The convection happens because hot water has a lower density and thus rises up to the surface where it cools and sinks down to the bottom again. When water starts boiling there is the additional effect of the formation of water vapor bubbles that have a much stronger buoyancy than hot water.
Convection currents are circular patterns that move air and liquids as a result of unequal heating and cooling. Hot air rises because it is less dense and then sinks as it cools. Hot water rises and then the cold water sinks.
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