So far, we have examined Earth's plates and how they move, as well as what occurs at convergent, divergent and transform boundaries. But why are the plates moving?
Tectonic plates are floating above a plastic-like layer of magma material in the mantle. The intense heat in this layer causes convection currents. Convection currents are circular patterns of heat in liquids and gases. Generally, hotter material is less dense and cooler material is more dense. As material nearest to the outer core is heated, it becomes less dense and moves upward above the cooler material above. As it moves closer to the crust, however, it cools, becomes more dense and begins to sink again, creating a circular current of heat movement. Depending on the direction of these currents, the plates slowly move together, apart, or past each other.
The phenomenon is visible in heated water, as well. It happens much more quickly, of course, but it makes for a good demonstration in class.
For this lab, we place beakers of water onto a lab burner. I usually place a few beakers on each burner so that the demonstration can be attempted a different temperatures or multiple times. As the water heats, students answer discussion questions on their lab sheet including, "which part of the beaker is the hottest?" and "which part of the mantle is hottest?"
As the water heats, add a drop or two of food dye into the water. The students must watch closely because the dye will become completely mixed into the water very quickly, especially if the water is already very hot. On their lab sheets, they should draw as accurate of a sketch as possible to show the circular currents of food dye in the water. Once the dye becomes completely mixed, the convection currents are still occurring, it is just no longer visible.
Below are some images of the circular currents in action: