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How are rocks formed?

Bare rock in the mountainsBare rock in the mountainsRocks are the hard materials that make up the Earth’s crust. They lie beneath the soil, beneath city streets and beneath ocean waters. You can see them in cliffs along the seashore, mountaintops or in quarries. Rocks are, themselves, made up from a solid mixture of chemical substances called minerals. There are three major types of rock: igneous rock, metamorphic rock and sedimentary rock.

Six-sided columns of basalt, an igneous rockSix-sided columns of basalt, an igneous rock

Igneous rocks

Igneous (or volcanic) rocks, such as granite and basalt, result from the cooling of magma, molten rock from beneath the Earth’s crust. Magma may rise through the crust but not erupt through volcanoes. Instead, it cools to form solid rock under ground. This is how granite is formed. When magma does reach the Earth's surface, it erupts as lava. This cools and turns hard, forming basalt.

Shards of granite, an igneous rockShards of granite, an igneous rock

Metamorphic rocks

The oldest rocks on Earth are found in northern Canada. There, gneiss, a metamorphic rock, has been dated to more than 4 billion years old.

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