Mantle plumes are places where a stable upwelling of hot magma can be detected. Many geologists and seismologists believe that they are caused by stable convection cells carrying heat away from the Earth's core.
Plumes were first hypothesized to explain trails of volcanos or volcanic islands, believed to be caused when the plume melts through the overlying crust as continental drift carries the crust over the plume. The most famous example is the island chain of Hawaii.
Later, less-dense plumes were discovered when seismologists used computerized seismic tomography to map the density of the interior of the earth.