What causes the convection in earths mantle?

Answer 1

Convection Currents

The mantle is made of much denser, thicker material, because of this the plates "float" on it like oil floats on water.

Many geologists believe that the mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling, sinking again and then heating, rising and repeating the cycle over and over. The next time you heat anything like soup or pudding in a pan you can watch the convection currents move in the liquid. When the convection currents flow in the mantle they also move the crust. The crust gets a free ride with these currents. A conveyor belt in a factory moves boxess like the convection currents in the mantle moves the plates of the Earth.

Convection currents are caused by the inner core, which is the Inner layer of the Earth.

Convection currents begin due to the extreme heat of the inner core. The inner core heats up the very bottom of the mantle. The heated up substances within the mantle has a higher temperature than those higher in the mantle and closer to the surface of the earth. The particles of the heated substances spread out because they are moving more quickly, and they become less dense and rise to the top of the mantle, pushing the cooler substances down towards the bottom of the mantle. Thus, the process of convection continues.
Convection currents are caused by the heat from the inner core. A layer in the mantle, called the asthenosphere, is fluid-like and is heated by the core, thus inducing convection currents to form.
Earths outer core.
The heat from the gradual decay of long lived radioisotopes creates areas of upwelling within the mantle material. The material that rises causes adjacent material to descend to fill the void. This motion occurs over very broad areas, and is what drives sea floor spreading and plate tectonics. It all boils down to nuclear decay.

Answer 2

The cooling of the Earth's interior.