As anything heats, it rises; As it rises, it cools; As it cools, it falls; As it falls, it heats
Convection currents are fueled by heat. In the mantle, these currents are created by the extreme temperatures in the core. You might have a convection oven in your kitchen. There are also convection currents in the atmosphere which are fueled by the sun. Remember this jingle:
As anything heats, it rises; As it rises, it cools; As it cools, it falls; As it falls, it heats