Internal structure of the Earth:

The Earth is shaped like an orange, spherical but slightly flattened at the poles. Its structure and surface is like a badly cracked egg. The ‘cracked shell’ is the very thin crust, the ‘white’ is the mantle and the ‘yolk’ is the core. These concentric layers increase in thickness, density and temperature towards the centre. Evidence for the Earth’s structure comes from various sources. Information about the crust comes from studying mines, volcanoes and earthquakes. Evidence for the mantle and core comes from the study of earthquakes, deep volcanoes, meteorites and the Earth’s magnetic field. The diagram shows a cross section of the Earth.

The Earth’s crust is composed of igneous, metamorphic and sedimentary rocks. The crust is very thin and is broken into large and small parts, called plates. These float on the denser mantle, parts of which are molten. Over long periods of geological time, changes occur in the crust. Immense internal forces cause the plates to move resulting in faults, rift valleys, volcanoes and earthquakes. In some parts, the crust is being stretched and is under tension. Deep cracks appear in the crust, rocks break and one side slips down filling the space created. This forms a fault. Rift valleys form where two normal faults lie alongside each other. Deep cracks in the Earth’s surface can also lead to volcanoes. In other parts of the Earth, the crust is being pushed together and compressed. This causes layers of the crust to ride over each other and crumple, producing fold mountains, also resulting in earthquakes and volcanic activity.