The three major types of rocks are igneous, sedimentary and metamorphic. Formed deep inside the Earth, igneous rocks form when a melted rock cools and solidifies. This liquid rock is magma, and the rate of cooling determines the exact type of igneous rock formed.
Formed on the Earth's surface, sedimentary rocks form in either water or on land. These rocks feature layers of sediment from bits of minerals, rocks, plant matter or animal matter. A metamorphic rock forms when an igneous or sedimentary rock comes under intense pressure or heat. These rocks occur deep in the Earth. Instead of melting the rock, this process changes the rock into an entirely new rock.