Is it possible to determine how much magma is beneath a volcano? In
other words, is there any new and wonderful technology that allows us to
'see' the size of a magma chamber inside the earth?
Marcy
Hi Marcy,
That's a good question. The technology isn't really that "new" or
"wonderful" but the best way to determine where a magma chamber is, and
how big it is, is to use seismicity. This can be done in two general
ways. First it is imortant to know that when seismic waves (which are really
only sound waves travelling through the Earth's body) encounter molten
material they are either slow way down or don't pass through at all.
One way to see if and where a magma chamber is present is to put
seismometers all around a volcano and record the seismic waves generated
by far-away earthquakes. You don't need the earthquakes to be
particularly big, and after not too long, your ring of seismometers have
collected data from earthquake waves coming in from all directions. This
means that some of the waves have traveled directly under your volcano.
If this happens, and there is a magma chamber present, then that
seismometer on the direct opposite side of the volcano from where the
earthquake originated won't see a seismic signal since the waves were
"blocked" by not being able to go through the magma chamber. The
seismometers on either side will see the waves from that particular
earthquake since they will have passed on either side of the magma chamber.
The other technique is to monitor the seismicity generated by the volcano
itself. Magma chambers often swell and contract, but again the small
earthquakes associated with this activity cannot pass through the magma
chamber itself so a detailed seismic map will show a "shadow zone" where
the magma chamber is.
Other techniques involve monitoring how the volcano swells prior to an
eruption. If you measure enough places on the surface, you can plug all
the data into computer programs that tell you how deep the chamber is and
how big it is.
These techniques have all been perfected at the Hawaiian Volcano
Observatory, and are now used all over the world.
I hope this helps to answer your question.
sincerely,
Scott Rowland
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