Igneous rocks are one of the three main rock groups (the other two being sedimentary and metamorphic).

Most igneous rocks are hard and impermeable. They tend to form upland areas because they are resistant to erosion. In the UK, igneous and metamorphic rocks are found to the north and west of the Tees-Exe line.

Formation[edit | edit source]

They are formed when magma cools and solidifies.

Intrusive igneous rocks are formed by cooling beneath the surface of the ground - the magma cools slowly and forms large crystals. Granite is an intrusive igneous rock - the large crystals of quartz, mica and feldspar in granite are visible to the naked eye.

Extrusive igneous rocks form when magma is released onto the Earth's surface (as at volcanos). Because the molten lava cools quickly, crystal sizes are very small. Basalt is an extrusive igneous rock.

Types of Igneous Rock[edit | edit source]

Common examples of igneous rock are basalt, granite, and obsidian.

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