Rock is a naturally formed solid substance that makes up the Earth's crust. Rocks are combinations of minerals that form solids with different properties, depending on the minerals present. Most geologists exclude soil, which comes from eroded and broken-down rocks, from this category.
Rocks are classified as igneous, sedimentary, or metamorphic according to how they formed. igneous rock forms from solidified molten magma, which comes from beneath the Earth's crust. Underground magma chambers may cool to form intrusive rocks such as diorite. If magma reaches the surface through a volcano, it becomes lava and forms extrusive rocks such as basalt or volcanic rocks such as tuff.
Sedimentary Rock forms by the solidification of accumulated sediments that have settled from a state of suspension in water or air or have been precipitated from a solution. Clastic sedimentary rocks are made from particles of older rocks that have been eroded and deposited elsewhere, such as shale. Chemically precipitated rocks include limestone and rock salt. Organic rocks, such as coal, form from the the remains of plants or animals.
Metamorphic Rock has undergone transformation through heat, pressure, or chemical changes, a result of folding of the crust in mountain building. A sedimentary rock such as shale may alter to slate due to slightly increased temperature and pressure; under greater temperature and pressure, slate may recrystallize into schist, an aggregate of platy and elongated minerals such as mica. Migrating fluids below the Earth's surface may supply new ingredients to convert schist to granite, also believed to be an intrusive igneous rock. Crustal uplift and erosion can expose this granite at the surface, and it will eventually erode and provide ingredients for new deposits of sedimentary rocks.