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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 as opposed to intrusive rock formation, 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.
Granite is a good example of an intrusive igneous rock. Extrusive rocks cool above the earth's surface and crystallize very quickly, as a result are finer grained. Extrusive rocks are released from the earth in the form of volcanic eruptions, which occur both on the continents and on the ocean floor.ntrusive igneous rocks is formed from magma that cools, solidifies, crystallizes, and hardens slowly within the earth's crust, and this makes its crystals large.