Rocks are naturally occurring solid material consisting of various minerals. Earth's crust is largely made up of rocks. Rocks are classified by mineral composition, how they were formed and other physical attributes such as texture. Igneous rocks are formed when molten magma cools. Sedimentary rocks are formed by deposition and hardening of clastic sediments, minerals precipitates and organic matter. Metamorphic rock forms when pre-existing rock undergoes changes due to high temperatures and pressures.