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Types of Rock

What is a rock?

A rock is a mixture of different minerals bonded together in a lump. We usually describe and classify rocks by how they are formed.

 

Igneous Rock

Imagine igneous rock as a hot-headed person whose temper can boil up, but will eventually cool down. Igneous rocks are formed when very hot liquid rock (magma) slowly cools down. 


This can happen slowly inside the Earth or more quickly after the magma reaches the surface, for example when a volcano erupts and a mixture of lava, hot gas, ash and rock travels down its sides.  Igneous rocks do not contain fossils.


Examples of igneous rocks include:

  • Granite
  • Lava
  • Obsidian
  • Pumice
  • Basalt

Sedimentary Rock

Particles (tiny pieces) of gravel, sand, mud, silt, dead animals and plants are constantly moving into stretches of water and being deposited there. They form 'sediment', which is the mud at the bottom of a lake and the sand under the sea.


After many thousands of years, the sediment is tightly compacted and cemented together, until it is so hard that it forms sedimentary rock. Fossils are often found in sedimentary rock. 


Examples of sedimentary rocks include:

  • Sandstone
  • Limestone
  • Flint 
  • Mudstone (shale)
  • Clay
  • Conglomerate
  • Breccia
  • Gypsum

Metamorphic Rock

If any type of rock travels deep inside the Earth, after millions of years, the vast amounts of pressure and heat beneath the Earth's surface will change it into a metamorphic rock.  Sometimes fossils are found in metamorphic rock, but they are often misshapen because of the pressure involved in creating the rock.


Examples of metamorphic rocks include:

  • Marble
  • Slate
  • Schists

Glossary:

Compacted - when something is closely packed together

Constantly - happening again and again

Deposit - when something is left or dropped somewhere

Fossils - remains of animals or plants preserved inside rock or another long-lasting natural material

Magma - hot, liquid rock under the surface of the Earth

Lava -hot, liquid rock on the surface of the Earth.

Outcome - the result of something

Particles - very small pieces of something

Vast - so big it is hard to measure


View other relevant My Learning resources or see the teachers' notes page for discussion and activity ideas. 


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Italian coin encased in a piece of lava
Piece of limestone, sedimentary rock
Piece of slate, metamorphic rock