There are three main rock groups:

Sedimentary rock is formed when sediment settles on the bottom of a body of water, or other liquid, is compacted and gradually converted to rock through pressure from the overlying material. Sedimentary rocks are laid down in layers. Each new layer forms horizontally over older layers. These layers can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressure that do not destroy fossil remains. Sedimentary rock covers 75-80% of the Earth's land.

Metamorphic rock is the transformation of pre-exisitng rock in the Earth's crust, changing in heat and/or pressure without melting. These pre-existing rocks can be sedimentary rock, igneous rock or older metamorphic rock. They are classified by texture and by chemical and mineral assemblage. Metamorphic rock makes up a large part of the Earth's crust.

Igneous rock is formed beneath the Earth's surface, though the cooling of molten rock and solidifies, changing from a liquid to a solid. It makes up about 95% of the upper part of the Earth's crust, hidden beneath a thin layer of sedimentary and metamorphic rock.