Boardworks igneous rock formation w8

Igneous Rock Formation
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How are rocks formed?
Rocks are classified into three groups: sedimentary,
metamorphic and igneous.
 Sedimentary rocks are made from
deposited sediments. They are quite soft,
easy to shape and may contain fossils.
marble

 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.
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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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