What was the cause of the eruption of Mount St. Helens eruption?
And why did so much pressure build up?
Joanna Means
Hi Joanna,
Well, a body of magma migrated upward from within the upper mantle, or
perhaps from storage within the crust, and it is what erupted. The magma
is produced in the upper mantle by melting associated with the subduction
of the Juan de Fuca plate under the N. American plate. It probably
collects for years in deep storage, where it is not detectable by
seismometers, until it has accumulated to the point that its buoyancy
causes it to rise. Once this starts, an eruption is pretty inevitable.
It is gases in the magma that causes the eruptions to be explosive. At
depth the gas molecules (mainly H2O, SO2, and CO2) are kept in solution
rather than being bubbles of pure gas. This is like when you look in the
bottle of an un-opened soda and don't see anything. As the magma moves
upward the pressure on it drops because there is less and less overlying
rock to provide pressure, and the gas starts to come out of solution to
form bubbles (just like after you open the soda you see bubbles forming).
These bubbles want to expand, and this causes the pressure within the
magma to go up. As the magma continues to move upward, more and more
bubbles grow and want to expand, causing the pressure to build. Because
the particular magma that was erupted in 1980 is a particularly viscous
type (called dacite), the bubbles couldn't really float through it to
escape out the top so they just kept building the pressure. Eventually,
the pressure of the magma exceeded the strength and weight of the
mountain, and it exploded (this may have been precipitated by an
earthquake). As soon as the weight of the top of the volcano was removed
the pressure dropped a whole lot and now way more gas could come out of
solution, form bubbles, and escape, almost a run-away process.
Since the 1980 eruption there have been smaller explosive eruptions. The
main reasons why they have been smaller is that smaller amounts of magma
have been involved, and also there is less mountain to allow the pressure
to build up so eruptions can occur after smaller amounts of gas have
exsolved.
I hope this has answered your question.
Sincerely,
Scott Rowland
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