Difference between revisions of "Ultramafic rock" - New World Encyclopedia

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'''Ultramafic''' (or ultrabasic) rocks are [[igneous rocks|igneous]] and meta-igneous rocks with very low [[silica]] content (less than 45%), generally >18% [[Magnesium|Mg]][[Oxygen|O]], high [[Iron|Fe]]O, low [[potassium]], and are composed of usually greater than 90% [[mafic]] [[mineral]]s (dark colored, high [[magnesium]] and [[iron]] content). The [[Earth's mantle]] is considered to be composed of ultramafic rocks.
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{{Claimed}}
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'''Ultramafic''' (or ultrabasic) rocks are [[igneous rocks|igneous]] and meta-igneous rocks with very low [[silica]] content (less than 45%), generally more than 18% [[Magnesium|Mg]][[Oxygen|O]], high [[Iron|Fe]]O, low [[potassium]], and are composed of usually greater than 90% [[mafic]] [[mineral]]s (dark colored, high magnesium and iron content). The [[Earth's mantle]] is considered to be composed of ultramafic rocks.
  
 
==Intrusive ultramafic rocks==
 
==Intrusive ultramafic rocks==
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Intrusive ultramafic rocks are often found in large, layered [[Ultramafic to mafic layered intrusions|ultramafic intrusion]]s where differentiated rock types often occur in layers. Such [[Cumulate rocks|cumulate]] rock types do not represent the chemistry of the magma from which they crystallized.
 
Intrusive ultramafic rocks are often found in large, layered [[Ultramafic to mafic layered intrusions|ultramafic intrusion]]s where differentiated rock types often occur in layers. Such [[Cumulate rocks|cumulate]] rock types do not represent the chemistry of the magma from which they crystallized.
 
   
 
   
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==Volcanic ultramafic rocks==  
 
==Volcanic ultramafic rocks==  
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Volcanic ultramafic rocks are rare outside of the [[Archaean]] and are essentially restricted to the [[Neoproterozoic]] or earlier, although some [[boninite]] lavas currently erupted within back-arc basins (Manus Trough, Philippines) verge on being ultramafic. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on [[Io (moon)|Io]] may be ultramafic, as evidenced by their temperatures which are higher than terrestrial [[mafic]] eruptions.
 
Volcanic ultramafic rocks are rare outside of the [[Archaean]] and are essentially restricted to the [[Neoproterozoic]] or earlier, although some [[boninite]] lavas currently erupted within back-arc basins (Manus Trough, Philippines) verge on being ultramafic. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on [[Io (moon)|Io]] may be ultramafic, as evidenced by their temperatures which are higher than terrestrial [[mafic]] eruptions.
  
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==Ultrapotassic ultramafic rocks==
 
==Ultrapotassic ultramafic rocks==
Technically [[Ultrapotassic igneous rocks|ultrapotassic rocks]] and [[melilitic]] rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-under-saturated rocks with >18% MgO. which can be considered "ultramafic".
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Most of these rocks occur as dykes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most Kimberlite and Lampproite occurrences are as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown.  
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Technically, [[Ultrapotassic igneous rocks|ultrapotassic rocks]] and [[melilitic]] rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-undersaturated rocks with greater than 18% MgO, which can be considered "ultramafic." Most of these rocks occur as dykes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most Kimberlite and Lampproite occurrences are as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown.
  
 
* [[Kimberlite]]
 
* [[Kimberlite]]
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==Distribution in space and time==
 
==Distribution in space and time==
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The majority of ultramafic rocks are exposed in [[orogenic]] belts, and predominate in [[Archaean]] and [[Proterozoic]] terranes. Ultramafic magmas in the [[Phanerozoic]] are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.  
 
The majority of ultramafic rocks are exposed in [[orogenic]] belts, and predominate in [[Archaean]] and [[Proterozoic]] terranes. Ultramafic magmas in the [[Phanerozoic]] are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.  
  
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==Ultramafic rocks and the regolith==
 
==Ultramafic rocks and the regolith==
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Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as [[calcium]], [[potassium]] and [[phosphorus]]) are exposed on the surface, the high metal content of the rocks creates unique vegetation. Examples are the ultramafic woodlands and ultramafic barrens of the [[Appalachian]] mountains and piedmont, the "wet maquis" of the [[New Caledonia rain forests]], and the ultramafic forests of [[Mount Kinabalu]] and other peaks in [[Sabah]], [[Malaysia]]. Vegetation is typically stunted, and is sometimes home to [[endemic (ecology)|endemic]] species adapted to the metallic soils.
 
Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as [[calcium]], [[potassium]] and [[phosphorus]]) are exposed on the surface, the high metal content of the rocks creates unique vegetation. Examples are the ultramafic woodlands and ultramafic barrens of the [[Appalachian]] mountains and piedmont, the "wet maquis" of the [[New Caledonia rain forests]], and the ultramafic forests of [[Mount Kinabalu]] and other peaks in [[Sabah]], [[Malaysia]]. Vegetation is typically stunted, and is sometimes home to [[endemic (ecology)|endemic]] species adapted to the metallic soils.
  
 
Often thick, [[magnesite]]-[[calcrete]] [[caprock]], clayey [[laterite]] and [[duricrust]] forms over ultramafic rocks in tropical and subtropical environments. Particular floral assemblages associated with highly [[nickel]]iferous ultramafic rocks are indicative tools for [[mineral exploration]].
 
Often thick, [[magnesite]]-[[calcrete]] [[caprock]], clayey [[laterite]] and [[duricrust]] forms over ultramafic rocks in tropical and subtropical environments. Particular floral assemblages associated with highly [[nickel]]iferous ultramafic rocks are indicative tools for [[mineral exploration]].
  
[[Category:Petrology]]
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== See also ==
[[Category:Igneous rocks]]
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[[Category:Metamorphic rocks]]
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* [[Igneous rock]]
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* [[Metamorphic rock]]
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* [[Rock (geology)]]
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==References==
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* 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.
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* Pellant, Chris. 2002. ''Rocks and Minerals''. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
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* Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. ''Rocks, Gems and Minerals''. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
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[[Category:Physical sciences]]
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[[Category:Earth sciences]]
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[[Category:Geology]]
  
[[de:Ultramafitit]]
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{{credit|104301096}}
[[ru:Ультраосновные горные породы]]
 

Revision as of 20:17, 11 May 2007

Ultramafic (or ultrabasic) rocks are igneous and meta-igneous rocks with very low silica content (less than 45%), generally more than 18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is considered to be composed of ultramafic rocks.

Intrusive ultramafic rocks

Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent the chemistry of the magma from which they crystallized.

  • Troctolite-Gabbro-Norite
  • Dunite-Peridotite
  • Anorthosite
  • Hornblendite and, rarely phlogopitite
  • Pyroxenite

Volcanic ultramafic rocks

Volcanic ultramafic rocks are rare outside of the Archaean and are essentially restricted to the Neoproterozoic or earlier, although some boninite lavas currently erupted within back-arc basins (Manus Trough, Philippines) verge on being ultramafic. Subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on Io may be ultramafic, as evidenced by their temperatures which are higher than terrestrial mafic eruptions.

Ultrapotassic, ultramafic igneous rocks such as lamprophyre, lamproite and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved. Vents of Proterozoic lamproite (Argyle diamond mine), and Cenozoic lamproite (Gaussberg, Antarctica) are known, as are vents of Devonian lamprophyre (Scotland, see Lamprophyre). Kimberlite pipes in Canada, Russia and South Africa have incompletely-preserved tephra and agglomerate facies.

These are generally diatreme events and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process to typical ultramafic rocks.

  • Komatiite
  • Picritic basalt
  • Lamprophyre
  • Kimberlite
  • Lamproite

Ultrapotassic ultramafic rocks

Technically, ultrapotassic rocks and melilitic rocks are considered a separate group, based on melting model criteria, but there are ultrapotassic and highly silica-undersaturated rocks with greater than 18% MgO, which can be considered "ultramafic." Most of these rocks occur as dykes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most Kimberlite and Lampproite occurrences are as volcanic and subvolcanic diatremes and maars; lavas are virtually unknown.

Metamorphic ultramafic rocks

Metamorphic ultramafic rocks are typically formed from ultramafic igneous protoliths. Examples include

  • Serpentinite
  • Soapstone

Distribution in space and time

The majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. Ultramafic magmas in the Phanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic.

Many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones.

Ultramafic rocks and the regolith

Where ultramafic rocks (in particular, the types which have low amounts of nutrient elements such as calcium, potassium and phosphorus) are exposed on the surface, the high metal content of the rocks creates unique vegetation. Examples are the ultramafic woodlands and ultramafic barrens of the Appalachian mountains and piedmont, the "wet maquis" of the New Caledonia rain forests, and the ultramafic forests of Mount Kinabalu and other peaks in Sabah, Malaysia. Vegetation is typically stunted, and is sometimes home to endemic species adapted to the metallic soils.

Often thick, magnesite-calcrete caprock, clayey laterite and duricrust forms over ultramafic rocks in tropical and subtropical environments. Particular floral assemblages associated with highly nickeliferous ultramafic rocks are indicative tools for mineral exploration.

See also

References
ISBN links support NWE through referral fees

  • 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.

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