The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt because of heat from the outer core. This lower down rock is heated till it is less dense than the rock above it and consequently rises with more dense rock taking its place. Once this less dense molten rock has almost reached the crust it begins to cool while moving along the bottom of the crust and then it sinks back to the core to be heated again. These are called convection currents.