What signs are examined to determine if a volcano is going to erupt?
Adam Luxenberg
Hi Adam,
There are lots of signs that are examined, depending on how closely
monitored the particular volcano is. Probably the most common type of
monitoring is by seismicity. Even one seismometer can tell if there is an
increase of seismic activity on a usually seismically-quiet volcano. If
you have at least 3 seismometers, and they are strategically placed, you
can triangulate on earthquakes to see if they are occurring in a place
that indicates perhaps magma movement. By examining the seismic data over
a period of time you may be able to determine if the earthquakes are
migrating towards the surface (suggesting that magma is also migrating
towards the surface since the earthquakes are probably being generated as
magma breaks rocks that are in its way).
Another type of data that is used is the study of ground deformation.
When magma moves up into the shallow plumbing of a volcano, it takes up
space and pushes the surrounding rock outward. This also causes the
surface of the volcano to deform. Some points move upward and any two
points will move farther apart. By using very accurate leveling and
distance-measuring techniques, these surface changes can be measured.
Usually the changes are a few mm over a distance of a few hundred meters,
but sometimes they are dramatic. For example prior to many eruptions at
Kilauea, the summit bulges 1-2 meters upward. In the last few days prior
to the big Mt. St. Helens eruption the northern flank was bulging outward
at a few meters per day!
Some people like to monitor volcanoes by constantly monitoring gases that
come out of fumaroles. Most active volcanoes have fumaroles where
volcanic gases escape to the surface. It is relatively easy to monitor
the temperatures of these gases, and an anomalous increase in temperature
might be a sign that magma has moved closer to the surface. Monitoring
the composition of the gases is more difficult to do, and changes in the
composition are way more difficult to interpret. Many times just visual
changes to fumarole areas are indications of impending activity. If the
area of active degasing gets larger, if the plants nearby die suddenly, if
the color of any lakes or ponds nearby changes...Many volcanoes have
summit lakes through which heat and gases rise to the surface and escape.
Many of these lakes have strange colors due to all the dissolved minerals
in them, and many of the colored ones change color, pH, temperature, etc.
These too, are signs of change below but are often difficult to interpret.
A number of people are studying ways in which to use satellite data to
monitor volcanoes. It is possible to obtain thermal images of volcanic
areas, and by comparing images on a monthly or bi-weekly basis, increases
or decreases in temperatures can be detected. Additionally, some new
technologies have allowed for the determination of very accurate
topography from satellite data. This technology may someday allow for the
remote monitoring of surface deformation assicated with sub-surface magma
movement. This process is still being developed. It usually
takes too long to get satellite data processed for this technique to be
useful in a rapidly-escalating crisis so it would be used over the long
term, in the years to months prior to an eruption rather than the hours prior.
Hopefully this will give you at least a quick look at how volcanoes are
monitored. There are books on this subject if you want even more details.
The sad fact is that many many volcanoes occur in poor countries and may
not even have a single seismometer on them. These also tend to be the
most dangerous volcanoes, especially those that erupt infrequently. In
those cases the population may not even know they're living on an active
volcano! On the other hand we have Kilauea. It almost never erupts
violently, is in a rich country, but is dotted with probably a couple
hundred monitoring devices, level lines, gas meters, etc. I don't mean to
sound cynical; many of the monitoring techniques that are today used on
dangerous volcanoes around the world could only have been developed in the
relatively safety of Kilauea.
Sincerely,
Scott Rowland, University of Hawaii
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