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Basalt rock

Basalt is a common gray to black volcanic rock.

It is usually fine-grained due to rapid cooling of lava on the Earth's surface.

It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria.

Unweathered basalt is black or gray.

Basalt magmas form by decompression melting of peridotite in the Earth's mantle, a process discussed in the entry for igneous rocks.

The crustal portions of oceanic tectonic plates are comprised predominantly of basalt, produced from upwelling peridotite in the mantle below ocean ridges.

Note:   The above text is excerpted from the Wikipedia article "Basalt rock", which has been released under the GNU Free Documentation License.
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