Voices: Dead or alive ... or neither? Why a dormant volcano is not a dead one

by Jessica Ball
Friday, January 20, 2012

Quite a lot of volcanic activity has been reaching the news lately — Sinabung in northern Sumatra, a possible eruption in the Congo and another Sumatran volcano called Seulawah Agam that may be “waking up” after 170 years of quiet. One thing that’s been mentioned in many of the news reports about these events is that the eruptions were “unexpected” or “surprising” because people had always assumed the volcanoes in question were dormant. But what does dormant really mean? And what does active or extinct mean?

The answer to this question varies significantly depending on whom you ask: There are no set definitions for active, dormant and extinct volcanoes.

Generally speaking, you could say that an active volcano is one that is currently erupting, or has erupted in the last 10,000 years. Kilauea, Stromboli and Mount St. Helens are all considered active, as are volcanoes such as Pacaya and Redoubt. A dormant volcano is one that is “sleeping” but could awaken in the future, such as Mount Rainier and Mount Fuji. An extinct volcano is “dead” — it hasn’t erupted in the past 10,000 years and is not expected to ever erupt again. Kohala on the Big Island of Hawaii could be considered an extinct volcano.

Extinct is probably the most exacting term in describing a volcano’s eruptive potential; it’s a term that is very cautiously used, given that there is almost always potential for a currently inactive volcano to begin erupting again. Extinct implies that there is no magmatic, seismic or degassing activity going on at the volcano, and that it’s never expected to have any in the future. That’s very difficult to prove, and requires field mapping to determine the volcano’s eruptive history. If it can be shown that the volcano has not erupted for periods of time much longer than its past eruptive recurrence intervals, it’s probably safe to call it extinct. For example, if a volcano’s eruptive history shows that it usually erupts every 10,000 years or so, and there hasn’t been an eruption for a million years, it may be called extinct.

The terms dormant and active are much more arbitrary. A nonscientist who lives near a volcano might assume the volcano is dormant — or may not even know it’s a volcano — because there have been no eruptions in living memory. (“Living memory” means that the oldest people in a community have seen the last activity at a volcano.) Historical records may add several hundred years to a community’s memory of eruptions, but that depends on people having knowledge of those records. Even if eruptions are known to have occurred in the past few centuries, a volcano may still not be considered active by the general public unless it is visibly erupting.

A volcanologist, however, has very different ideas of what constitutes a dormant or active volcano — but even volcanologists don’t agree on the exact definitions. Some volcanologists consider a volcano active if it has erupted within the last 10,000 years; they would consider it dormant if it had erupted more than 10,000 years ago, but still has access to a magma source that could fuel future eruptions. (Sometime more than 10,000 years before present marks the beginning of the Holocene epoch, which is used as an arbitrary cutoff because it marks roughly the point in time when the last ice age ended worldwide). Other volcanologists call a volcano active only if it has erupted in “historical time” and is considered likely to do so in the near future.

But what does historical time even mean? Whose history are they referring to? Do they count myths and legends as historical records, or do they have to be written? What about areas of the world that have no written records, but may still have oral histories going back thousands of years? “Historical time” varies widely depending on what region of the world you’re looking at and what are accepted as accurate historical records, and like the Holocene cutoff, it’s an arbitrary way of describing a time period. If you see “historical” being used to describe an eruption, it’s always wise to check up on the record-keeping in that part of the world.

The lack of an overarching consensus on the meaning of these terms is a sticky spot in volcanology, and a source of confusion when it comes to dealing with nonscientists. Volcanologists may be hesitant to declare a volcano dormant for scientific purposes, but they must consider short-term concerns: Which volcanoes most urgently need to be studied and monitored? Will nonscientists support the work if they consider the volcano to be dormant? Will the public listen to outreach and education campaigns about potentially dangerous volcanoes if it doesn’t seem like they will erupt?

Seulawah Agam and Sinabung haven’t erupted since the 19th century (and scientists aren’t even positive about that for Sinabung), which is more than enough time for people living near those volcanoes to forget that they are volcanoes. It’s a dangerous situation, because being prepared for a volcanic eruption is the best way to avoid the danger.

But whether or not a volcano has been historically active, it’s important to remember that natural phenomena operate on geologic as well as human timescales; what seems like a quiet, forested mountain one day may become a violently exploding volcano the next — though there are usually precursory signs. Volcanologists do their best to reconcile geologic and human time, scientific and layperson’s terminology, and the differing perceptions of what a dormant volcano is, but it’s a difficult situation and likely always will be.

For now, it’s best to keep an eye on anything near you that a scientist calls a volcano — because you never know when it might decide to make your life interesting.

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