Extrusive rocks, such as basalts, are formed when molten material from within the earth is expelled onto the surface. Because of the difference in temperature between the hot interior and relatively cool exterior of the earth, the molten materials cool rapidly. Very fine crystals, characteristic of this type of rock, form as a result of the sudden cooling.
Extrusive 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. This is opposed to intrusive rock formation, in which magma does not reach the surface.