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. The scientific study of ultramafic rocks has been revealing some of the geological processes in the Earth's history.
Ultramafic rocks are generally composed of more than 90 percent mafic minerals—that is, they have a high content of magnesium oxide (more than 18 percent MgO) and iron oxide (FeO). Their silica content is less than 45 percent, and their potassium content is low.
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.
- Hornblendite and, rarely phlogopitite
Volcanic ultramafic rocks
Volcanic ultramafic rocks are rare outside of the Archean and are essentially restricted to the Neoproterozoic or earlier periods. There are, however, some recently erupted boninite lavas (such as in Manus Trough, Philippines) that verge on being ultramafic. Subvolcanic ultramafic rocks and dikes persist longer but are rare. Many of the lavas being produced on Jupiter's moon Io may be ultramafic, as their temperatures are higher than terrestrial mafic eruptions.
Ultrapotassic, ultramafic igneous rocks—such as lamprophyre, lamproite, and kimberlite—are known to have reached the Earth's surface. Although no modern eruptions have been observed, analogs are preserved. Vents of Proterozoic lamproite (Argyle diamond mine), and Cenozoic lamproite (Gaussberg, Antarctica) are known, as are vents of Devonian lamprophyre (in Scotland). 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. They represent low-volume volatile melts and attain their ultramafic chemistry through a process different from that which generates typical ultramafic rocks.
- Picritic basalt
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 more than 18 percent MgO, and they can be considered "ultramafic." Most of these rocks occur as dikes, diatremes, lopoliths, or laccoliths, and very rarely, intrusions. Most kimberlite and lamproite 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:
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 few true ultramafic lavas in the Phanerozoic have been recognized.
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 indicators for mineral exploration.
- 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.
- Ultrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks. These rocks are defined by a molar ratio of K2O/Na2O of greater than three in much of the scientific literature; but in other papers, they are defined as rocks with a weight-percent ratio of K2O/Na2O greater than two.
- A diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion.
- Farndon, John. 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, 2006. ISBN 0754815412.
- Pellant, Chris. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley, 2002. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press, 2001. ISBN 1582381321.
All links retrieved April 4, 2020.
- Geological Terms Beginning With "U" Geology.com.
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