Convection Currents

Heat is the transfer of kinetic energy from one substance to another. In the image, all three hands are being heated, but through different processes; the one on the left is being heated through radiation (infrared), the one above the flame is being heated through convection currents in the air, and the one holding the spoon is gaining energy through conduction.

In the interior of the Earth, thermal energy is created through the process of radioactive decay as well as the extreme pressure that occurs at the Earth's core. This energy spreads outward through conduction and convection. In conduction, molecules transfer energy through direct contact with one another, without movement of the material. If you grab a spoon that's been in a pot of boiling soup, you'll feel that the handle of the spoon is very hot even though it wasn't actually in the soup. The fast moving molecules of the hot soup bump into the atoms of the spoon, which makes the metal atoms vibrate faster. As they vibrate, they bump into neighboring atoms, which bump into additional metal atoms, and so on - until all of the atoms in the spoon are vibrating. Solid portions of the Earth - like the inner core and the mesosphere - transfer heat in this manner.

Convection is a process by which energy is carried from one location to another by molecules in motion. As molecules become more energized (hotter), they vibrate more, which causes them to spread out from neighboring molecules. The overall effect of this is that the substance becomes less dense as it becomes hotter. In the case of fluids, we have seen that they will arrange themselves in order of density, so a liquid or gas will rise to the top as it becomes less dense. As a warm fluid rises upwards, cooler fluid move in to take the place that it formerly occupied. However, as a fluid rises upwards, it moves further away from the heat source, and begins to cool off. As it cools, it becomes denser and sinks back down towards the bottom. These rising and falling movements of fluids due to heating and cooling are known as convection currents. Lava lamps are, again, an excellent visual example of convection currents in motion.

Convection currents occur in the liquid outer core, as well as in the asthenosphere. The asthenosphere is a "plastic", or ductile, layer; it appears to be solid, but is actually understood to be circulating at an extremely slow rate (centimeters/yr). This layer is approximately 700 km (400 miles) thick, so it may take a million years or longer for rock in the asthenosphere to make one complete cycle!

Oceanic Spreading

IMAGES CREATED BY GAVS OR OPENSOURCE