What is the theory that explains the type of eruptions
observed in the following volcano:"Mt. St. Helens"
Andi
Hi Andi,
I'm not sure if there is one "theory" that explains the Mt. St. Helens
eruptions, but rather we understand a number of processes. First, as the
Juan de Fuca plate is subducted under the North American Plate, parts of
it and/or the N. American plate start to melt. This melted rock (=magma)
rises toward the surface to feed the Cascade volcanoes.
Every once in a while enough magma accumulates under a volcano that it
starts to expand and rise the last few km to the surface. This magma is
very viscous. In the case of the big 1980 eruption, the magma accumulated
high in the volcano and as it did it bulged the north flank very
perceptibly outward.
Once the pressure becomes so great that the overlying rock of the volcano
can no longer contain it the eruption starts. There is lots of gas
dissolved within the magma and it is kept in solution by the overlying
pressure (just like the fact that you don't see bubbles in a soda before
you open it). However, as soon as the top of the volcano is blasted off
the pressure is lowered. This allows the gas bubbles to expand and force
even more magma out, which further lowers the pressure, which allows more
bubbles to expand and on and on and on. This rapid depressurization is
the reason for the high explosivity of the eruption.
Hopefully this helps to answer your question.
Sincerely,
Scott Rowland
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