Scientist think that plates move by the same basic process that occurs when you heat soup. Soup that is cooking in a pan on the stove contains currents caused by an unequal distribution of heat the pan. Hot, less dense soup is forced upward by the surrounding, cooler soup. As the hot soup reaches the surface, it cools and sinks back down into the pan. This entire cycle of heating, rising, cooling and sinking is called convection current. A version of this same process, occurring in the mantle, is thought to be the force behind plate tectonics. Scientist suggest that differences in density cause hot, plastic like rock to be forced upward to the surface.