Meteorite Dictionary
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Igneous Rocks:

Rocks that form by heat and melting. The two types of igneous rocks are volcanic and plutonic. Volcanic rocks as the name implies are eruptive rock, like lavas. Plutonic igneous rocks form below the surface by the slow cooling of rocks at great depth. Volcanic rocks are fine grained because of quick cooling. Not time for crystals to grow more than microscopic in size.  Plutonic igneous rocks have larger crystals through very slow cooling and long growth times.