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Igneous Rock Formation

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How are rocks formed?

Metamorphic rocks have been changed by heat and pressure in the Earth’s crust. They are hard.

Igneous rocks are formed from cooled magma from the mantle of the Earth. These rocks are very hard and difficult to shape.

Sedimentary rocks are made from deposited sediments. They are quite soft, easy to shape and may contain fossils.

Rocks are classified into three groups: sedimentary, metamorphic and igneous.

marble

limestone

granite

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Igneous rocks

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Types of magma

Magma that is high in iron and low in silica tends to be quite runny. When a volcano erupts, this type of magma flows out of the volcano in a fairly ‘safe’ eruption.

Magma that is low in iron and high in silica is thick and causes violent eruptions. Pumice, volcanic ash and volcanic bombs are produced.

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Igneous rock crystals

Magma is full of minerals that turn into crystals under the right conditions. The size of the crystals in an igneous rock is related to the rate at which the molten magma cools.

If magma cools quickly, the crystals do not have very much time to form and so are small in size. Basalt has small crystals.

On the other hand, if magma cools slowly, the crystals have more time to grow and so are large. Granite has large crystals.

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Igneous rock examples

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The formation of crystals

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