Occasionally, however, masses of hotter-than-normal rock rise independently of the broad flow, like bubbles through a flowing stream. These masses of very hot rock form rising columns with rounded tops, called plumes.he rising hot rock comes in contact with cold rocks near the surface of Earth where it gives off its heat, cools, and sinks again. Most of the rock in the mantle moves in this broad cyclic flow, indicated by the arrows in the figure.