Extrusive rocks are igneous volcanic rocks which formed when hot magma extruded from inside the Earth (flew out) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff, and then solidified as rocks when this magma cooled off. Extrusive rocks are opposed to intrusive or plutonic rocks, in which magma does not reach 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. Often, a residual portion of the matrix fails to crystallize at all, instead becoming an interstitial natural glass or obsidian. If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt.