The Earth is made up of different layers, or strata. It is similar to a balloon. When a balloon is cut in two, a hard rubber core can be seen. Around this are hard, compressed layers. These layers, less hard than the core, form a thin layer on the outside. Earth's hard layer is called its 'crust'. Its thickness? From 15 to 50 km (10 to 30 miles)


In comparison, the Earth's crust is much thinner than the skin of a balloon. Under the first layer of stone is another which forms the Earth's 'mantle'. This substance, far from being rigid changes shape under great pressures. Earth's mantle is 2900 km (1800 miles) deep and surrounds the Earth's core. There is an inner core and an outer core. Both are made of metal, mostly iron. The outer core is solid. The temperature is about 80000C at the core.

The total distance from the Earth's surface to its centre is about 6400 km (4000 miles). No scientific implement has reached deeper than 3 to 6 km (18 to 40 miles)! By studying land movements, much can be learned about the inside of the Earth. Thousands of earth tremors yearly send shock waves which reach into the Earth's depths.