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Obsidian is an extrusive igneous rock because it is formed from the rapid cooling of magma after eruption ontoEarth's surface. The rapid cooling of lava results in a fine-grained texture, or in this case (with obsidian), a glassy texture, where cooling takes place so rapidly that mineral crystals do not have time to form. Rock that is formed from slow cooling magma beneath the surface is intrusive igneous rock, and exhibits visible mineral crystals.
Obsidian is an extrusive igneous rock.

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14y ago

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