Igneous
The oldest type of all rocks is the igneous rock (IG nee us). The word "igneous" comes from a Greek word for fire. Deep inside the earth, the temperature is very high and the minerals there are in liquid form called magma. As the magma pushes towards the earth's surface, it starts to cool and turns into solid igneous rock.
The earth's surface is constantly being eroded. This means that rocks are broken up into smaller pieces by weathering agents such as wind, water, and ice. These small pieces of rock turn into pebbles, gravel, sand, and clay. They tumble down rivers and streams. These pieces settle in a new place and begin to pile up and the sediments form flat layers. Over a long period of time, the pieces become pressed together and form solid rock called sedimentary rock. Most sedimentary rocks form under water. Most of the earth has been covered by water some time in the past. 70% of the earth is covered by water now. So sedimentary rocks are common all over the world. Sedimentary rocks are often rich in fossils.
Metamorphic
Heat and pressure can change many things. They can even change rocks. The name for rocks that has been changed is metamorphic (met uh MOR fik) rocks. Metamorphic comes from Greek words meaning "change" and "form".
Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemical reactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at 12-16 kilometers beneath the earth's surface. They begin changing at temperatures of 100 degrees Celsius to 800 degrees Celsius. If you squeeze and heat a rock for a few million years, it can turn into a new kind of rock.
All igneous rocks do not cool the same way. That is why they do not look all the same. Some cool slowly, deep under the earth's surface. These are called intrusive igneous rocks. The slow cooling formed rocks with large crystals. Granite is an example of a rock that cooled slowly and has large crystals.
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