Difference between revisions of "Gneiss" - New World Encyclopedia

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Gneiss

Gneiss is a widely distributed type of metamorphic rock. It is formed by high-grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks usually have medium to coarse foliation (aligned by directed pressure) and are largely recrystallized. They do not, however, carry large quantities of platy minerals such as micas or chlorites. The term gneissose is used to describe rocks with properties similar to gneiss.

The study of gneiss has helped scientists understand the geological processes by which this metamorphic rock can be formed. Also, using a radiometric dating technique, researchers have identified the Acasta Gneiss (in Northwest Territories, Canada) as the oldest known crustal rock outcrop in the world.

Etymology

The word "gneiss" is from an old Saxon mining term that seems to have meant decayed, rotten, or possibly worthless material.

Composition and classification

Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and so forth. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, and so forth. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.

Gneiss resembles granite, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes. If a rock shows minerals occurring in distinct bands, it is probably gneiss.

Augen gneiss

Augen gneiss from Rio de Janeiro, Brazil.

Augen gneiss (from the German augen, meaning "eyes") is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite. It contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts within the layering of bands of quartz, biotite, and magnetite.

Acasta Gneiss

The Acasta Gneiss is the oldest known crustal rock outcrop in the world. It is an Archaean tonalite gneiss in the Slave craton in Northwest Territories, Canada.

The Acasta Gneiss is named for the nearby Acasta River east of Great Slave Lake some 350 kilometers (km) north of Yellowknife. It is found in a remote area of the Tlicho land settlement. The rock exposed in the outcrop formed just over four billion (4 x 10⁹) years ago; an age based on radiometric dating of zircon crystals (Bowring and Williams, 1999). Based on this estimate, it is the oldest known rock in the world.

In 2003, a team from the Smithsonian Institute collected a four-tonne boulder of Acasta Gneiss for display outside the National Museum of the American Indian in Washington, DC.

References
ISBN links support NWE through referral fees

Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W.H. Freeman. ISBN 0716724383.

Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.

Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.

Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.

Bowring, S.A., and I.S. Williams. 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology 134:3-16.

Stern, R.A., and W. Bleeker. 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. The Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada 25:27-31.

External links

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@@ Line 2: / Line 2: @@
 [[Image:Gneiss.jpg|thumb|Gneiss]]
-'''Gneiss''' is a common and widely distributed type of [[Rock (geology)|rock]] formed by high-grade regional [[metamorphic rock|metamorphic]] processes from preexisting formations that were originally either [[igneous rock|igneous]] or [[Sedimentary rock|sedimentary]] rocks. Gneissic rocks are usually medium to coarse [[Foliation (geology)|foliated]] (aligned by directed pressure) and largely [[Recrystallization|recrystallized]] but do not carry large quantities of [[mica]]s, [[Chlorite group|chlorite]] or other platy [[mineral]]s.
+'''Gneiss''' is a widely distributed type of [[metamorphic rock]]. It is formed by high-grade regional [[metamorphic rock|metamorphic]] processes from preexisting formations that were originally either [[igneous rock|igneous]] or [[Sedimentary rock|sedimentary]] rocks. Gneissic rocks usually have medium to coarse [[Foliation (geology)|foliation]] (aligned by directed pressure) and are largely [[Recrystallization|recrystallized]]. They do not, however, carry large quantities of platy [[mineral]]s such as [[mica]]s or [[Chlorite group|chlorites]]. The term '''gneissose''' is used to describe rocks with properties similar to gneiss.
-Gneisses that are metamorphosed igneous rocks or their equivalent are termed [[granite]] gneisses, [[diorite]] gneisses, and so forth. Depending on their [[composition]], they may also be called [[garnet]] gneiss, [[biotite]] gneiss, [[albite]] gneiss, and so forth. ''Orthogneiss'' designates a gneiss derived from an igneous rock, and ''paragneiss'' is one from a sedimentary rock. ''Gneissose'' is used to describe rocks with properties similar to gneiss.
+The study of gneiss has helped scientists understand the geological processes by which this metamorphic rock can be formed. Also, using a [[radiometric dating]] technique, researchers have identified the '''Acasta Gneiss''' (in [[Northwest Territories]], [[Canada]]) as the oldest known crustal [[Rock (geology)|rock]] [[outcrop]] in the world.
-Gneiss resembles granite, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes. If a rock shows minerals occurring in distinct bands, it is probably gneiss.
+== Etymology ==
 The word "gneiss" is from an old [[Saxony|Saxon]] mining term that seems to have meant decayed, rotten, or possibly worthless material.
+== Composition and classification ==
+Gneisses that are metamorphosed igneous rocks or their equivalent are termed [[granite]] gneisses, [[diorite]] gneisses, and so forth. Depending on their [[composition]], they may also be called [[garnet]] gneiss, [[biotite]] gneiss, [[albite]] gneiss, and so forth. ''Orthogneiss'' designates a gneiss derived from an igneous rock, and ''paragneiss'' is one from a sedimentary rock.
+Gneiss resembles granite, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes. If a rock shows minerals occurring in distinct bands, it is probably gneiss.
 ===Augen gneiss===
 [[Image:Augen-gneiss-2.jpg|right|thumb|250px| Augen gneiss from Rio de Janeiro, Brazil.]]
-Augen gneiss is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar [[porphyroclast]]s, normally [[microcline]], within the layering of the quartz, biotite and magnetite bands.
+Augen gneiss (from the German ''augen'', meaning "eyes") is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite. It contains characteristic elliptic or lenticular shear-bound feldspar [[porphyroclast]]s within the layering of bands of quartz, biotite, and magnetite.
-Etymology: from the German ''augen'', meaning "eyes".
 == Acasta Gneiss ==
@@ Line 21: / Line 25: @@
 The '''Acasta Gneiss''' is the oldest known crustal [[Rock (geology)|rock]] [[outcrop]] in the world. It is an [[Archaean]] [[tonalite]] gneiss in the [[Slave craton]] in [[Northwest Territories]], [[Canada]].
-The Acasta Gneiss is named for the nearby [[Acasta River]] east of [[Great Slave Lake]] some 350 km north of [[Yellowknife, Northwest Territories|Yellowknife]]. The rock exposed in the outcrop formed just over four billion (4 x 10<sup>9</sup>) years ago; an age based on [[radiometric dating]] of [[zircon]] crystals (4.03 Ga orthogneisses (Bowring & Williams, 1999), which are the oldest rocks in the world so far).
+The Acasta Gneiss is named for the nearby [[Acasta River]] east of [[Great Slave Lake]] some 350 kilometers (km) north of [[Yellowknife, Northwest Territories|Yellowknife]]. It is found in a remote area of the [[Tlicho]] land settlement. The rock exposed in the outcrop formed just over four billion (4 x 10<sup>9</sup>) years ago; an age based on [[radiometric dating]] of [[zircon]] crystals (Bowring and Williams, 1999). Based on this estimate, it is the oldest known rock in the world.
-In 2003 a team from the [[Smithsonian Institute]] collected a four-tonne boulder of Acasta Gneiss for display outside the [[National Museum of the American Indian]] in [[Washington, D.C]].
-The Acasta outcrop is found in a remote area of the [[Tlicho]] land settlement.
-According to some timings of the geological periods, it was formed in the [[Basin Groups]] unofficial period of the [[Hadean]] eon, which came before the Archean: see [[Timetable of the Precambrian]].
+In 2003, a team from the [[Smithsonian Institute]] collected a four-tonne boulder of Acasta Gneiss for display outside the [[National Museum of the American Indian]] in [[Washington, DC]].
 == See also ==
@@ Line 35: / Line 35: @@
 * [[Schist]]
 * [[Zircon]]
-* [[Age of the Earth]]
 ==References==
@@ Line 47: / Line 46: @@
 * Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. ''Rocks, Gems and Minerals''. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
-*Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3-16.
+* Bowring, S.A., and I.S. Williams. 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. ''Contributions to Mineralogy and Petrology'' 134:3-16.
-*Stern, R.A., Bleeker, W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, p. 27-31
+* Stern, R.A., and W. Bleeker. 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. The Acasta gneiss complex, Northwest Territories, Canada. ''Geoscience Canada'' 25:27-31.
 == External links ==