thumb|upright=1.3|[[IUGS classification of aphanitic extrusive igneous rocks to their relative alkali (Na<sub>2</sub>O + K<sub>2</sub>O) and silica (SiO<sub>2</sub>) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot.

Original source:

  • (ed.); 1989: A classification of igneous rocks and glossary of terms, Blackwell Science, Oxford. ]]

thumb|A volcanic rock from Italy with a relatively large six-sided [[phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope]]

Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contrast, intrusive rock refers to rocks formed by magma which cools below the surface.

The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of crystals. Any larger crystals visible to the human eye, called phenocrysts, form earlier while slowly cooling in the magma reservoir. When igneous rocks contain two distinct grain sizes, the texture is porphyritic, and the finer crystals are called the groundmass.

Extrusive bodies and rock types

Shield volcanoes are large, slow forming volcanoes that erupt fluid basaltic magma that cools to form the extrusive rock basalt. Basalt is composed of minerals readily available in the planet's crust, including feldspars and pyroxenes.