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 | + | '''Ultramafic''' (or ultrabasic) rocks are dark-colored [[igneous rocks|igneous]] and meta-igneous rocks that are rich in minerals containing magnesium and iron ("mafic" minerals) and have a relatively low content of [[silica]]. The [[Earth's mantle]] is thought to be composed of ultramafic rocks. Most of the exposed ultramafic rocks have been found in [[orogenic]] (mountain-forming) belts. |
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| + | == Composition == | ||
| + | |||
| + | Ultramafic rocks are generally composed of more than 90% [[mafic]] [[mineral]]s—that is, they have a high content of magnesium oxide (MgO) and iron oxide (FeO).<ref>The term ''mafic'' combines the first letters of "[[magnesium]]" and "ferrum," the [[Latin]] word for [[iron]]: ma(gnesium) + f(errum) + ic. In addition, mafic magmas are rich in [[calcium]] and [[sodium]].</ref> Their silica content is less than 45 percent. | ||
==Intrusive ultramafic rocks== | ==Intrusive ultramafic rocks== | ||
| − | Intrusive ultramafic rocks are often found in large, layered [[Ultramafic to mafic layered intrusions| | + | Intrusive ultramafic rocks are often found in large, layered [[Ultramafic to mafic layered intrusions|intrusion]]s where differentiated rock types often occur in layers. These rock types do not represent the chemistry of the magma from which they crystallized. Examples are listed below. |
* [[Troctolite]]-[[Gabbro]]-[[Norite]] | * [[Troctolite]]-[[Gabbro]]-[[Norite]] | ||
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==Volcanic ultramafic rocks== | ==Volcanic ultramafic rocks== | ||
| − | Volcanic ultramafic rocks are rare outside of the [[ | + | Volcanic ultramafic rocks are rare outside of the [[Archean]] and are essentially restricted to the [[Neoproterozoic]] or earlier periods. 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. |
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. | 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. | ||
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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-undersaturated rocks with greater than 18% MgO, which can be considered "ultramafic." Most of these rocks occur as | + | 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 dikes, 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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==Metamorphic ultramafic rocks== | ==Metamorphic ultramafic rocks== | ||
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[[Metamorphic rock|Metamorphic]] ultramafic rocks are typically formed from ultramafic igneous [[protolith]]s. Examples include | [[Metamorphic rock|Metamorphic]] ultramafic rocks are typically formed from ultramafic igneous [[protolith]]s. Examples include | ||
*[[Serpentinite]] | *[[Serpentinite]] | ||
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==Distribution in space and time== | ==Distribution in space and time== | ||
| − | The majority of ultramafic rocks are exposed in [[orogenic]] belts, and predominate in [[ | + | The majority of ultramafic rocks are exposed in [[orogenic]] (mountain-forming) belts, and predominate in [[Archean]] 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 [[obduction|obducted]] onto [[continental crust]] along and above [[subduction]] zones. | Many surface exposures of ultramafic rocks occur in [[ophiolite]] complexes where deep mantle-derived rocks have been [[obduction|obducted]] onto [[continental crust]] along and above [[subduction]] zones. | ||
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==Ultramafic rocks and the regolith== | ==Ultramafic rocks and the regolith== | ||
| − | Where ultramafic rocks (in particular, the types | + | Where ultramafic rocks (in particular, the types that 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]]. |
== See also == | == See also == | ||
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* [[Metamorphic rock]] | * [[Metamorphic rock]] | ||
* [[Rock (geology)]] | * [[Rock (geology)]] | ||
| + | |||
| + | == Notes == | ||
| + | <references/> | ||
==References== | ==References== | ||
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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. | * Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. ''Rocks, Gems and Minerals''. Rev. ed. New York: St. Martin's Press. ISBN 1582381321. | ||
| + | |||
| + | == External links == | ||
| + | |||
| + | * [http://www2.nature.nps.gov/GEOLOGY/usgsnps/noca/nocageol2a.html North Cascades Geology.] ''U.S. Geological Survey, National Park Service''. Retrieved May 22, 2007. | ||
[[Category:Physical sciences]] | [[Category:Physical sciences]] | ||
Revision as of 15:57, 22 May 2007
Ultramafic (or ultrabasic) rocks are dark-colored igneous and meta-igneous rocks that are rich in minerals containing magnesium and iron ("mafic" minerals) and have a relatively low content of silica. The Earth's mantle is thought to be composed of ultramafic rocks. Most of the exposed ultramafic rocks have been found in orogenic (mountain-forming) belts.
Composition
Ultramafic rocks are generally composed of more than 90% mafic minerals—that is, they have a high content of magnesium oxide (MgO) and iron oxide (FeO).[1] Their silica content is less than 45 percent.
Intrusive ultramafic rocks
Intrusive ultramafic rocks are often found in large, layered intrusions where differentiated rock types often occur in layers. These rock types do not represent the chemistry of the magma from which they crystallized. Examples are listed below.
- Troctolite-Gabbro-Norite
- Dunite-Peridotite
- Anorthosite
- Hornblendite and, rarely phlogopitite
- Pyroxenite
Volcanic ultramafic rocks
Volcanic ultramafic rocks are rare outside of the Archean and are essentially restricted to the Neoproterozoic or earlier periods. 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 dikes, 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
- Lamproite
- Lamprophyre
- Carbonatite
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 (mountain-forming) belts, and predominate in Archean 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 that 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
Notes
ReferencesISBN 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.
External links
- North Cascades Geology. U.S. Geological Survey, National Park Service. Retrieved May 22, 2007.
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