Ultramafic rock

Ultramafic (or ultrabasic) rocks are igneous rocks with very low silica content (less than 45%), generally >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 crystallised.

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 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 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 >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.

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 into 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 ultramaic rocks are indicative tools for mineral exploration.

Classification

Petrology | Igneous rocks