Mantle Convection

What forces would lead to the movement of the plates over the surface of the earth? At this time it is believed this movement results from mantle convection. Deep within the earth there is a large amount of heat coming from two sources: the magma remnants of the Great Bombardment, and radiation from radioactive elements. This heat, in accordance with the 2 ^ndlaw of thermodynamics, must flow to a cooler place, and subsequently rises to the surface of the Earth. If the Earth were smaller, this heat would flow by conduction, but because of the relatively large size of the Earth, convection also takes place. Thus, as in heating a pot of water on a stove, convection currents are established within the earth, which bring warm material to the surface and send cooler material to the interior.

 

Figure 14.6: Earth's convection currents

The time scale involved for rocks to rise from the lower mantle to the surface, get cooled, and return to the interior is estimated to be around 200 million years. The resulting convection currents around the earth's surface is thought to be the driving force behind the movement of the plates.