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The Rock Cycle

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The rock cycle: how rocks are formed

large rock with chimney in background

There are three main types of rock: igneous, metamorphic and sedimentary.

Igneous rock

Igneous rocks are formed when hot, molten rock (magma) has cooled and hardened. There are two types of Igneous rock.

1.                   Extrusive Igneous Rock

When a volcano erupts, magma and volcanic ash explodes out and flows above the surface as lava and then cools quickly on the ground. Forming rock with small crystals.

This forms Extrusive Igneous Rock.

2.                   Intrusive Igneous Rock

Sometimes molten rock/magma comes up from deep underground but gets stuck under the earth’s crust. It stays trapped underground just underneath the Earth’s surface, where it cools slowly. Forming rock with large crystals.

 

Igneous rocks contain randomly arranged crystals. The crystals are large if the rock cooled slowly and small if it cooled quickly. Some rocks have tightly interlocking crystals, which make the rock hard, like Granite.

Over 90% of the Earth’s crust is made from Igneous rock.

Sedimentary rock

Sedimentary rocks start to form on the surface of the earth by beaches, lakes and rivers or anywhere that sand, silt, mud and other material collect, most are made from fragments of older rocks or remain of living things, cemented together. Material is washed down into lakes and seas where it settles, layer upon layer. As the material builds up, layers are buried deeper and deeper, drying and hardening into sedimentary rock.   

Metamorphic rock

Metamorphic rocks are those that have been changed from one type of rock to another, by heat and pressure - often as rocks are pushed deeper underground. This change is called metamorphism. Slate is formed from mudstone or clay subjected to such intense pressures.

 


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