The upper part of the mantle is solid but at greater depths the heat causes the rock to behave more like a liquid. The high pressure, however, stops the rock from melting. It becomes 'plastic' at temperatures in excess of 1000°C and can flow. The rocks are mainly composed of iron (Fe), magnesium (Mg), aluminium (Al), silicon (Si), and oxygen (O) compounds.
The crustal plates that form the surface of the Earth are formed not just of crustal rocks but also from the top part of the mantle. Plate movements extend 72 - 100 km into the top of the mantle,in a semi-molten layer called the asthenosphere.
Very hot rocks in the lower part of the mantle rise up through the cooler, upper, layers. As the rocks rise they cool and flow sideways. They then sink down again towards the core. This movement is called convection, and each area of rising and cooling rocks is known as a Convection Cell.
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