Basalte

Basalte is a dark, fine-grained, mafic extrusive igneous rock that forms when basaltic lava erupts at or near the surface and cools rapidly. It is a common constituent of the oceanic crust and appears in many continental lava flows. The rock typically contains plagioclase feldspar and pyroxene, with olivine in many samples, and magnetite as an accessory mineral. Its texture is usually aphanitic, though some basalts are porphyritic with larger crystals; vesicular and glassy varieties also occur.

Basalte forms from low-viscosity magma produced by partial melting of mantle peridotite. It is commonly produced at mid-ocean ridges and hotspot settings and can also occur in continental flood basalts and volcanic islands. When lava flows cool slowly or thicken, features such as pillow basalts (underwater) and columnar jointing may develop.

In geological terms, basalt is the dominant rock type of the oceanic crust and a frequent component of continental lava flows. It has practical significance as well: crushed basalt is widely used as aggregate for construction and road bases; it is employed as decorative stone in some applications. Basalt fiber and rock wool are produced from processed basalt for reinforcement and insulation, respectively.

Basalte varieties include tholeiitic basalt, which is common in oceanic settings, and alkaline basalt, which tends to occur in continental rift zones and some intraplate areas. These distinctions reflect differences in chemical composition and tectonic setting but share the same basic rock-forming processes.