What is the most common volcanic rock in Earth’s crust?

Basalt is the most common volcanic rock in Earth’s crust.

Basalt forms when low-viscosity mafic lava cools at or near Earth’s surface. It is rich in iron and magnesium and relatively low in silica compared with rocks such as rhyolite. Its usual dark gray to black color reflects its mineral composition.

Most oceanic crust consists of basaltic rocks. At mid-ocean ridges, mantle material partially melts and produces magma that erupts or crystallizes to create new seafloor. Basalt also makes up many volcanic islands, including large portions of Hawaii and Iceland.

Basalt commonly contains fine-grained plagioclase feldspar and pyroxene, with olivine present in some varieties. Because lava cools quickly at the surface, its crystals are usually too small to see easily. Granite is also widespread, but it is a coarse-grained intrusive rock that crystallizes underground and is more typical of continental crust.