1. High Melting Point: Ultramafic rocks have very high melting points. They are derived from the Earth's mantle, which is incredibly hot, but still, the magma generated is often not hot enough to erupt onto the surface.
2. Rapid Cooling: If ultramafic magma does manage to reach the surface, it cools very quickly due to its low silica content. This rapid cooling doesn't give the magma enough time to crystallize into large crystals, forming volcanic rocks with a distinctive texture.
3. Chemical Reactions: When ultramafic magma interacts with crustal rocks, it can undergo chemical reactions that change its composition. These reactions often result in the formation of more silica-rich, intermediate magmas that are more likely to erupt.
4. Partial Melting: Ultramafic rocks in the mantle are rarely completely melted. Instead, they often undergo partial melting, meaning only a portion of the rock melts. The resulting magma is typically a more silica-rich composition than the original ultramafic rock.
5. Intrusive Formation: Most ultramafic rocks are found as intrusive rocks, known as peridotites, formed when magma cools and crystallizes beneath the Earth's surface.
Where we can find ultramafic extrusive rocks:
While rare, ultramafic extrusive rocks do exist in some specific geological settings:
* Ophiolites: These are fragments of oceanic crust that have been thrust onto continents. They often contain ultramafic rocks, including komatiites, which are thought to have formed from very hot, primitive magmas.
* Volcanic Settings with Extremely Hot Magmas: Some volcanic settings, like those associated with hotspots, might produce extremely hot magmas that can erupt ultramafic rocks.
In summary: The combination of high melting points, rapid cooling, chemical reactions, partial melting, and their tendency to form intrusive rocks make ultramafic extrusive rocks a rarity on Earth.
