A “mantle plume” is a bubble of material which rises to the surface layers from the deep interior of the planet. The plume is the red portion shown in the drawing to the left. Such plumes are thought to form when a portion of the deep mantle is heated to several hundred degrees hotter than its surroundings. This portion of the mantle is now less dense than its surroundings and begins to rise, like a hot air balloon.
As it nears the surface, the plume may take on a mushroom-like shape, as shown here. The plume may push the surface upward, forming a rise. It also begins to generate magma, some of which escapes to the surface to form volcanoes, lava flows, and rift valleys. Rises like this may be found on Venus and Mars.
Hot plumes which rise from the interior to the surface help a planet to cool off.