What is a covection current caused by diffences in what?

Answer 1

Convection currents are caused by differences in density. Warmer fluid is less dense, so a fixed volume of the warmer fluid is lighter. Colder fluid is denser, and so heavier in a fixed volume. The colder, denser fluid presses down and falls below the less dense fluid, which floats up to the top of the fluid. Often, there is a heat source that the bottom of the container, so the fluid at the bottom, which was more dense, warms up and becomes less dense. At the same time, the fluid at the top cools down and becomes more dense, and so the cycle repeats, forming a convection current.