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What are the different categories of rocks?

Rocks are divided into three groups: igneous, sedimentary, and metamorphic.

Igneous rocks are formed by the solidification of magma (molten rock), which exists beneath or within the Earth's crust and is ejected during volcanic eruptions. The texture and properties of the thousands of types of igneous rocks vary greatly, depending upon the composition of the magma and the conditions under which the magma solidified.

Granite, for example, which has large crystals of quartz, feldspars, and mica, is formed by the slow cooling of magma underground. Other examples of igneous rocks include pegmatite, rhyolite, obsidian, gabbro, and basalt. Igneous rocks constitute 95 percent of the Earth's crust.

Sedimentary rocks are produced by the accumulation and hardening of sediments. The sediments are either particles eroded from the surfaces of pre-existing...

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