Overall Classifications

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Overall Classifications

Igneous rocks

Igneous rocks are fire formed

They originate from the magma in the mantle of the Earth.

Extrusive igneous rocks are formed by being thrown out during a volcanic eruption. They cool very quickly once they reach the surface.

An example of an extrusive igneous rock is basalt.

Intrusive igneous rocks are formed under the surface of the earth, cooling very slowly to form a large area of rock under the surface of the earth.

An example of an intrusive igneous rock is granite.

Sedimentary rocks

Over millions of years the actions of rivers, glaciers and the wind produce large amounts of small particles called sediments.

These accumulated on sea floors in layers, and due to the pressure of material above they were compressed to form rocks.

Examples include:

  • Sandstone, which is made from compressed sand;
  • Clay, which is made from compressed mud;
  • Coal, which is made from the compressed remains of ancient forests;
  • Chalk and limestone, which is made from the compressed remains of plants and animals.

Rocks which have been changed in shape or form, usually by heat or pressure, are called metamorphic rocks.

They begin as either igneous or sedimentary rocks before being changed, usually in areas of high pressure and intense heat, such as plate margins.

Examples include slate (which used to be clay) and marble (which used to be limestone).

The Rock Cycle