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A convection current is caused by the expansion of a liquid or gas due to its rise in temperature. The material becomes less dense and rises. As it gets colder, it starts to fall or sink. This starts the cycle of a convection current. This only happens to fluids that are near an energy source that causes it to expand. For example, the movement of water in a pan is driven by a localized 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.

Another example are the clouds and the earth. The precipitation goes down, and the heat source makes it rise into the clouds again, which becomes a continuous cycle. The mass of fluid flowing in a circle is a convection cell. The atmosphere of Earth has many convection cells. They make the climate and weather.

Heat from within the earth creates convection currents which cause the plates to move, perhaps by a few centimeters a year. Plates may either move away from, towards, or sideways past, neighboring plates. It is also the transformation of water to vapor due to the Earth's crust.

After the fluid has expanded, it becomes cooler and cooler, and it reaches a point where it just turns to water vapour, and the particles of the liquid in the fluid becomes a gas. Most of the information about "heat" is usually related to "the particle theory", so you may keep that in mind.

A good example is a central heating system. The heat will rise if it is less dense and will fall if it is cold and more dense. Convection is the 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 occurring. this happens in the earths outer core.