Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary. Igneous rocks crystallize from magma. Metamorphic rocks form by the deformation and/or recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry. Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock.
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good info !
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