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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.
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What is a Volcanic Hotspot? (Educational)

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羅紹桀 2015 年 7 月 25 日 に公開

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