Convection is the heat transfer mode that occurs within fluids which involves the bulk motion of material at different temperatures. Hotter material will flow towards cooler material to try and get the system in thermal equilibrium.
The Earth's 2,890 km thick mantle is made of solid rock and its heat transfer mechanism of choice is convection.
HOLD ON! Those sound like two ideas in direct conflict with each other: solid rock is not something we typically think of flowing in response to thermal instabilities. But that's what it does. I am considering working on a page that goes into the physics of mantle convection which will include how we know it's convecting, how it's convecting and why the hell heat doesn't just conduct like normal solid material. However, my market research suggests that that's not what the people want! The people want colorful pictures and animated gifs, so the physics will have to come later.
Anyways, we can model the convection of the mantle by solving numerically the equations of conservation of mass, conservation of momentum, and conservation of energy.
The following images are results of a numerical model constructed using the COMSOL Multiphysics finite element package. The colors represent temperatures, where red is hottest and blue is coolest and the velocity field is shown in white. This temperature profile is uniform and linearly increasing with depth, and does not represent the temperature structure in the mantle at all! It is simply convenient for demonstrative purposes.
Density depends on temperature, with cooler material compacting due to less particle motions, so this system is highly unstable! The cooler, denser material is at the top. This kind of instability is called a Rayleigh-Taylor type instability, and hot, less-dense material from the bottom is rising in response to the instability.
It could also happen the other way, where cooler, denser material sinks to the bottom. In this case it appears as though two convective cells are forming, for the time being.
The steady state of the system tends to look like this: one convective cell. Steady state does not mean everything has stopped moving (see how there is still a velocity field?), it simply means the instabilities have been overcome and the motion is predictable. How did it get that way? You'll have to see the awesome animations on the next page.
NEXT: SEE IT IN ACTION HERE!