Extrusive (geology)
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Extrusive refers to a mode of igneous 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.
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 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 and small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt.
External links
- Igneous and volcanic textures - images (http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html)
- Vesicular and Amygdaloidal Textures (http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/7VesicularAmygdaloidal.html)