Large masses cool slowly to form a polygonal joint pattern.
Basalt is an extrusive volcanic rock that contains olivine and pyroxene minerals that make the rock dark-colored. It is usually fine-grained because of its rapid cooling of lava on the Earth's surface. Flood basalts are commonplace, formed by lava pouring out and solidifying.
Unweathered basalt is black or grey. Basalt blocks are often used for roads and pavements (sidewalks). It is hard-wearing and heat resistant.
On Earth, most basalt magmas have formed by melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta.
The crustal portions of oceanic tectonic plates are composed mostly of basalt, produced from upwelling mantle below mid-ocean ridges.
Basalt can be made into fiber. Basalt fiber is often used as a safe replacement for asbestos.
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