Basalt

Igneous - volcanic

Basalt is a dark-colored mafic volcanic rock composed mostly of plagioclase feldspar and clinopyroxene. Many times green olivine is present as large phenocrysts within basalt. The Hawaiian volcanoes erupt olivine basalt; basalt erupts from shield volcanoes that are much less explosive than cone-shaped stratovolcanoes (like Mt. St. Helens in Washington) and are much hotter - basaltic lavas are about 1200° C versus rhyolitic lavas that are about 800° C. The Franciscan Complex contains sea-floor basalts; in some locations in the Marin Headlands, one can find "pillow" basalts that form when basalts are erupted underwater.