Rock Cycle

There are three main classes or types of rock: igneous, sedimentary, and metamorphic. Igneous rocks form when magma cools and hardens. Sedimentary rocks are made of particles of materials such as shells, sand, and pebbles. Metamorphic rocks form under the surface of the earth as a result of intense pressure and heat. Sedimentary rocks are made of sand, shells, pebbles, and other particles, and they often crumble easily. Igneous and metamorphic rocks are both much harder and more difficult to break.