Main Page | See live article | Alphabetical index 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. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.