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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.
When magma cools either above or underground, igneous rock is formed. What is the difference between intrusive and extrusive igneous rocks? Since Intrusive igneous rocks were formed underground, they had more time to release air and the size of the crystals is bigger. Extrusive rocks cool quickly so they don't have much time to compress together with all the crystals.