These materials were worn away from the land. They then collected in low places, layer upon layer, and hardened into rock. Many sedimentary rocks contain shells, bones, and other remains of living things. Such remains, or the impressions of remains in sedimentary rocks, are called fossils. Metamorphic rocks are formed deep in the crust when igneous and sedimentary rocks are changed by heat and the weight of the crust pressing on them.
Rock formation is a slow process that has been occurring throughout geologic time.
World Book photos All rocks on the Earth's surface are made of minerals, the most common solid materials found on the Earth. Minerals are themselves made up of basic chemical substances called elements. The Earth's crust consists of the continental crust and the oceanic crust. The continental crust makes up the continents. It is thick, and it consists largely of granite, a type of hard, coarsely crystalline rock.