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Ultramafic is a geological term used to describes igneous and metamorphic rocks which are mostly composed of mafic minerals. Ultramafic rocks form when silicate minerals crystalize at the highest temperatures in Bowen's Reaction Series, giving ultramafic rock a very low silica content and high levels in both iron oxide and magnesium oxide (or potassium, though such rocks are technically a separate thing). 

An ultramafic rock may be either intrusive, volcanic, ultrapotassic, or metamorphic.

The mantle of Earth is composed of ultramafic rock. That is to say, the layer of rock below the crust is a thick layer of very hot rock which is also the primary source of magma. The mantle is thought to be the driving force behind plate tectonics. In certain areas of Earth there have been eruptions from the inner layers of the earth, due to ancient volcanic activity and continental shifts over a long period of time. None such eruptions have been observed during recorded history although evidence of such occurences from more volatile times has been found. This activity is known as an intrusion.

As an intrusion, ultramafic rock forms as one of many different layers of other mafic rocks. These sorts of "layered intrusions" may be found in recent areas of geological activity, such as Montana, Antartica, Greenland, South Africa, West Australia, Minnesota, and parts of Canada. Intrusive rock is also known as Plutonic.

The planet Mercury contains volcanic ultramafic rocks. On the earth, volcanic ultramafic dates back to the Archaean era or earlier. A common volcanic ultramafic rock is komatiite.   

Ultrapotassic rocks are technically different than other types of ultramafic rocks as they undergo a different chemical process in their formation. Ultrapotassic rocks can be found in dikes, diatremes, lopoliths, or laccoliths and the rare vent. Sometimes diamonds and other rare substances are found at the vents.

Ultramafic rock is said to be metamorphic when it is formed in the presence of either water or carbon dioxide.

The presence of ultramafic rocks changes the soil composition of a given area, so that only certain types of plants can grow.

Ultramafic rocks are also called ultrabasic.

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