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Extrusive or volcanic rocks crystallize from lava at the earth's surface. The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals.
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Extrusive or volcanic rocks form from lava on the earth's surface. The texture of igneous rocks is determined by the speed of the cooling process. Slow cooling results in larger crystals, while fast cooling leads to smaller crystals.