Convention currents occur in

Convection currents occur in the mantel due to density. Cold air sinks and hot air rises. As the cold air sinks, it gets heated up and travels upward. Then as it cools down it sinks back down and gets heated up again. The cycle then repeats. These currents are what move tectonic plates.

Convection currents in the magma drive plate tectonics. Heat generated from the radioactive decay of elements deep in the interior of the Earth creates magma (molten rock) in the aesthenosphere. The aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the Earth that extends to 2900 km.