Lava contains 45% to 75% silica (SiO₂). The more silica, the more viscous the lava. Viscosity is also determined by the temperature of the lava. Viscous lavas are associated with more violent eruptions than are fluid lavas. Volcanic domes form from the extrusion of very viscous lavas.

A mafic lava, relatively low in silica, crystallizes into basalt, the most abundant extrusive igneous rock. Basalt, which is dark in color, is composed of minerals that are relatively high in iron, magnesium, and calcium.

Rhyolite, a light-colored rock, forms from silicic lavas that are high in silica but contain little iron, magnesium, or calcium. Because potassium and sodium are important elements in rhyolite, its constituent minerals are mostly potassium- and sodium-rich feldspars and quartz.

A lava with a composition between mafic and silicic crystallizes to andesite, a moderately dark rock. Andesite contains about equal amounts of ferromagnesian minerals and sodium- and calcium-rich feldspars.

Extrusive rocks are characteristically fine-grained. Porphyritic rock contains some larger crystals in an otherwise fine-grained rock. Rocks that solidified too rapidly for crystals to develop form a natural glass called obsidian. Gas trapped in rock forms vesicles.

Pyroclasts are the result of volcanic explosions. Tuff is volcanic ash that has consolidated into a rock. If large pyroclastic fragments have reconsolidated, the rock is a volcanic breccia.

A cinder cone is composed of loose pyroclastic material that forms steep slopes as it falls back around the crater. Cinder cones are not as large as the other two major types of cones.

A shield volcano is built up by successive eruptions of mafic lava. Its slopes are gentle but its volume generally large.

Composite cones are made of alternating layers of pyroclastic material and solidified lava flows. They are not as steep as cinder cones but steeper than shield volcanoes. Young composite volcanoes, predominantly composed of andesite, are aligned along the circum-Pacific belt and, less extensively, in the Mediterranean belt.

Plateau basalts are thick sequences of lava floods. Columnar jointing develops in solidified basalt flows. Basalt that erupts underwater forms a pillow structure. Pillow basalts are common along the crests of mid-oceanic ridges.