Convection currents in Earth's mantle are caused by the rise of hot material rising towards the crust, becoming cooler and sinking back down. This process occurs repeatedly, causing the currents to constantly flow. The movement of the currents plays a factor in the movement of the mantle.
The convection currents also help transfer heat from Earth's core, where magma is created through radioactive decay, to the mantle. This heat transfer continues through the layers of the crust until it reaches the surface. The convection currents, as they carry heat upward with the magma, break apart the plates at areas referred to as divergent plate boundaries.
The plates spread at the breaking point, but this pushes the edges of the ocean plates down into the trenches or subduction zones where it is reclaimed or recycled back into the asthenosphere. This occurs because the crust is broken up into multiple tectonic plates. The plates grow when the convection currents cause the spreads, but the spread forces the edges of the plates down into the subduction zones where it is melted once more and turned back into magma. This process keeps the crust in a constant state of movement, which in turn causes Earth's crust to essentially float on the currents of magma below the surface.