That sounds like an easy question to answer. If a volcano has been quiet long enough, it is definitely extinct. Or is it not? So, when is a volcano considered dead?
It’s not that simple for several reasons. First of all, there is the human understanding of time. If a volcano doesn’t erupt for 10,000 years, then one tends to believe that it is dead. 10,000 years, that sounds like a very long period of time but only in the human imagination. Geologically speaking it looks completely different as 10,000 years is nothing.
A Volcano Indicates Activity
That is why we cannot simply declare the volcanoes dead. Time plays a rather subordinate role in assessing volcanic activity. Rather, geophysicists look for so-called activity parameters, i.e. for indications that something volcanic is still happening in a mountain. In the Eifel maars, for example, these are bubbles. One could assume that these are digester gases from some kind of decomposition process in the water. Our investigations have shown, however, that it is volcanic carbon dioxide that is seeping out of the earth there. So volcanic degassing takes place in these springs.
And so the scientists come to the conclusion that the Eifel volcanoes that have not erupted for 10,000 years are by no means dead. It cannot be predicted when the next eruption will take place or whether one will take place at all. But the volcano is definitely still active in the Eifel.
Seismic surveys are also carried out to determine volcanic activity. The speed at which elastic waves travel through the ground is analyzed. In volcanic regions, the seismic speed is decreased. In addition, uplifts and subsidence, and volcanic tremors are among the physical parameters that speak for an active volcanic system.
Volcanoes are embedded in a system. For example, Iceland lies in the mid-Atlantic ridge, where North America and Europe move away from each other. The Icelandic Eyjafjallajokull volcano, now well known to us, last erupted 190 years ago. It is a very short period of time to conclude geological developments. In this so-called spreading zone, tremors and volcanoes arise because the gap created by the movement is filled with magma.
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Once Dead, Always Dead – When Is A Volcano Considered Dead?
However, volcanoes can also die due to plate movements. When tectonic plate boundaries shift, volcanoes are cut off from their magma sources and become extinct. However, these are processes that extend over a much longer period of time than the previously thought and it can take hundreds of thousands of years.
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The Dhinodhar Hills in Gujarat, India, is volcanic rock, but nothing geophysically has changed there, there are no anomalies and no signs of volcanic activity. And it stays that way because, once the volcano is dead, it is dead.