Earth's thin crust is made of different, uneven layers of stones. These stones are always under pressure, mainly from forces inside the Earth. These forces are so great that they can fold the rocky layers which sometimes break in the middle, like a stick. The pieces return to their original position. This sudden, violent shock starts the Earth's crust moving. It is an earthquake.

It is possible to divide the different stones in the Earth's crust into three categories: solidified rock, sedimentary rock and igneous rock. Solidified rock is formed from liquid substances of the Earth's centre called 'magma'. Sedimentary and igneous rock are formed from pieces of splintered. Solidified rock, dead plants (coal), or animals (oil). The substances are moved around by water, ice or wind.

Igneous rock changes its form and composition at great temperatures and under immense pressure exerted over a very long time. The Earth's crust is like the bark of a tree: the planet's sap is fire!