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1. n. [Geology]
An aggregate of minerals or organic matter (in the case of coal, which is not composed of minerals because of its organic origin), or volcanic glass (obsidian, which forms a rock but is not considered a mineral because of its amorphous, noncrystalline nature). Rocks can contain a single mineral, such as rock salt (halite) and certain limestones (calcite), or many minerals, such as granite (quartz, feldspar, mica and other minerals). There are three main types of rocks. Sedimentary rocks like sandstone and limestone form at the Earth's surface through deposition of sediments derived from weathered rocks, biogenic activity or precipitation from solution. Igneous rocks originate deeper within the Earth, where the temperature is high enough to melt rocks, to form magma that can crystallize within the Earth or at the surface by volcanic activity. Metamorphic rocks form from other preexisting rocks during episodes of deformation of the Earth at temperatures and pressures high enough to alter minerals but inadequate to melt them. Such changes can occur by the activity of fluids in the Earth and movement of igneous bodies or regional tectonic activity. Rocks are recycled from one type to another by the constant changes in the Earth.