Convection Currents
Depiction of the internal structure of the Earth. Image courtesy of USGS.
Image is in the public domain.
audio
Scientists hypothesize that convection currents in Earth's mantle are the driving force behind tectonic plate movement. The idea is that over time, magma in the mantle circulates, a little like boiling water. Magma near the mantle/outer core boundary becomes heated and rises towards the lithosphere because it is less dense. As the magma flows beneath the lithosphere, it causes the plates to split apart. Therefore, the heating & rising part of the convection cell is associated with divergent plate boundaries.
As the plates move away from each other, the magma underneath begins to cool. Eventually the now cold, dense rocks are pulled back down toward the mantle at subduction zones. Therefore, the cooling & sinking part of the convection cell is associated with convergent plate boundaries.
This continuous cycle of rocks heating and rising and cooling and sinking sets up continuous convection currents in the mantle that move the plates around the surface of the Earth. As a result, the Earth is constantly renewing itself with plates being created, then subducted back into the Earth again. The whole process takes millions and millions of years.
Last modified: Wednesday, 14 December 2011, 9:46 AM