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Rocks are classified into three major classifications: igneous, metamorphic, and sedimentary. Igneous rocks are formed by the cooling of liquid lava or magma. A major subdivision of igneous rocks is how and where the rock forms. They can be classified as intrusive or extrusive. Intrusive rocks form within the Earth's crust. As a result, the magma cools relatively slowly and the grain of the rock is more coarse. Granite is an example of an intrusive igneous rock, formed deep within mountains. Extrusive rocks form on the exterior of the Earth's crust. As a result, the lava cools more quickly and the grain of the rock is more fine. Basalt lava flows are an example of an extrusive igneous rock.
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