Outline the differences between the three different types of rock.

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Igneous rocks

Igneous rocks are distinguishable by the fact that they are made up of crystals, which vary in size depending on where the rock was fomed.

Intrusive rocks consist of larger crystals - formed inside the earth by the cooling of liquid magma (lava inside the earth), e.g. in a magma chamber.  Examples include granites and gabbros.

Extrusive rocks have smaller crystals and form when the lava cools on the earth's surface e.g. in a volcanic eruption.  Examples include basalts and andesites.

Composition of both intrusive and extrusive rocks varies depending on the original composition of the magma.

Sedimentary

Sedimentary rocks can form in rivers, lakes and oceans.

Form from particles of pre-existing rocks or clay material - sediments.

May contain shells of organisms that lived in the water column that can be preserved as fossils.

Examples include sandstones and mudstones.

Metamorphic

Can be formed from sedimentary or igneous rocks.

The rock is subjected to high pressures and/or temperatures, resulting in the rock changing to remain stable.

May be deformed.

Very dense rocks.

Limestone metamorphoses to marble.

 

Laura S. GCSE Geography tutor

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