Convection is the process by which currents are driven by temperature differences within fluid bodies. When fluid is heated, it expands, lowering the density of the heated material, causing it to rise through the cooler fluid. As it rises, leaving the vicinity of the heat source, in this case, hot magma near the earth's surface, it will cool. When it becomes more dense (because it is cooler) than the surrounding fluid, it will begin to sink. This behavior sets up circulation currents in the fluid resevoir beneath the earth's surface, driving the hot springs.