithosphere by rift valleys such as the famous East African [[Great Rift Valley]]. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the [[fracture zone]]s, many bearing names, that are a major source of [[submarine]] earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by [http://pubs.usgs.gov/publications/text/baseball.html linear features] perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and [[subsidence]]).
  
 
It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne [[Earth's magnetic field|geomagnetic]] surveys showed a strange pattern of symmetrical [[magnetic reversal]]s on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying [[Geomagnetic reversal|polar reversal]]s and the link was made by [[Lawrence Morley|Lawrence W. Morley]], [[Fred Vine|Frederick John Vine]] and [[Drummond Matthews|Drummond Hoyle Matthews]] in the [[Morley-Vine-Matthews hypothesis]]. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.
 
It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne [[Earth's magnetic field|geomagnetic]] surveys showed a strange pattern of symmetrical [[magnetic reversal]]s on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying [[Geomagnetic reversal|polar reversal]]s and the link was made by [[Lawrence Morley|Lawrence W. Morley]], [[Fred Vine|Frederick John Vine]] and [[Drummond Matthews|Drummond Hoyle Matthews]] in the [[Morley-Vine-Matthews hypothesis]]. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.
Line 89: Line 60:
 
The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an [[oceanic trench]] on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of [[South America]] where the oceanic Nazca Plate is being subducted beneath the continental [[South American Plate]].
 
The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an [[oceanic trench]] on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of [[South America]] where the oceanic Nazca Plate is being subducted beneath the continental [[South American Plate]].
  
While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts ([[magma]]) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider [[Mount St. Helens]]). The melts rise to the surface and cool forming long chains of [[volcano]]es inland from the continental shelf and parallel to it.{{Fact|date=January 2008}} The continental spine of western [[South America]] is dense with this type of volcanic [[orogeny|mountain building]] from the subduction of the [[Nazca plate]]. In North America the [[Cascade Range|Cascade mountain range]], extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy [[cinders]], followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as ''The Ring of Fire''.
+
While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts ([[magma]]) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider [[Mount St. Helens]]). The melts rise to the surface and cool forming long chains of [[volcano]]es inland from the continental shelf and parallel to it. The continental spine of western [[South America]] is dense with this type of volcanic [[orogeny|mountain building]] from the subduction of the [[Nazca plate]]. In North America the [[Cascade Range|Cascade mountain range]], extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy [[cinders]], followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as ''The Ring of Fire.''
  
Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the [[Himalaya]]s and the [[Tibetan Plateau]] beyond. It may have also pushed nearby parts of the Asian continent aside to the east.<ref name="butler">Butler, Rob (October 2001). [http://www.see.leeds.ac.uk/structure/dynamicearth/himalayas/deformation/index.htm Where and how do the continents deform?], [http://www.see.leeds.ac.uk/structure/dynamicearth/himalayas/index.htm Himalayan tectonics], [http://www.see.leeds.ac.uk/structure/dynamicearth/ Dynamic Earth]. School of Earth Sciences, University of Leeds. Accessed [[2008-01-29]].</ref>
+
Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the [[Himalaya]]s and the [[Tibetan Plateau]] beyond. It may have also pushed nearby parts of the Asian continent aside to the east.<ref name="butler">Rob Butler, 2001. [http://www.see.leeds.ac.uk/structure/dynamicearth/himalayas/deformation/index.htm Where and how do the continents deform?], [http://www.see.leeds.ac.uk/structure/dynamicearth/himalayas/index.htm Himalayan tectonics], [http://www.see.leeds.ac.uk/structure/dynamicearth/ Dynamic Earth]. ''School of Earth Sciences, University of Leeds''. Retrieved October 18, 2008.</ref>
  
 
When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be [[Japan]] and the [[Aleutian Islands]] in Alaska.
 
When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be [[Japan]] and the [[Aleutian Islands]] in Alaska.
Line 105: Line 76:
  
 
==Driving forces of plate motion==
 
==Driving forces of plate motion==
Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater [[density]] of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.<ref>
+
Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater [[density]] of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.<ref>Pedro Mendia-Landa, [http://www.yale.edu/ynhti/curriculum/units/2007/4/07.04.13.x.html Myths and Legends on Natural Disasters: Making Sense of Our World.] Retrieved October 18, 2008.</ref>
{{cite web |author=Pedro Mendia-Landa
+
Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.
|url=http://www.yale.edu/ynhti/curriculum/units/2007/4/07.04.13.x.html |title=Myths and Legends on Natural Disasters: Making Sense of Our World |accessdate=2008-02-05
 
}}</ref>
 
Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.
 
  
Two and three-dimensional imaging of the Earth's interior ([[seismic tomography]]) shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is [[mantle convection]] from buoyancy forces.<ref>
+
Two and three-dimensional imaging of the Earth's interior ([[seismic tomography]]) shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is [[mantle convection]] from buoyancy forces.<ref>Tanimoto Toshiro, and Lay Thorne. 2000. Mantle dynamics and seismic tomography. ''Proceedings of the National Academy of Science'' 97(23):12409–12410.</ref> How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this [[energy]] must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: [[friction]] and [[gravity]].
{{cite journal |doi= 10.1073/pnas.210382197 |author=Tanimoto Toshiro, Lay Thorne |title=Mantle dynamics and seismic tomography |journal=Proceedings of the National Academy of Science |date=2000-11-07 |volume=97 |issue=23 |pages=12409–12410 |pmid=11035784
 
}}</ref> How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this [[energy]] must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: [[friction]] and [[gravity]].
 
  
 
===Friction===
 
===Friction===
Line 120: Line 86:
  
 
===Gravitation===
 
===Gravitation===
:Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.
+
:[[Gravity|Gravitational]] sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.
  
:Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push". This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:
+
:Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push." This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:
::1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges.
+
::1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear [[topology|topographical]] feature that can offset or at least affect the influence of topographical ocean ridges.
 
::2. [[Mantle plume]]s impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.
 
::2. [[Mantle plume]]s impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.
  
;Slab-pull : Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches.<ref>
+
;Slab-pull: Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches.<ref>C.P. Conrad, C. Lithgow-Bertelloni. 2002. How Mantle Slabs Drive Plate Tectonics. ''Science'' 298(5591):L45.</ref> There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.
{{cite journal|author=Conrad CP, Lithgow-Bertelloni C |year=2002 |title=How Mantle Slabs Drive Plate Tectonics |journal=Science |volume=298|issue=5591
 
| pages = L45
 
| doi = 10.1126/science.1074161 <!--Retrieved from CrossRef by DOI bot—>
 
}}</ref> There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.
 
  
 
===External forces===
 
===External forces===
In a study published in the January-February 2006 issue of the ''Geological Society of America Bulletin'', a team of Italian and U.S. scientists argued that the westward component of plates is from Earth's rotation and consequent tidal friction of the moon. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why Venus and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars.<ref>
+
In a study published in the January-February 2006 issue of the ''Geological Society of America Bulletin,'' a team of Italian and U.S. scientists argued that the westward component of plates is from Earth's rotation and consequent [[tide|tidal]] friction of the [[Moon]]. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why [[Venus]] and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars.<ref>Richard A. Lovett, 2006. [http://news.nationalgeographic.com/news/2006/01/0124_060124_moon.html Moon Is Dragging Continents West, Scientist Says.] ''National Geographic News''. Retrieved October 18, 2008.</ref> This is not, however, a new argument.
{{cite journal|author=Lovett Richard A |title=Moon Is Dragging Continents West, Scientist Says |journal=National Geographic News |date=2006-01-24 |url=http://news.nationalgeographic.com/news/2006/01/0124_060124_moon.html
 
}}</ref> This is not, however, a new argument.
 
  
 
It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist [[Harold Jeffreys]] who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.
 
It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist [[Harold Jeffreys]] who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.
  
 
===Relative significance of each mechanism===
 
===Relative significance of each mechanism===
[[Image:Global plate motion 2008-04-17.jpg|thumb|350px|Plate motion based on Global Positioning System (GPS) satellite data from NASA [http://sideshow.jpl.nasa.gov/mbh/series.html JPL]. Vectors show direction and magnitude of motion.]]
+
[[Image:Global plate motion 2008-04-17.jpg|thumb|350px|Plate motion based on [[Global Positioning System]] (GPS) [[satellite]] data from NASA [http://sideshow.jpl.nasa.gov/mbh/series.html JPL]. Vectors show direction and magnitude of motion.]]
 
The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem regarding what degree each process contributes to the motion of each tectonic plate.
 
The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem regarding what degree each process contributes to the motion of each tectonic plate.
  
The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.{{Fact|date=January 2008}}
+
The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.
  
One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.{{Fact|date=January 2008}}
+
One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.
  
 
The driving forces of plate motion are, nevertheless, still very active subjects of on-going discussion and research in the geophysical community.
 
The driving forces of plate motion are, nevertheless, still very active subjects of on-going discussion and research in the geophysical community.
  
==Major plates==
+
==Major Plates==
 
The main plates are
 
The main plates are
 
*[[African Plate]] covering [[Africa]] - Continental plate
 
*[[African Plate]] covering [[Africa]] - Continental plate
Line 176: Line 136:
 
{{seesubarticle|Continental drift}}
 
{{seesubarticle|Continental drift}}
  
''Continental drift'' was one of many ideas about tectonics proposed in the late 19th and early 20th centuries. The theory has been superseded and the concepts and data have been incorporated within plate tectonics.
+
''Continental drift'' was one of many ideas about tectonics proposed in the late nineteenth and early twentieth centuries. The theory has been superseded and the concepts and data have been incorporated within plate tectonics.
  
 
By 1915, Alfred Wegener was making serious arguments for the idea in the first edition of ''The Origin of Continents and Oceans.'' In that book, he noted how the east coast of [[South America]] and the west coast of [[Africa]] looked as if they were once attached. Wegener wasn't the first to note this ([[Abraham Ortelius]], [[Francis Bacon]], [[Benjamin Franklin]], [[Snider-Pellegrini]], [[Roberto Mantovani]] and [[Frank Bursley Taylor]] preceded him), but he was the first to marshal significant [[fossil]] and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as [[Alex du Toit]]). However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that propelled continental drift.
 
By 1915, Alfred Wegener was making serious arguments for the idea in the first edition of ''The Origin of Continents and Oceans.'' In that book, he noted how the east coast of [[South America]] and the west coast of [[Africa]] looked as if they were once attached. Wegener wasn't the first to note this ([[Abraham Ortelius]], [[Francis Bacon]], [[Benjamin Franklin]], [[Snider-Pellegrini]], [[Roberto Mantovani]] and [[Frank Bursley Taylor]] preceded him), but he was the first to marshal significant [[fossil]] and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as [[Alex du Toit]]). However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that propelled continental drift.
  
Wegener's vindication did not come until after his death in 1930. In 1947, a team of scientists led by [[Maurice Ewing]] utilizing the [[Woods Hole Oceanographic Institution]]’s research vessel ''Atlantis'' and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions.<ref>
+
Wegener's vindication did not come until after his death in 1930. In 1947, a team of scientists led by [[Maurice Ewing]] utilizing the [[Woods Hole Oceanographic Institution]]’s research vessel ''Atlantis'' and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions.<ref>Laurence Lippsett, 2001. [http://www.columbia.edu/cu/alumni/Magazine/Winter2001/ewing.html Maurice Ewing and the Lamont-Doherty Earth Observatory.] ''Living Legacies''. Retrieved October 18, 2008.</ref>
{{cite journal |author=Lippsett Laurence |url=http://www.columbia.edu/cu/alumni/Magazine/Winter2001/ewing.html  
 
|title=Maurice Ewing and the [[Lamont-Doherty]] Earth Observatory] |journal=Living Legacies |year=2001 |accessdate=2008-03-04
 
}}</ref>
 
  
Beginning in the 1950s, scientists including Harry Hess, using magnetic instruments ([[magnetometer]]s) adapted from airborne devices developed during [[World War II]] to detect [[submarine]]s, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that [[basalt]]—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral ([[magnetite]]) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the [[Earth's magnetic field]] at the time.
+
Beginning in the 1950s, scientists including Harry Hess, using magnetic instruments ([[magnetometer]]s) adapted from airborne devices developed during [[World War II]] to detect [[submarine]]s, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that [[basalt]]—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral ([[magnetite]]) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late eighteenth century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the [[Earth's magnetic field]] at the time.
  
 
As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.
 
As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.
Line 201: Line 158:
  
 
===Plate tectonic theory===
 
===Plate tectonic theory===
Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist [[Harry Hammond Hess]] ([[Robert S. Dietz]] published the same idea one year earlier in ''Nature''. However, priority belongs to Hess, since he distributed an unpublished manuscript of his 1962 article already in 1960). Hess suggested that instead of continents moving ''through'' oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In the same year, [[Robert R. Coats]] of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little-noted (and even ridiculed) at the time, has since been called "seminal" and "prescient". In 1967, [[W. Jason Morgan]] proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, [[Xavier Le Pichon]] published a complete model based on 6 major plates with their relative motions.
+
Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist [[Harry Hammond Hess]] ([[Robert S. Dietz]] published the same idea one year earlier in ''Nature''. However, priority belongs to Hess, since he distributed an unpublished manuscript of his 1962 article already in 1960). Hess suggested that instead of continents moving ''through'' oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In the same year, [[Robert R. Coats]] of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little-noted (and even ridiculed) at the time, has since been called "seminal" and "prescient." In 1967, [[W. Jason Morgan]] proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, [[Xavier Le Pichon]] published a complete model based on 6 major plates with their relative motions.
  
 
====Explanation of magnetic striping====
 
====Explanation of magnetic striping====
 
[[Image:Oceanic.Stripe.Magnetic.Anomalies.Scheme.gif|thumb|Seafloor magnetic striping.]]
 
[[Image:Oceanic.Stripe.Magnetic.Anomalies.Scheme.gif|thumb|Seafloor magnetic striping.]]
The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New [[magma]] from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new [[oceanic crust]]. This process, later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:
+
The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New [[magma]] from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new [[oceanic crust]]. This process, later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:
 
# at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
 
# at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
 
# the youngest rocks at the ridge crest always have present-day (normal) polarity;
 
# the youngest rocks at the ridge crest always have present-day (normal) polarity;
 
# stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has reversed many times.
 
# stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has reversed many times.
By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the [[Earth's magnetic field]].
+
By explaining both the zebra like magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the [[Earth's magnetic field]].
  
 
====Subduction discovered====
 
====Subduction discovered====
 
A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists, most notably S. Warren Carey, who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "[[Expanding Earth theory]]" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?
 
A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists, most notably S. Warren Carey, who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "[[Expanding Earth theory]]" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?
  
This question particularly intrigued Harry Hess, a [[Princeton University]] geologist and a Naval Reserve Rear Admiral, and [[Robert S. Dietz]], a scientist with the [[U.S. Coast and Geodetic Survey]] who first coined the term ''seafloor spreading''. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the [[oceanic trench]]es&nbsp;— very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is consumed in the trenches, new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.
+
This question particularly intrigued Harry Hess, a [[Princeton University]] geologist and a Naval Reserve Rear Admiral, and [[Robert S. Dietz]], a scientist with the [[U.S. Coast and Geodetic Survey]] who first coined the term ''seafloor spreading.'' Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the [[oceanic trench]]es&nbsp;—very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the [[Atlantic Ocean]] was expanding while the [[Pacific Ocean]] was shrinking. As old oceanic crust is consumed in the trenches, new [[magma]] rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little [[sediment]] accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.
  
 
====Mapping with earthquakes====
 
====Mapping with earthquakes====
During the 20th century, improvements in and greater use of seismic instruments such as [[seismograph]]s enabled scientists to learn that [[earthquake]]s tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as [[Wadati-Benioff zone]]s, or simply [[Benioff zone]]s, in honor of the seismologists who first recognized them, [[Kiyoo Wadati]] of [[Japan]] and [[Hugo Benioff]] of the [[United States]]. The study of global seismicity greatly advanced in the 1960s with the establishment of the [http://neic.usgs.gov/neis/station_book/WWSS_NETWORK.html Worldwide Standardized Seismograph Network] (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration world wide.
+
During the twentieth century, improvements in and greater use of seismic instruments such as [[seismograph]]s enabled scientists to learn that [[earthquake]]s tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, [[seismology|seismologists]] were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as [[Wadati-Benioff zone]]s, or simply [[Benioff zone]]s, in honor of the seismologists who first recognized them, [[Kiyoo Wadati]] of [[Japan]] and [[Hugo Benioff]] of the [[United States]]. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN)<ref>[http://neic.usgs.gov/neis/station_book/WWSS_NETWORK.html Worldwide Standardized Seismograph Network].Retrieved October 29, 2008. </ref> to monitor the compliance of the 1963 treaty banning above-ground testing of [[nuclear weapon]]s. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration world wide.
  
 
===Geological paradigm shift===
 
===Geological paradigm shift===
The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) may be compared to the Copernican revolution in [[astronomy]] (see [[Nicolaus Copernicus]]). Within a matter of only several years [[geophysics]] and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?". Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.
+
The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) may be compared to the Copernican revolution in [[astronomy]] (see [[Nicolaus Copernicus]]). Within a matter of only several years [[geophysics]] and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?." Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.
  
 
However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable [[scientific journal]] was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.
 
However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable [[scientific journal]] was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.
  
One simply has to read the pre-plate descriptions of why the [[Alps]] or [[Himalaya]] exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the [[Dolomites]]?", or "How did the convex and concave margins of the Alpine chain form?", any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.
+
One simply has to read the pre-plate descriptions of why the [[Alps]] or [[Himalaya]] exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the [[Dolomites]]?," or "How did the convex and concave margins of the Alpine chain form?," any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.
  
 
With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift the sea floor to great heights. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.
 
With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift the sea floor to great heights. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.
Line 233: Line 190:
  
 
==Biogeographic implications on biota==
 
==Biogeographic implications on biota==
Continental drift theory helps biogeographers to explain the disjunct [[Biogeography|biogeographic]] distribution of present day life found on different continents but having similar [[Common descent|ancestors]].<ref>
+
Continental drift theory helps biogeographers to explain the disjunct [[Biogeography|biogeographic]] distribution of present day [[life]] found on different continents but having similar [[Common descent|ancestors]].<ref>S.J. Moss, and M.E.J. Wilson. 1998. [http://searg.rhul.ac.uk/publications/books/biogeography/biogeog_pdfs/Moss_Wilson.pdf "Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo"], in Hall, R., and J.D. Holloway eds. ''Biogeography and Geological Evolution of SE Asia.'' (Leiden, NL: Backhuys. ISBN 9073348978), 133-163. Retrieved October 18, 2008.</ref> In particular, it explains the [[Gondwana]]n distribution of [[ratite]]s and the [[Antarctica|Antarctic]] [[flora]].
{{cite book |author=Moss SJ, Wilson MEJ |editor=Hall R, Holloway JD (eds) |title=Biogeography and Geological Evolution of SE Asia |accessdate=2008-01-29 |year=1998 |publisher=Backhuys |location=Leiden, The Netherlands |isbn=9073348978 |pages=133-163 |chapter=Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo |chapterurl=http://www.gl.rhul.ac.uk/searg/publications/books/biogeography/biogeog_pdfs/Moss_Wilson.pdf |format=PDF
 
}}</ref> In particular, it explains the [[Gondwana]]n distribution of [[ratite]]s and the [[Antarctic flora]].
 
  
 
==Plate tectonics on other planets==
 
==Plate tectonics on other planets==
The appearance of plate tectonics on [[terrestrial planet]]s is related to planetary mass, with [[Super-Earth|more massive planets than Earth]] expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water.<ref>
+
The appearance of plate tectonics on [[terrestrial planet]]s is related to planetary mass, with [[Super-Earth|more massive planets than Earth]] expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water.<ref>Diana Valencia, Richard J. O'Connell, and Dimitar D. Sasselov. 2007. [http://arxiv.org/abs/0710.0699v1 Inevitability of Plate Tectonics on Super-Earths.] ''Astrophysical Journal Letters'' 670(1):L45–L48.</ref>
{{cite journal |author=Valencia Diana, O'Connell Richard J, Sasselov Dimitar D |month=November | year=2007 |title=Inevitability of Plate Tectonics on Super-Earths |journal=Astrophysical Journal Letters |volume=670 |issue=1 |pages=L45–L48 |doi=10.1086/524012 |url=http://arxiv.org/abs/0710.0699v1
 
}}</ref>
 
  
 
===Venus===
 
===Venus===
{{seealso|Geology of Venus}}
+
 
 
Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved [[impact crater]]s have been utilized as a [[dating methods|dating method]] to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 to 750 Ma, although ages of up to ~1.2 Ga have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.
 
Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved [[impact crater]]s have been utilized as a [[dating methods|dating method]] to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 to 750 Ma, although ages of up to ~1.2 Ga have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.
  
One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.<ref>
+
One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.<ref>Henry Bortman, 2004. [http://www.space.com/scienceastronomy/venus_life_040826.html Was Venus alive? 'The Signs are Probably There']. ''Astrobiology Magazine''. Retrieved October 18, 2008.</ref><ref>J.F. Kasting, 1988. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. ''Icarus'' 74(3):472–494.</ref> The Earth's crust is soaked with water, and water plays an important role in the development of [[shear zone]]s. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.
{{cite web
 
|url=http://www.space.com/scienceastronomy/venus_life_040826.html
 
|title=Was Venus alive? 'The Signs are Probably There'
 
|author=Bortman Henry
 
|date=2004-08-26
 
|accessdate=2008-01-08
 
|publisher=Astrobiology Magazine
 
}}</ref><ref>
 
{{cite journal
 
| title = Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus
 
| author = Kasting JF
 
| journal = Icarus
 
| volume = 74
 
| issue = 3
 
| pages = 472–494
 
| year = 1988
 
| doi = 10.1016/0019-1035(88)90116-9
 
}}</ref> The Earth's crust is soaked with water, and water plays an important role in the development of [[shear zone]]s. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.
 
  
 
===Mars===
 
===Mars===
{{seealso|Geology of Mars}}
 
Unlike Venus, the crust of Mars has water in it and on it (mostly in the form of ice). This planet is considerably smaller than the Earth, but shows some  indications that could suggest a similar style of tectonics. The gigantic volcanoes in the [[Tharsis]] area are linearly aligned like volcanic arcs on Earth; the enormous canyon [[Valles Marineris]] could have been formed by some form of crustal spreading.
 
  
As a result of observations made of the [[magnetic field]] of Mars by the ''[[Mars Global Surveyor]]'' spacecraft in 1999, large scale patterns of magnetic striping were discovered on this planet. To explain these magnetisation patterns in the Martian crust it has been proposed that a mechanism similar to plate tectonics may once have been active on the planet.<ref>
+
Unlike Venus, the crust of Mars has water in it and on it (mostly in the form of ice). This planet is considerably smaller than the Earth, but shows some indications that could suggest a similar style of tectonics. The gigantic volcanoes in the [[Tharsis]] area are linearly aligned like volcanic arcs on Earth; the enormous canyon [[Valles Marineris]] could have been formed by some form of crustal spreading.
{{cite journal |author=Connerney JEP, Acuña MH, Wasilewski PJ, Ness NF, Rème H, Mazelle C, Vignes D, Lin RP, Mitchell DL, Cloutier PA |year=1999 |title=Magnetic Lineations in the Ancient Crust of Mars |journal=Science |volume=284 |page=794-798 |doi=10.1126/science.284.5415.794
+
 
| pages = 794 |pmid=10221909
+
As a result of observations made of the [[magnetic field]] of Mars by the ''[[Mars Global Surveyor]]'' spacecraft in 1999, large scale patterns of magnetic striping were discovered on this planet. To explain these magnetization patterns in the Martian crust it has been proposed that a mechanism similar to plate tectonics may once have been active on the planet.<ref>J.E.P. Connerney, M.H. Acuña, P.J. Wasilewski, N.F. Ness, H. Rème, C. Mazelle, D. Vignes, R.P. Lin, D.L. Mitchell, and P.A. Cloutier. 1999. Magnetic Lineations in the Ancient Crust of Mars. ''Science'' 284:794.</ref><ref>J.E.P. Connerney, M.H. Acuña, N.F. Ness, G. Kletetschka, D.L. Mitchell, R.P. Lin, and H. Rème. 2005. Tectonic implications of Mars crustal magnetism. ''Proceedings of the National Academy of Sciences'' 102:14970–14975.</ref> Further data from the ''[[Mars Express]]'' orbiter's ''High Resolution Stereo Camera'' in 2007 clearly showed an example in the [[Aeolis (Mars)|Aeolis Mensae]] region.<ref>[http://www.esa.int/SPECIALS/Mars_Express/SEMF399OY2F_0.html Tectonic signatures at Aeolis Mensae.] ''European Space Agency''. Retrieved October 18, 2008.</ref>
}}</ref><ref>
 
{{cite journal |author=Connerney JEP, Acuña MH, Ness NF, Kletetschka G, Mitchell DL, Lin RP, Rème H | year=2005 |title=Tectonic implications of Mars crustal magnetism |journal=Proceedings of the National Academy of Sciences |volume=102 |pages=14970–14975 |doi=10.1073/pnas.0507469102 |pmid=16217034
 
}}</ref> Further data from the ''[[Mars Express]]'' orbiter's ''High Resolution Stereo Camera'' in 2007 clearly showed an example in the [[Aeolis (Mars)|Aeolis Mensae]] region.<ref>
 
{{cite web |url=http://www.esa.int/SPECIALS/Mars_Express/SEMF399OY2F_0.html |title=Tectonic signatures at Aeolis Mensae |publisher=European Space Agency |date=2007-06-28 |accessdate=2008-01-29
 
}}</ref>
 
  
 
===Galilean satellites===
 
===Galilean satellites===
Line 283: Line 211:
  
 
===Titan===
 
===Titan===
Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005.<ref>
+
Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005.<ref>Laurence A. Soderblom, Martin G. Tomasko, Brent A. Archinal, Tammy L. Becker, Michael W. Bushroe, Debbie A. Cook, Lyn R. Doose, Donna M. Galuszka, Trent M. Hare, Elpitha Howington-Kraus, Erich Karkoschka, Randolph L. Kirk, Jonathan I. Lunine, Elisabeth A. McFarlane, Bonnie L. Redding, Bashar Rizk, Mark R. Rosiek, Charles See, and Peter H. Smith. 2007. Topography and geomorphology of the Huygens landing site on Titan. ''Planetary and Space Science'' 55(13):2015.</ref>
{{cite journal |author=Soderblom Laurence A, Tomasko Martin G, Archinal Brent A, Becker Tammy L, Bushroe Michael W, Cook Debbie A, Doose Lyn R, Galuszka Donna M, Hare Trent M, Howington-Kraus Elpitha, Karkoschka Erich, Kirk Randolph L, Lunine Jonathan I, McFarlane Elisabeth A, Redding Bonnie L, Rizk Bashar, Rosiek Mark R, See Charles, Smith Peter H |year=2007 |title=Topography and geomorphology of the Huygens landing site on Titan |journal=Planetary and Space Science |volume=55 |issue=13 |page=2015-2024 |doi=10.1016/j.pss.2007.04.015
 
| pages = 2015
 
}}</ref>
 
  
 
==See also==
 
==See also==
*[[List of plate tectonics topics]]
+
 
*[[List of tectonic plates]]
+
* [[Alps]]
*[[List of tectonic plate interactions]]
+
* [[Continental drift]]
*[[Geosyncline|Geosyncline theory]], obsolete explanation of mountain-building
+
* [[Earth]]
*[[Plume tectonics]], an extension of plate tectonics that attempts to explain other aspects of the field
+
* [[Great Rift Valley]]
 +
* [[Himalaya]]
 +
* [[Lithosphere]]
 +
 
 +
== Notes ==
 +
<references/>
  
 
==References==
 
==References==
<!-- ----------------------------------------------------------
 
  See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a
 
  discussion of different citation methods and how to generate
 
  footnotes using the <ref>, </ref> and <reference /> tags
 
----------------------------------------------------------- —>
 
{{reflist|2}}
 
  
==Further reading==
+
* Atkinson L., and C. Sancetta. 1993. Hail and farewell. ''Oceanography'' 6(34).
* {{cite book|author=McKnight Tom |year=2004 |title=Geographica: The complete illustrated Atlas of the world |publisher=Barnes and Noble Books |place=New York |isbn=076075974X}}
+
* Conrad C.P., C. Lithgow-Bertelloni. 2002. How Mantle Slabs Drive Plate Tectonics. ''Science'' 298(5591):L45.
* {{cite book|author=Oreskes, Naomi (ed) |year=2003 |title=Plate Tectonics: An Insider's History of the Modern Theory of the Earth |publisher=Westview |isbn=0813341329}}
+
* Frankel, Henry, 1978-07. Arthur Holmes and Continental Drift. ''The British Journal for the History of Science'' 11(2):130–150.
* {{cite book|Schubert G, Turcotte DL, Olson P |year=2001 |title=Mantle Convection in the Earth and Planets |publisher=Cambridge University Press |place=Cambridge |isbn=052135367X}}
+
* Hancock, Paul L. and Brian J Skinner. 2000. ''The Oxford Companion to The Earth.'' Oxford, UK; New York, NY: Oxford University Press. ISBN 0198540396.
* {{cite book|author=Stanley Steven M |year=1999 |title=Earth System History |publisher=W.H. Freeman | pages=211–228 |isbn=0716728826}}
+
* Lyman, J., and R.H. Fleming. 1940. Composition of Seawater. ''J Mar Res.'' 3:134–146.
* {{cite journal |author=Tanimoto Toshiro, Lay Thorne |year=2000 |title=Mantle dynamics and [[seismic tomography]] |journal=Proceedings of the National Academy of Science |doi= 10.1073/pnas.210382197 |volume=97 |pages=12409 |pmid=11035784}}
+
* McKnight, Tom. 2004. ''Geographica: The complete illustrated Atlas of the world.'' New York, NY: Barnes and Noble Books. ISBN 076075974X.
* {{cite book |author=Thompson Graham R, Turk Jonathan |year=1991 |title=Modern Physical Geology |publisher=Saunders College Publishing |isbn=0030253985}}
+
* Oreskes, Naomi ed. 2003. ''Plate Tectonics: An Insider's History of the Modern Theory of the Earth.'' Boulder, CO: Westview. ISBN 0813341329.
* {{cite book|author=Turcotte DL, Schubert G |year=2002 |title=[[Geodynamics]]: Second Edition |publisher=John Wiley & Sons |place=New York |isbn=0521666244}}
+
* Read, H.H. and Janet Watson. 1975. ''Introduction to Geology.'' London, UK: Macmillan; New York, NY: St. Martin's Press.
* {{cite book |author=Winchester, Simon |year=2003|title=[[Krakatoa]]: The Day the World Exploded: [[August 27]], [[1883]] |publisher=HarperCollins |isbn=0066212855}}
+
 
* {{cite journal |author=Atkinson L, Sancetta C |year=1993 |title=Hail and farewell |journal=Oceanography |volume=6 |issue=34}}
+
* Schubert, Gerald, Donald Lawson Turcotte, Peter Olson, and NetLibrary, Inc. 2001. ''Mantle Convection in the Earth and Planets.'' Cambridge, UK; New York, NY:Cambridge University Press. ISBN 052135367X.
* {{cite journal |author=Lyman J, Fleming RH |year=1940 |title=Composition of Seawater |journal=J Mar Res |volume=3 |pages=134–146}}
+
* Stanley, Steven M. 1999. ''Earth System History.'' New York, NY: W.H. Freeman. ISBN 0716728826.
* {{cite book |author=Sverdrup HU, Johnson MW, Fleming RH |year=1942 |title=The Oceans: Their physics, chemistry and general biology |publisher=Prentice-Hall |place=Englewood Cliffs |pages=1087 }}
+
* Sverdrup, H.U., M.W. Johnson, and R.H. Fleming. 1942. ''The Oceans: Their physics, chemistry and general biology.'' New York, NY: Prentice-Hall.
* {{cite journal |author=Vine FJ, Matthews DH |year=1963 |title=Magnetic anomalies over oceanic ridges |journal=Nature |volume=199 |pages=947–949 | doi = 10.1038/199947a0 <!--Retrieved from CrossRef by DOI bot—>}}
+
* Tanimoto, Toshiro, and Thorne Lay. 2000. Mantle dynamics and seismic tomography. ''Proceedings of the National Academy of Science'' 97:12409.
</div>
+
* Thompson, Graham R, and Jonathan Turk. 1991. ''Modern Physical Geology.'' Philadelphia, PA: Saunders College Publishing. ISBN 0030253985.
 +
* Toshiro, Tanimot,o and Lay Thorne. 2000. Mantle dynamics and seismic tomography. ''Proceedings of the National Academy of Science'' 97(23):12409–12410.
 +
 
 +
* Turcotte, D.L., and G. Schubert. 2002. ''Geodynamics,'' Second Ed. Hoboken, NJ: John Wiley & Sons. ISBN 0521666244.
 +
* Vine, F.J., and D.H. Matthews. 1963. Magnetic anomalies over oceanic ridges. ''Nature'' 199:947–949.
 +
* Wegener, Alfred. (original 1915) 1966. ''The Origin of Continents and Oceans,'' translated by John Biram. reprint ed. Mineola, NY: Courier Dover. ISBN 0486617084
 +
* Winchester, Simon. 2003. ''Krakatoa: The Day the World Exploded: August 27, 1883.'' New York, NY: Harper-Collins. ISBN 0066212855.
  
 
==External links==
 
==External links==
{{commonscat|Plate tectonics}}
+
All links retrieved May 6, 2015.
* [http://www.ig.utexas.edu/research/projects/plates/ The PLATES Project], a comprehensive resource with reconstructions, movies, images, list of publications, and teaching resources, from the University of Texas Institute for Geophysics at the [[Jackson School of Geosciences]].
+
* [http://www.ig.utexas.edu/research/projects/plates/ The PLATES Project], a comprehensive resource with reconstructions, movies, images, list of publications, and teaching resources, from the University of Texas Institute for Geophysics at the [[Jackson School of Geosciences]].  
 
* [http://www.scotese.com/ The Paleomap Project], [[Christopher Scotese]]'s website with reconstructions in the past and future, [[palaeogeography|paleogeographies]], teaching material etc.
 
* [http://www.scotese.com/ The Paleomap Project], [[Christopher Scotese]]'s website with reconstructions in the past and future, [[palaeogeography|paleogeographies]], teaching material etc.
* [http://www.ucmp.berkeley.edu/geology/tectonics.html Movie showing 750 million years of global tectonic activity]
+
* [http://www.ucmp.berkeley.edu/geology/tectonics.html Movie showing 750 million years of global tectonic activity.]  
* [http://geology.com/nsta/ Easy-to-draw illustrations for teaching plate tectonics]
+
* [http://geology.com/nsta/ Easy-to-draw illustrations for teaching plate tectonics.]  
* [http://www.tectonic-forces.org An explanation of tectonic forces]
+
* [http://www.tectonic-forces.org An explanation of tectonic forces.]  
* [http://element.ess.ucla.edu/publications/2003_PB2002/2003_PB2002.htm Bird, P. (2003) An updated digital model of plate boundaries], also available as a [http://element.ess.ucla.edu/publications/2003_PB2002/2001GC000252.pdf large (13 mb) PDF file]
+
* [http://snobear.colorado.edu/Markw/Mountains/03/week3.html Map of tectonic plates.]  
* [http://snobear.colorado.edu/Markw/Mountains/03/week3.html Map of tectonic plates]
+
* [http://www.mantleplumes.org/ MantlePlumes.org], a website debating the existence of deep mantle plumes.
* [http://www.mantleplumes.org/ MantlePlumes.org], a website debating the existence of deep mantle plumes
+
* [http://pubs.usgs.gov/publications/text/understanding.html USGS site on plate motions.]  
* [http://pubs.usgs.gov/publications/text/understanding.html USGS site on plate motions]
+
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999JGR...104.7405M&amp;db_key=PHY&amp;data_type=HTML&amp;format= Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision.]  
* [http://www.onderzoekinformatie.nl/en/oi/nod/onderzoek/OND1273645/ The geodynamics of the North-American/Eurasian/African plate boundaries]
+
* [http://hypertextbook.com/facts/ZhenHuang.shtml Speed of the Continental Plates.] The Physics Factbook.
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999JGR...104.7405M&amp;db_key=PHY&amp;data_type=HTML&amp;format= Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision]
+
* [http://www.impacttectonics.org/ ImpactTectonics.org], examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets.
* {{cite web|url=http://hypertextbook.com/facts/ZhenHuang.shtml|title=Speed of the Continental Plates| autor=Huang Zhen Shao |year=1997 |work=The Physics Factbook}}
+
----
* [http://www.impacttectonics.org/ ImpactTectonics.org], examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets
 
* [http://www.bbc.co.uk/radio4/history/inourtime/rams/inourtime_20080124.ram BBC Radio 4 ''In Our Time'' - Plate Tectonics]
 
 
 
 
{{Tectonic plates}}
 
{{Tectonic plates}}
 
{{earthsinterior}}
 
{{earthsinterior}}

Revision as of 19:32, 6 May 2015

The tectonic plates of the world were mapped in the second half of the twentieth century.

Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") describes the large scale motions of Earth's lithosphere. The theory encompasses the older concepts of continental drift, developed during the first half of the twentieth century, and seafloor spreading, understood during the 1960s.

The outermost part of the Earth's interior is made up of two layers: above is the lithosphere, comprising the crust and the rigid uppermost part of the mantle. Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure.

The lithosphere is broken up into what are called tectonic plates —in the case of Earth, there are seven major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50—100 mm/a.[1]

Synopsis of the development of the theory

Detailed map showing the tectonic plates with their movement vectors.

In the late nineteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by geosynclinal theory. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean —or, more precisely, the edges of the continental shelves —have similar shapes and seem once to have fitted together.[2] Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain.[3]

The discovery of radium and its associated heating properties in 1896 prompted a re-examination of the apparent age of the Earth,[4] Those calculations implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.

Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener in 1912[5] and expanded in his 1915 book The Origin of Continents and Oceans. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of more dense basalt.[6][7] But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science proved theories proposed by English geologist Arthur Holmes in 1920 that their junctions might actually lie beneath the sea and Holmes' 1928 suggestion of convection currents within the mantle as the driving force.[8][9][3]

The first evidence that crust plates did move around came with the discovery of variable magnetic field direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. Initially theorized as an expansion of the global crust,[10] later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of seafloor spreading and magnetic field reversals by Harry Hess and Ron G. Mason[11][12][13][14] pinpointed the precise mechanism which accounted for new rock upwelling.

Following the recognition of magnetic anomalies defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a mid-ocean ridge, plate tectonics quickly became broadly accepted. Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power.

Study of the deep ocean floor was critical to development of the theory; the field of deep sea marine geology accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.

Key Principles

The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on mechanical differences and in the ways that heat is transferred. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The lithosphere contains both crust and some mantle. A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10-40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows).[15][16]

The plates are around 100 km (60 miles) thick and consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminum). The two types of crust differ in thickness, with continental crust considerably thicker than oceanic (50 km vs. 5 km).

One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below.

Tectonic plates can include continental crust or oceanic crust, and a single plate typically carries both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").[17] As a result, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust projects above sea level (see isostasy for explanation of this principle).

Types of plate boundaries

Three types of plate boundary.

Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:

  1. Transform boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is one example.
  2. Divergent boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries.
  3. Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.

Transform (conservative) boundaries

John Tuzo Wilson recognized that because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California).

Divergent (constructive) boundaries

Bridge across the Álfagjá rift valley in southwest Iceland, the boundary between the Eurasian and North American continental tectonic plates.

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and subsidence).

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley-Vine-Matthews hypothesis. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries

The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate.

While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts (magma) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider Mount St. Helens). The melts rise to the surface and cool forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of western South America is dense with this type of volcanic mountain building from the subduction of the Nazca plate. In North America the Cascade mountain range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire.

Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. It may have also pushed nearby parts of the Asian continent aside to the east.[18]

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska.

Oceanic / Continental
Continental / Continental
Oceanic / Oceanic

Plates may collide at an oblique angle rather than head-on to each other (e.g. one plate moving north, the other moving south-east), and this may cause strike-slip faulting along the collision zone, in addition to subduction or compression.

Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far northeastern Siberia, plus a substantial portion of the Atlantic Ocean.

Driving forces of plate motion

Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. The current view, although it is still a matter of some debate, is that excess density of the oceanic lithosphere sinking in subduction zones is the most powerful source of plate motion. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.[19] Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists.

Two and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is mantle convection from buoyancy forces.[20] How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

Friction

Basal drag
Large scale convection currents in the upper mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.
Slab suction
Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Gravitation

Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.
Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push." This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example:
1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges.
2. Mantle plumes impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.
Slab-pull
Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches.[21] There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The overall driving force for plate motion and its energy source remain subjects of ongoing research.

External forces

In a study published in the January-February 2006 issue of the Geological Society of America Bulletin, a team of Italian and U.S. scientists argued that the westward component of plates is from Earth's rotation and consequent tidal friction of the Moon. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why Venus and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars.[22] This is not, however, a new argument.

It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist Harold Jeffreys who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

Relative significance of each mechanism

Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. Vectors show direction and magnitude of motion.

The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem regarding what degree each process contributes to the motion of each tectonic plate.

The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.

One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.

The driving forces of plate motion are, nevertheless, still very active subjects of on-going discussion and research in the geophysical community.

Major Plates

The main plates are

  • African Plate covering Africa - Continental plate
  • Antarctic Plate covering Antarctica - Continental plate
  • Australian Plate covering Australia - Continental plate
  • Indian Plate covering Indian subcontinent and a part of Indian Ocean - Continental plate
  • Eurasian Plate covering Asia and Europe - Continental plate
  • North American Plate covering North America and north-east Siberia - Continental plate
  • South American Plate covering South America - Continental plate
  • Pacific Plate covering the Pacific Ocean - Oceanic plate

Notable minor plates include the Arabian Plate, the Caribbean Plate, the Juan de Fuca Plate, the Cocos Plate, the Nazca Plate, the Philippine Plate and the Scotia Plate.

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea; Pangaea eventually broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

Related article
  • List of tectonic plates
Plate tectonics map

Historical development of the theory

Continental drift

For more details on this topic, see Continental drift.

Continental drift was one of many ideas about tectonics proposed in the late nineteenth and early twentieth centuries. The theory has been superseded and the concepts and data have been incorporated within plate tectonics.

By 1915, Alfred Wegener was making serious arguments for the idea in the first edition of The Origin of Continents and Oceans. In that book, he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener wasn't the first to note this (Abraham Ortelius, Francis Bacon, Benjamin Franklin, Snider-Pellegrini, Roberto Mantovani and Frank Bursley Taylor preceded him), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that propelled continental drift.

Wegener's vindication did not come until after his death in 1930. In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution’s research vessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. All these new findings raised important and intriguing questions.[23]

Beginning in the 1950s, scientists including Harry Hess, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late eighteenth century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials recorded the Earth's magnetic field at the time.

As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping.

When the rock strata of the tips of separate continents are very similar it suggests that these rocks were formed in the same way implying that they were joined initially. For instance, some parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.

Floating continents

The prevailing concept was that there were static shells of strata under the continents. It was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt. It was apparent that a layer of basalt underlies continental rocks.

However, based upon abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later during study of Himalayan gravitation, and seismic studies detected corresponding density variations.

By the mid-1950s the question remained unresolved of whether mountain roots were clenched in surrounding basalt or were floating like an iceberg.

In 1958 the Tasmanian geologist Samuel Warren Carey published an essay The tectonic approach to continental drift in support of the expanding earth model.

Plate tectonic theory

Significant progress was made in the 1960s, and was prompted by a number of discoveries, most notably the Mid-Atlantic ridge. The most notable was the 1962 publication of a paper by American geologist Harry Hammond Hess (Robert S. Dietz published the same idea one year earlier in Nature. However, priority belongs to Hess, since he distributed an unpublished manuscript of his 1962 article already in 1960). Hess suggested that instead of continents moving through oceanic crust (as was suggested by continental drift) that an ocean basin and its adjoining continent moved together on the same crustal unit, or plate. In the same year, Robert R. Coats of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little-noted (and even ridiculed) at the time, has since been called "seminal" and "prescient." In 1967, W. Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, in 1968, Xavier Le Pichon published a complete model based on 6 major plates with their relative motions.

Explanation of magnetic striping

Seafloor magnetic striping.

The discovery of magnetic striping and the stripes being symmetrical around the crests of the mid-ocean ridges suggested a relationship. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years continues to form new ocean floor all across the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence:

  1. at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest;
  2. the youngest rocks at the ridge crest always have present-day (normal) polarity;
  3. stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has reversed many times.

By explaining both the zebra like magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field.

Subduction discovered

A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists, most notably S. Warren Carey, who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "Expanding Earth theory" hypothesis was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years. Still, the question remained: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth?

This question particularly intrigued Harry Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spreads away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches —very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is consumed in the trenches, new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Mapping with earthquakes

During the twentieth century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN)[24] to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration world wide.

Geological paradigm shift

The acceptance of the theories of continental drift and sea floor spreading (the two key elements of plate tectonics) may be compared to the Copernican revolution in astronomy (see Nicolaus Copernicus). Within a matter of only several years geophysics and geology in particular were revolutionized. The parallel is striking: just as pre-Copernican astronomy was highly descriptive but still unable to provide explanations for the motions of celestial objects, pre-tectonic plate geological theories described what was observed but struggled to provide any fundamental mechanisms. The problem lay in the question "How?." Before acceptance of plate tectonics, geology in particular was trapped in a "pre-Copernican" box.

However, by comparison to astronomy the geological revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly accepted within a few short years in the 1960s and 1970s. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons, however, today sound much like pre-Copernican astronomy.

One simply has to read the pre-plate descriptions of why the Alps or Himalaya exist to see the difference. In an attempt to answer "how" questions like "How can rocks that are clearly marine in origin exist thousands of meters above sea-level in the Dolomites?," or "How did the convex and concave margins of the Alpine chain form?," any true insight was hidden by complexity that boiled down to technical jargon without much fundamental insight as to the underlying mechanics.

With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift the sea floor to great heights. The cause of marine trenches oddly placed just off island arcs or continents and their associated volcanoes became clear when the processes of subduction at converging plates were understood.

Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre-tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift, similar to the Great Rift Valley in northeastern Africa, had split apart a single continent, eventually forming the Atlantic Ocean, and the forces were still at work in the Mid-Atlantic Ridge.

We have inherited some of the old terminology, but the underlying concept is as radical and simple as was "The Earth moves" in astronomy.

Biogeographic implications on biota

Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present day life found on different continents but having similar ancestors.[25] In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora.

Plate tectonics on other planets

The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundant water.[26]

Venus

Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet's distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that the Venusian lithosphere has thickened greatly over the course of several hundred million years) has made constraining the course of its geologic record difficult. However, the numerous well-preserved impact craters have been utilized as a dating method to approximately date the Venusian surface (since there are thus far no known samples of Venusian rock to be dated by more reliable methods). Dates derived are the dominantly in the range ~500 to 750 Ma, although ages of up to ~1.2 Ga have been calculated. This research has led to the fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at least once in its distant past, with the last event taking place approximately within the range of estimated surface ages. While the mechanism of such an impressionable thermal event remains a debated issue in Venusian geosciences, some scientists are advocates of processes involving plate motion to some extent.

One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water to be present.[27][28] The Earth's crust is soaked with water, and water plays an important role in the development of shear zones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or was once active on this planet.

Mars

Unlike Venus, the crust of Mars has water in it and on it (mostly in the form of ice). This planet is considerably smaller than the Earth, but shows some indications that could suggest a similar style of tectonics. The gigantic volcanoes in the Tharsis area are linearly aligned like volcanic arcs on Earth; the enormous canyon Valles Marineris could have been formed by some form of crustal spreading.

As a result of observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999, large scale patterns of magnetic striping were discovered on this planet. To explain these magnetization patterns in the Martian crust it has been proposed that a mechanism similar to plate tectonics may once have been active on the planet.[29][30] Further data from the Mars Express orbiter's High Resolution Stereo Camera in 2007 clearly showed an example in the Aeolis Mensae region.[31]

Galilean satellites

Some of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although the materials and specific mechanisms may be different from plate-tectonic activity on Earth.

Titan

Titan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe, which landed on Titan on January 14, 2005.[32]

See also

Notes

  1. H.H. Read, and Janet Watson. 1975. Introduction to Geology. (London, UK: Macmillan; New York, NY: St. Martin's Press), 13-15.
  2. W.J. Kious, and R.I. Tilling. 1996. "Historical perspective". This Dynamic Earth: the Story of Plate Tectonics. Retrieved October 29, 2008. (Washington, DC: U.S. Geological Survey. ISBN 0160482208), Retrieved October 18, 2008. “Abraham Ortelius in his work Thesaurus Geographicus … suggested that the Americas were "torn away from Europe and Africa … by earthquakes and floods …. The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents].”
  3. 3.0 3.1 Henry Frankel, 1978-07. Arthur Holmes and Continental Drift. The British Journal for the History of Science. 11(2):130–150.
  4. J. Joly, 1909. Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History. Archibald Constable.
  5. Patrick Hughes, Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle. Earth Observatory, NASA. Retrieved October 18, 2008. "… on January 6, 1912, Wegener … proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography."
  6. Alfred Wegener. (1915 original) 1966. The Origin of Continents and Oceans, translated by John Biram. reprint ed. (Mineola, NY: Courier Dover. ISBN 0486617084), 246.
  7. Patrick Hughes, Alfred Wegener (1880-1930): The Origin of Continents and Oceans. Earth Observatory, NASA. Retrieved October 18, 2008. "By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands), …"
  8. Arthur Holmes, 1928. Radioactivity and Earth Movements. Transactions of the Geological Society of Glasgow 18:559–606.
  9. Arthur Holmes. 1978. Principles of Physical Geology, 3rd. (Hoboken, NJ: Wiley. ISBN 0471072516), 640-641.
  10. S.W. Carey, 1958. Continental drift, a symposium … held in the Geology Department of the University of Tasmania, in March, 1956. (Hobart, AU: Geology Dept., University of Tasmania), 177-363.
  11. Ben J. Korgen, 1995. A Voice From the Past: John Lyman and the Plate Tectonics Story. Oceanography 8(1):19–20. Retrieved October 18, 2008.
  12. Fred Spiess, and William Kuperman. 2003. The Marine Physical Laboratory at Scripps. Oceanography 16(3):45–54. Retrieved October 18, 2008.
  13. R.G. Mason A.D. Raff. 1961. Magnetic survey off the west coast of the United States between 32°N latitude and 42°N latitude. Bulletin of the Geological Society of America. 72:1259–1266.
  14. A.D. Raff, R.G. Mason. 1961. Magnetic survey off the west coast of the United States between 40°N latitude and 52°N latitude. Bulletin of the Geological Society of America 72:1267–1270.
  15. Glenn Elert, (ed.) 1997. Speed of the Continental Plates. The Physics Factbook.
  16. Paul L. Hancock, and Brian J Skinner. 2000. The Oxford Companion to The Earth. (Oxford, UK; New York, NY: Oxford University Press. ISBN 0198540396.)
  17. Victor A. Schmidt, and William Harbert. [http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html "The Living Machine: Plate Tectonics"], Planet Earth and the New Geosciences, third ed. (Dubuque, IA: Kendall/Hunt Pub. Co. ISBN 0787242969.) Retrieved October 18, 2008.
  18. Rob Butler, 2001. Where and how do the continents deform?, Himalayan tectonics, Dynamic Earth. School of Earth Sciences, University of Leeds. Retrieved October 18, 2008.
  19. Pedro Mendia-Landa, Myths and Legends on Natural Disasters: Making Sense of Our World. Retrieved October 18, 2008.
  20. Tanimoto Toshiro, and Lay Thorne. 2000. Mantle dynamics and seismic tomography. Proceedings of the National Academy of Science 97(23):12409–12410.
  21. C.P. Conrad, C. Lithgow-Bertelloni. 2002. How Mantle Slabs Drive Plate Tectonics. Science 298(5591):L45.
  22. Richard A. Lovett, 2006. Moon Is Dragging Continents West, Scientist Says. National Geographic News. Retrieved October 18, 2008.
  23. Laurence Lippsett, 2001. Maurice Ewing and the Lamont-Doherty Earth Observatory. Living Legacies. Retrieved October 18, 2008.
  24. Worldwide Standardized Seismograph Network.Retrieved October 29, 2008.
  25. S.J. Moss, and M.E.J. Wilson. 1998. "Biogeographic implications from the Tertiary palaeogeographic evolution of Sulawesi and Borneo", in Hall, R., and J.D. Holloway eds. Biogeography and Geological Evolution of SE Asia. (Leiden, NL: Backhuys. ISBN 9073348978), 133-163. Retrieved October 18, 2008.
  26. Diana Valencia, Richard J. O'Connell, and Dimitar D. Sasselov. 2007. Inevitability of Plate Tectonics on Super-Earths. Astrophysical Journal Letters 670(1):L45–L48.
  27. Henry Bortman, 2004. Was Venus alive? 'The Signs are Probably There'. Astrobiology Magazine. Retrieved October 18, 2008.
  28. J.F. Kasting, 1988. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. Icarus 74(3):472–494.
  29. J.E.P. Connerney, M.H. Acuña, P.J. Wasilewski, N.F. Ness, H. Rème, C. Mazelle, D. Vignes, R.P. Lin, D.L. Mitchell, and P.A. Cloutier. 1999. Magnetic Lineations in the Ancient Crust of Mars. Science 284:794.
  30. J.E.P. Connerney, M.H. Acuña, N.F. Ness, G. Kletetschka, D.L. Mitchell, R.P. Lin, and H. Rème. 2005. Tectonic implications of Mars crustal magnetism. Proceedings of the National Academy of Sciences 102:14970–14975.
  31. Tectonic signatures at Aeolis Mensae. European Space Agency. Retrieved October 18, 2008.
  32. Laurence A. Soderblom, Martin G. Tomasko, Brent A. Archinal, Tammy L. Becker, Michael W. Bushroe, Debbie A. Cook, Lyn R. Doose, Donna M. Galuszka, Trent M. Hare, Elpitha Howington-Kraus, Erich Karkoschka, Randolph L. Kirk, Jonathan I. Lunine, Elisabeth A. McFarlane, Bonnie L. Redding, Bashar Rizk, Mark R. Rosiek, Charles See, and Peter H. Smith. 2007. Topography and geomorphology of the Huygens landing site on Titan. Planetary and Space Science 55(13):2015.

References
ISBN links support NWE through referral fees

  • Atkinson L., and C. Sancetta. 1993. Hail and farewell. Oceanography 6(34).
  • Conrad C.P., C. Lithgow-Bertelloni. 2002. How Mantle Slabs Drive Plate Tectonics. Science 298(5591):L45.
  • Frankel, Henry, 1978-07. Arthur Holmes and Continental Drift. The British Journal for the History of Science 11(2):130–150.
  • Hancock, Paul L. and Brian J Skinner. 2000. The Oxford Companion to The Earth. Oxford, UK; New York, NY: Oxford University Press. ISBN 0198540396.
  • Lyman, J., and R.H. Fleming. 1940. Composition of Seawater. J Mar Res. 3:134–146.
  • McKnight, Tom. 2004. Geographica: The complete illustrated Atlas of the world. New York, NY: Barnes and Noble Books. ISBN 076075974X.
  • Oreskes, Naomi ed. 2003. Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Boulder, CO: Westview. ISBN 0813341329.
  • Read, H.H. and Janet Watson. 1975. Introduction to Geology. London, UK: Macmillan; New York, NY: St. Martin's Press.
  • Schubert, Gerald, Donald Lawson Turcotte, Peter Olson, and NetLibrary, Inc. 2001. Mantle Convection in the Earth and Planets. Cambridge, UK; New York, NY:Cambridge University Press. ISBN 052135367X.
  • Stanley, Steven M. 1999. Earth System History. New York, NY: W.H. Freeman. ISBN 0716728826.
  • Sverdrup, H.U., M.W. Johnson, and R.H. Fleming. 1942. The Oceans: Their physics, chemistry and general biology. New York, NY: Prentice-Hall.
  • Tanimoto, Toshiro, and Thorne Lay. 2000. Mantle dynamics and seismic tomography. Proceedings of the National Academy of Science 97:12409.
  • Thompson, Graham R, and Jonathan Turk. 1991. Modern Physical Geology. Philadelphia, PA: Saunders College Publishing. ISBN 0030253985.
  • Toshiro, Tanimot,o and Lay Thorne. 2000. Mantle dynamics and seismic tomography. Proceedings of the National Academy of Science 97(23):12409–12410.
  • Turcotte, D.L., and G. Schubert. 2002. Geodynamics, Second Ed. Hoboken, NJ: John Wiley & Sons. ISBN 0521666244.
  • Vine, F.J., and D.H. Matthews. 1963. Magnetic anomalies over oceanic ridges. Nature 199:947–949.
  • Wegener, Alfred. (original 1915) 1966. The Origin of Continents and Oceans, translated by John Biram. reprint ed. Mineola, NY: Courier Dover. ISBN 0486617084
  • Winchester, Simon. 2003. Krakatoa: The Day the World Exploded: August 27, 1883. New York, NY: Harper-Collins. ISBN 0066212855.

External links

All links retrieved May 6, 2015.


Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.