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.
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.
Intrusive - Slow Cooling
In geology, an intrusion is a body of igneous rock that has crystallized from a molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Roman god of the underworld. Correspondingly, rocks of this kind are also referred to as igneous plutonic rocks or igneous intrusive rocks. This is to be contrasted with extrusive rocks. The rock surrounding a pluton is called country rock.
Source: Wikipedia.Org
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