Remember that there are three main types of rock:

• Sedimentary, for example chalk, limestone, sandstone and shale;

• Igneous, for example basalt and granite;

• Metamorphic, for example slate and marble

Continual change

The Earth's rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements. The rocks are gradually recycled over millions of years. This is called the rock cycle.

For example sedimentary rocks can be changed into metamorphic rocks, and these can be weathered and the pieces transported away. These pieces could be deposited in lakes or seas and eventually form new sedimentary rock. Many routes through the rock cycle are possible.

The rock cycle

The processes in the rock cycle are shown in this diagram. Make sure that you understand how each type of rock forms, and be ready to give an example of each type of rock.

• Sedimentaion creates layers or rock particles

• Compaction and cementation presses the layers and sticks the particles together. This creates sedimentary rock.

• Rocks underground that get heated and put under pressure are changed into metamorphic rock.

• Rocks underground that get heated so much they melt turn into magma. Magma is liquid rock. Magma also comes from deeper inside the Earth, from an region called the mantle.

• Pressure can forces magma out of the ground. This creates a volcano. When the magma cools it turns into solid rock, called extrusive igneous rock.

• Magma that cools underground forms solid rock called intrusive igneous rock.

• Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming underneath. This is called uplift.

• Weathering breaks down rocks on the surface of the Earth. There are three types of weathering - physical, chemical and biological.

• Wind and water move the broken rock particles away. This is called erosion.

• Rivers and streams transport rock particles to other places.

• Rock particles are deposited in lakes and seas, where they build up to form layers. This starts the process of sedimentation which will create sedimentary rock.