Figure 6-11Convection and the Mechanism of Plate Tectonics (a and b) Heat supplied by burning gas or a heating coil warms the water at the bottom of a pot. The heated water consequently expands, decreasing its density. This lower-density water rises (like bubbles in soda) and transfers its heat to the cooler surroundings. When the hot rising water gets to the top of the pot, it loses a lot of heat into the room, becomes denser, and sinks back to the bottom of the pot to repeat the process. (c) Convection currents in Earth’s interior are responsible for pushing around rigid plates on its crust. New crust forms in oceanic rifts, where magma oozes upward between separating plates. Mountain ranges and deep oceanic trenches are formed where plates collide, and crust sometimes sinks back into the interior. Note that not all tectonic plates move together or apart—some scrape against each other.