Plumes are narrow, low-viscosity conduits in a variable-viscosity fluid, up which buoyant material preferentially rises. The function of thermal plumes is to carry heat upward. It seems likely that mantle plumes exist, and serve to carry heat from the core upward through the mantle. A starting mantle plume produces a distinctive signature on the surface of the Earth, consisting of a flood basalt and a chain of progressively younger volcanoes terminating at a hotspot that is currently volcanically active. Plumes have been implicated in continental breakup, mass extinctions and reversals of the geomagnetic field, but evidence of these associations is not yet conclusive.