Basalt is a mafic volcanic rock consisting primarily of plagioclase feldspar and pyroxene minerals. Common accessory minerals can include other pyroxenes, olivine, quartz, and nepheline. Basalt is the volcanic equivalent of the plutonic rock gabbro, and as such has a low silica content (48%–52%). Like other volcanic rocks, basalt cools quickly after it erupts and therefore generally contains less than 50% visible crystals floating in a matrix of glass or microscopic crystals. Pillow basalt, consisting of lobes of lava emplaced and solidified on top of each other, is the result of undersea eruptions such as those along divergent oceanic plate boundaries. Basalt is also known to occur on the moon.

Because of its low silica content, which translates into a high melting point and low viscosity, basaltic lava erupts at a higher temperature (2,012–2,192°F; 1,100–1,250°C) and flows more easily across low slopes that do more silicic lava types. Under some conditions, basaltic lava can flow more than 12.5 miles (20 km) from the point of eruption. The low viscosity of molten basalt also means that dissolved volcanic gasses can escape relatively easily as the magma travels to the surface and confining pressure is reduced. Thus, basalt eruptions tend to be quiet and effusive (as typified by Hawaiian volcanoes) as compared to the explosive eruptions often associated with more viscous and silica-rich lava (as typified by Mount St. Helens). Lava fountains can, however, reach heights of several hundred meters during basaltic eruptions.

Lava flows that solidify with a smooth or ropy surface are often described using the Hawaiian term pahoehoe, whereas those which solidify with a jagged or blocky surface are described by the Hawaiian term aa. The former is pronounced "pa-hoy-hoy" and the latter is pronounced "ah-ah."

Another characteristic of many basalt flows is the presence of polygonal columnar joints, which are understood to form by contraction of the lava as it cools. The result is a system of nearly vertical joints that form a polygonal pattern when viewed from above and break the rock into slender prismatic columns.

See Also

Divergent Plate Boundary; Extrusive Cooling; Joint and Jointing; Rate Factors in Geologic Processes; Rifting and Rift Valleys; Sea-Floor Spreading