The earth has several major layers – a hot metallic core, a less hot liquid mantle, and the solid lithosphere and crust on top (see background section of Journey Through Earth for more information). The hot metallic core causes the mantle immediately above to heat up. As the liquid rock in the mantle heats up, it rises because a heated liquid (or gas) expands and becomes less dense than the cooler liquid (or gas) nearby. When this hot liquid reaches the top of the mantle layer, it gets pushed aside by more hot mantle rising below it, spreading out under the solid lithosphere above like a cloud of steam hitting the ceiling of the kitchen. As it spreads out, it cools. Cool liquids (and gases) shrink in volume and are more dense than the warmer liquids (or gases) nearby. Therefore, the cooled mantle sinks to the bottom of the mantle layer where it gets heated by the core and begins the cycle anew.
This process through which heat energy is transferred through currents within a liquid or gas is called convection. The cyclical nature of the process in an enclosed system like the mantle of the Earth results in convection cells – local regions of liquid or gas that form a relatively stable cycle (heating, rising, moving aside, cooling, and sinking in roughly the same location over and over again).
Convection takes place in many other systems. A pot of water boiling on the stove is a good example of convection. Watch spaghetti boiling in a large pot and you will see the noodles rise near the middle of the pot above the flames, spread out over the surface, and fall again near the edges where it is cool. In the Earth’s atmosphere, convection results in regional weather patterns and thermals (rising columns of heated air). Eagles and hang gliders both take advantage of thermals to stay aloft. In the Earth’s oceans, the warm ocean water near the equator tends to follow currents towards the poles while cold polar ocean water follow currents back again to the equator.
A key concept is that hot fluid and gas rises and cold fluid and gas sinks. To demonstrate this principle, you can create a hot air balloon in the classroom using a dry cleaner bag and a hand-held hair dryer.
Students should have learned about the Earth’s layers. Ideally, they will also have learned about density and the relationship between temperature, volume, and molecular motion. However, this lesson is written assuming that students don’t know about density and heat yet.