basalt

Basalt is a common dark-colored, fine-grained igneous rock of mafic composition. It forms when low-viscosity basaltic lava erupts or extrudes at or near the Earth’s surface and crystallizes rapidly. The typical mineral assemblage includes plagioclase feldspar and pyroxene, with olivine present in many basalts. Its texture is mainly aphanitic (fine-grained), though some varieties are vesicular or possess small crystals (phenocrysts) in an aphanitic matrix.

Basalt occurs most prominently in oceanic crust and in volcanic regions such as mid-ocean ridges, hot spots, and continental rift zones. It also forms large igneous provinces through flood basalts, where extensive lava flows cover vast areas. The intrusive, coarse-grained counterpart to basalt is gabbro. Eruptions are typically effusive, producing lava flows that can form surfaces of pahoehoe or aa, lava tubes, and pillow basalts when erupted underwater. Basalt can also be vesicular when gases are trapped during rapid cooling.

Chemically and mineralogically, basalt is mafic, with relatively low silica content (roughly 45–52 wt% SiO2) and high iron and magnesium. Common mineralogy includes plagioclase feldspar and pyroxene, often with olivine; accessory minerals may include magnetite and apatite. Basalt is important geologically as a major constituent of the oceanic crust and a key product of mantle melting and plate tectonics. It is widely used as construction material, including crushed stone and aggregate. Basaltic rocks weather to iron-rich soils and contribute to the chemical and mineralogical diversity of the crust.