How do you know if a volcano is about to erupt?

Q. The geologists are saying all is normal with (insert the name of your volcano here) at the moment. But I wonder at what point it becomes “abnormal”…what the indicators are.

A. Okay, the sorts of indicators that seismologists, geologists and volcanologists look for in regard to “non-normal” activity (i.e. 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 prior 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 too warm, but on the other hand they can still measure temperature changes in the streams. A quick up-tick 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, automatically 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” (i.e. seismic 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 recently. But even if most other factors are absent, the “volcanic tremors” — also called “harmonic 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 that the doctor can very confidently say what is 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 is 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, the 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.

Hope that answers your question.

Guest post by Mike

(Many thanks Mike – much appreciated and good answer ~David)

14 Responses to How do you know if a volcano is about to erupt?

  1. Yolo says:

    One more thing, the title ” how do you know if a volcano is about to erupt?” is probably the reason of why people mistook this for “how a volcano erupts” instead of ” the signs of a volcanic eruption.”

  2. Yolo says:

    This is great but we really need to look deeper on the signs of volcanic eruption, like the specific reason for water heating up and decreasing its oxygen content because of a volcanic eruption. But, thankyou for the ideas.

  3. Mary O says:

    Keeping track of HVO Kilauea Status via USGS, as of late I’ve noticed an insurmountable ongoing number of quakes presently; impossible to count. Where a year ago I could count a half a dozen or less daily by the end of 2014 I was amazed at the increase 75 plus. As of recent I noticed that the Hazard Summary that was posted daily is now only accessed through a separate designated site. Should I be reading between the lines here?

  4. hi says:

    this is dumb

    • PuterMan says:

      You are, of course, entitled to your opinion however your comment might carry some weight if you had the courtesy to explain why you think it is dumb. Simply sniping at someone else’s work without qualifying your comment leaves it empty and hollow and without basis. In effect it is not not worth the electronic space it occupies unless you back it up with facts. I look forward to hearing why you consider this response to a question ‘dumb’.

  5. marlene says:

    are they’re any signs on how it will going to erupt?
    i need 10

    • PuterMan says:

      Not quite sure what you are asking. 10 signs of whether a volcano is going to erupt? This will be different for each volcano, and some may only be visible to scientists through measurements such as magma chamber extent, deformation etc. In addition how far ahead are you talking about?

      One of the most immediate signs right before an eruption would be harmonic tremor. Of course earthquakes at the depth of the magma chamber are also an indication, but may indicate the formation of a new dyke rather than a surface eruption. Dyke formation does not always breach the surface.

      Venting and incandescence are other immediate signs. Some may call incandescence an eruption, but it is not necessarily the case. Generally an eruption is the presence of an ash cloud in excess of 1km in height that is grey (ash bearing) rather then white (steam or gasses).

      The growth of a lava cone on the volcano is another sign that en eruption may occur but this could be a very long time in the future, and the same goes for deformation. As an example Yellowstone rises and falls over years but that does not mean it is going to erupt imminently.

      Can you get 10 out of that!?

  6. isam1234 says:

    nice and intresing

  7. nancy says:

    this is awsome but not what i was looking 4

    • nancy says:

      yea thats realy true i needed to know how a volcano erupts but this didint tell me how

      • PuterMan says:

        Well Nancy the title of the post is “How do you know if a volcano is about to erupt?” and I believe Mike the guest author answered the question quite well. Of course as you point out this is not HOW a volcano erupts. You could start here and then work your way up to the more technical descriptions.

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