Geology

The kind of rock an extrusive lava makes is largely dependent on the chemistry of the venting magma (Figure 1).

Basalt, andesite, and rhyolite. Basalt, usually the first lava to form, contains a high percentage of ferromagnesian minerals and about 25 to 50 percent silica, making it dark green, gray, or black. Andesite is a lighter greenish‐gray and has more silica and plagioclase feldspar and less ferromagnesian minerals than basalt. Rhyolite is the most silicious of the extrusive rocks, containing at least 65 percent silica (mostly in feldspar minerals and quartz) and few ferromagnesian minerals. This chemistry gives it a tan, pink, or cream color. Dacite has a composition that falls between those of andesite and rhyolite— it has slightly less potassium feldspar and quartz and slightly more ferromagnesian minerals than rhyolite. Dacite is generally a light grayish‐green and often difficult to distinguish from rhyolite in the field.

Mafic, felsic, and intermediate extrusive rocks. More general terms for these rocks are mafic, felsic, and intermediate. Mafic rocks have about 50 percent silica and high amounts of iron, magnesium, and calcium and are dark in color. A common mafic rock is basalt. Felsic rocks are rich in silica, potassium, sodium, and aluminum and contain only small amounts of iron, magnesium, and calcium. Typical felsic rocks are dacite and rhyolite. Felsic magmas are the most viscous because of their high silica content. Intermediate rocks, such as andesite, fall between the mafic and felsic classifications.

Ultramafic extrusive rocks. A less common group are the ultramafic rocks, which consist almost entirely of ferromagnesian minerals and have no feldspars or quartz. They contain less than 45 percent silica, and are believed to originate from the mantle. These are some of the least viscous lavas because of their low silica content. A komatiite is a typical ultramafic extrusive rock that is mostly olivine and pyroxene, with lesser feldspar.