||The English used in this article may not be easy for everybody to understand. (December 2011)|
A convection current is caused by the expansion (becoming bigger) of a liquid or gas due to its rise in temperature. The material becomes less heavy than the material around it and rises. As it gets colder, it starts to get heavier, which causes it to fall or sink. This starts the cycle (process) of a convection current. This only happens to fluids that are near an energy source that causes it to become bigger. For example, the movement of water in a pan is driven by a fixed heat source. When cool water goes to an edge of a pie pan, the water is dense. It flows around and back to the center. As the water nears the hot metal, it warms again and rises as steam to repeat the cycle. The hot fluid becomes less dense and rises so molecules are carried to a new location. Energy in hot fluid transfers away, fluid contracts becoming more dense, and the fluid flows downward again.
Clouds are another example. The precipitation goes down, and the resulting heat makes the air rise again, which becomes a continuous cycle. The mass or amount of fluid flowing in a circle is a convection cell. The atmospheric circulation of Earth has many convection cells. They make the climate and weather.
Heat from within the earth creates convection currents in the outer core, which cause tectonic plates to move, perhaps by a few centimeters a year. The plates rub against neighboring plates, making earthquakes and other tectonic events.
After a fluid has become bigger it becomes cooler and cooler, and it reaches a point where it sinks. A good example is a central heating system. The warm air will rise because it is less heavy and will fall where it is cold and more heavy.
Convection is a fluid dynamics process. It is a continuous cycle in which a liquid or gas is heated then becoming less dense therefore rising but as it rises and comes further away from the heat source it cools again becoming more dense therefore sinking again, once it has sunk the process just keeps happening.