a isolated shallow magma reservoir beneath the crust, nor is it a pipe of magma that

streams from the outer core. One theory holds that hot spots may begin

as a blowtorch-like thermal perturbation in a zone between the liquid outer core and overlying

mantle about 2900 km deep. The thermal plume allows solid, yet mobile

mantle to rise very slowly and convect outward. Convection is the process by which heated

material rises and cooler material sinks. Although magma may be generated as deep as

1500 kilometers, individual blobs do not traverse the entire mantle.

Let’s zoom in to look at hotspot volcanism beneath a moving plate.

As each pocket of melt stalls, its heat is transferred to adjacent rock. This process

continues to the base of the tectonic plate where decreased pressure facilitates rock

melting. The magma that forms at the base of the plate

rises through the plate in a network of cracks and shallow chambers and erupts on the surface.

Over 100's of thousand years large volcanoes built atop the plate; the weight of the volcanoes

bends the plate downward. Volcanoes that spent their constructive life

over the thermal plume slowly get rafted away on the moving plate and new volcanoes build

in their place. Multiple dikes can feed several volcanoes

from separate conduits. The moving plate drags the thermal plume with

it. This can explain why volcanoes can erupt again

after centuries of quiescence, even after they have moved off the center of the hotspot.

However, erosion greatly outpaces volcanism as eruptions wane and the buoyant effect of

the plume diminishes allowing the volcano to subside.

Though heat is being transferred by these rising blobs, little magma is created.

Critics of the plume model have argued that the magma in hot spot volcanoes comes from

relatively shallow depths in the upper mantle (less than 660 kilometers), not deep plumes,

but the anomaly observed by the PLUME researchers extends to at least 1,500 kilometers. Rock

within the anomaly is also calculated to be significantly hotter than its surroundings,

as predicted by the plume model.