The Extrusive (geology) reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Extrusive (geology)

Extrusive refers to a mode of igneous rock (geology) formation, in which hot magma from inside the earth flows out (extrudes) onto the surface.

The main effect of extrusion is that the magma can cool much more quickly in the open air, and there is little time for the growth of crystals. Often, a residual portion of the groundmass (geology) fails to crystalize at all, instead becoming an interstitial glass.

If the magma contains free gas, which is common, then it may cool with large and small bubble shaped cavities, or the rock may be porous.

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