* High Melting Point: Ultra-basic rocks are composed of minerals like olivine and pyroxene, which have very high melting points. This means they solidify quickly as they rise from the mantle, often before they can reach the surface.
* Rapid Crystallization: The rapid cooling at the surface leads to fine-grained textures, making identification of these rocks difficult. They might be misidentified as other rock types.
* Chemical Weathering: Ultramafic rocks are very reactive with water and air. They are easily weathered and eroded, making them less likely to be preserved on the surface for long periods.
* Formation Environment: Ultramafic rocks are predominantly formed in environments associated with divergent plate boundaries (mid-ocean ridges) and mantle plumes. In these environments, the rocks are often exposed to hydrothermal alteration, further contributing to their weathering and alteration.
What you *do* find:
* Komatiites: These are ancient extrusive ultramafic rocks, often found in Archean-aged terrains. They are remnants of a time when Earth's mantle was hotter.
* Peridotites: These are intrusive ultramafic rocks, commonly found in ophiolites, which are remnants of ancient ocean crust.
Important Note: The rarity of extrusive ultramafic rocks at the surface doesn't mean they don't exist. They are prevalent deep within the Earth's mantle, forming the majority of the Earth's interior.
