- Extrusive
Extrusive refers to the mode of
igneous volcanic rock formation in which hotmagma from inside theEarth flows out (extrudes) onto the surface aslava or explodes violently into the atmosphere to fall back as pyroclastics ortuff . This is opposed tointrusive 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 ofcrystal s. Often, a residual portion of the matrix fails to crystallize at all, instead becoming aninterstitial natural glass orobsidian .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 vesicularbasalt .External links
* [http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html Igneous and volcanic textures - images]
* [http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/7VesicularAmygdaloidal.html Vesicular and Amygdaloidal Textures]
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