The rising of hotter material and sinking of cooler material in the mantle is caused by:

This is caused by the plates at our planet’s surface which move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to form. It moves in a pattern called a convection cell that forms when warm material rises, cools, and eventually sink down. As the cooled material sinks down, it is warmed and rises again.