Classification of rocks

Rocks found on the Earth’s surface actually come from inside the Earth – so they tell us a lot about the Earth’s interior. They are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks.

Rock types

 

Type of rock Example
Igneous rock
Lichen on granite

Granite

Sedimentary rock
Navajo Sandstone formations in Utah

Sandstone

Metamorphic rock Slate in the Lake District

Slate

Igneous rock

Igneous rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms. If magma cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering.

Examples of igneous rocks include basalt and granite.

Example of igneous rocks

Type of rock Example
Basalt
Basalt colums in the Giant's Causeway

Basalt

Granite
A granite outcrop on Bodmin Moor

Granite

Metamorphic rock

Slate Quarry, Cullipool

Slate Quarry, Cullipool

Metamorphic rocks have been subjected to tremendous heat and/or pressure, causing them to change into another type of rock. They are usually resistant to weathering and erosion and are therefore very hard-wearing.

Examples of metamorphic rocks includemarble, which originates from limestone,slate, which originates from clay, and schistsformed from sandstone or shale (sedimentary rocks).

Sedimentary rock

Limestone pavement on top of Malham Cove

Limestone pavement on top of Malham Cove

Sedimentary rocks are formed from sediments that have settled at the bottom of a lake, sea or ocean, and have been compressed over millions of years. The sediment comes from eroded rocks carried there by rivers or ice, and from the skeletons of sea creatures.

Examples of sedimentary rocks includesandstone, limestone, chalk and clay.

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