Convection occurs within the mantle of the Earth. The lower portion of the mantle, the aesthenosphere, heats from the high temperatures of the Earth's core. As it heats, this part of the mantle gains kinetic energy and expands in volume. Thus, it becomes less dense and begins to rise. As the particles rise and move away from the Earth's core, kinetic energy is lost. Thus, the particles slow down, contract, become more dense and sink. The pattern of falling, rising and falling is a cycle we know as a convection current. Scientists think that it's currents such as these that result in seafloor spreading along the Mid- Atlantic ridge.
Downers Grove Grade School, Illinois, explains convection currents and how they relate to seafloor spreading along the Mid-Atlantic ridge.