A hotspot is a localized source of high heat
energy that sustains volcanism. It is not 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.