It first must be noted that the hypothesis of mantle plumes is not universally accepted and everything we know about it is hypothetical and used to explain volcanic regions that cant be explained by plate tectonics. For example, the islands of Hawaii were created (and are still being created) by an active volcano that lies far from tectonic plate boundaries. So how did they get there? The mantle plume theory is a way of explaining this. According to this theory, convection in the mantle transports heat from the center of the Earth (the core) to the Earths surface in thermal diapirs. Mantle plumes carry heat upward in narrow, rising columns as a result of heat exchange across the core-mantle boundary (the core is much hotter than the mantle and this temperature difference causes a lot of energy to be released up through the mantle plume). The entire structure is actually thought to resemble something more like a mushroom than a column, with a long, thin channel connecting the top of the plume to its base, and a bulbous head that expands in size as the plume rises. So the plume is narrow only through the middle because the hot material moves upward through the channel faster than the main larger part of the plume.