The Earth is almost a sphere. These are its main layers, starting with the outermost:

• crust - relatively thin and rocky
• mantle - has the properties of a solid, but can flow very slowly
• outer core - made from liquid nickel and iron
• Inner core - made from solid nickel and iron

The Earth's lithosphere is the rigid outer layer that is made up of the crust and the part of the mantle just below it. The lithosphere is made up of a mixture of minerals.

Wegener suggested that mountains formed when the edge of a drifting continent collided with another, causing it to crumple and fold. For example, the Himalayas formed when India came into contact with Asia.

It took more than 50 years for Wegener’s theory to be accepted. One of the reasons was that it was difficult to work out how whole continents could move. It was not until the 1960s that enough evidence was discovered to support the theory fully.

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth’s mantle. These are driven by the heat produced by the natural decay of radioactive elements in the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

Igneous Rock

Igneous Rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms.

When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly

A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.

The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.

These are the different processes in order:

sedimentation → compaction → cementation

Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock include: chalk, limestone, sandstone, shale

Metamorphic rock has been subjected to tremendous heat, pressure or both. This has caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard-wearing.

Examples of metamorphic rock include:

• marble - which originates from chalk or limestone
• slate - which originates from clay
• schists – which have formed from sandstone or shale (sedimentary rocks)

Limestone - which is a sedimentary rock- is a valuable resource from the Earth’s crust. It has many uses. For example:

• it can be used as a building material
• it is a major ingredient in toothpaste
• it can be used as a food additive to provide calcium ions for strong teeth and bones
• it can be processed as a useful raw material in the chemical industry

Most limestone is obtained by quarrying, where the rock is blasted out of the ground in huge pits.

Cement is made by heating powdered limestone with clay.

Cement is an ingredient in mortar and concrete. Mortar - used to join bricks together - is made by mixing cement with sand and water. Concrete is made by mixing cement with sand, water and aggregate (crushed rock).