reply to post by zenius
I can answer that one, if that's cool with you.
Okay, the sorts of indicators that seismologists, geologists and volcanologists look for in regard to "non-normal" activity (ie not "abnormal", which
means something quite different) are:
-- an increase in the number of tremors in and around the volcano. Even if they're only seismic tremors, something is causing them and that something
might be the volcano building up to blow.
-- an increase in the release of certain gases that are associated with impending eruption. This is well known by volcanologists and is something they
are always aware of and some of these gases are extremely toxic. Their volumes of release can suddenly change from a few cubic metres per hour to
several magnitudes beyond that prio to an eruption and their toxicity can be fatal (never mind the fact that some are extremely hot), so the experts
would then watch closely for other signs.
-- any significant rise in observed surface temperatures within the caldera or close to it: this can be measured directly (and often is with
"worrying" volcanoes), but can also be deduced by rises in flow of streams -- in winter, in colder climes -- that indicate increased melt on the
volcano's slopes. Obviously this doesn't apply to volcanoes in places like Indonesia as a rule since it's to warm, but on the hand they can still
measure temperature changes in the streams. A quick uptick of a couple of degrees is cause for concern. The river that connects with Lake Yellowstone
is also temp-monitored year round for the same reason. Water getting heated beyond what the environment would allow, automaticaly implies a volcanic
type of heat source and so it's worth keeping an eye on.
-- a change in the character of the tremors from "regular" (ie sesimic type) tremors to "volcanic" tremors, which have a very distinctive pattern and
only occur in actively volcanic areas: this always
gets the attention of the watching experts, and if some of the other factors also indicate
that the volcano is building up to something, it will often trigger a raise in status to "eruption imminent" and authorities will be informed to begin
evacuating people. This was the case with Merapi in Indonesia some weeks back. But even if most other factors are absent, the "volcanic tremors" are
so strongly indicative of trouble that they may raise the status to "eruption imminent" anyway.
It's like a doctor diagnosing a patient. There may be several symptoms, but while some are not too worrying, for a major illness there is always at
least one that is so definitive, the doctor can very confidently say what's wrong. These scientists do the same.
So, when they start issuing their guarded statements of "concern", we need to read between the lines because these people don't get all concerned over
most of what goes on, even when it looks rather alarming to most of us. When they start muttering publicly about "increased activity" and "watching
the situation closely", that when we need to sit up and take notice.
It's a dilemma for the experts, actually, because if they announce an "eruption imminent" for a US volcano, then some people will get panicky and
there could be accidents on the roads as people flee the area and so forth. At the very least, people will skip work to grab the kids and dog and
start packing valuables before bugging out.
The trouble is, the experts could get it wrong and the volcano just bubbles away for a few days then quietens down again. It happens. Just like
sometimes a doctor can misdiagnose a patient. However if people get injured or killed while fleeing on the basis of the warning, then you know someone
is going to wind up getting sued. That's what happens in the US and many other countries.
So, they experts have to be very careful and very, very sure. This also means that they have to wait longer before issuing official warnings.
Smart folks will read between the lines of the "guarded" statements and be packed and ready to go well before the official warnings come out.
Anyway. Hope that answers your question.
edit on 30/12/10 by JustMike because: typos