The lithosphere
The Earth has three layers: a heavy, extremely hot core in the centre, a thick mantle around it and a thin solid crust on the outside. The lithosphere (“litos” means stone) consists of the crust and the uppermost solid part of the mantle. The crust varies in thickness: about 5 km under the ocean floor, 30–40 km on the continents and even more under high mountains. The lithosphere is not one piece but is split into large plates; the plates move very slowly over the mantle — a few centimetres a year. Where plates collide the ground shakes — an earthquake — and their edges rise to form mountain ranges. The upper part of the crust is made of sedimentary rocks (sand, clay, gravel, limestone) that formed over millions of years as deposits were pressed together. Below lies a granite layer and below that a basalt layer. The very top soft and fertile layer is the soil.
Model: lay two coloured plasticine layers on the table and slowly push them from both ends; watch what happens. Explain how this resembles the formation of mountains. Show the Earth’s layers with a cut-away drawing or a circle diagram.