Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, he is now a post-doc at the University of Johannesburg. 
In the wake of this weeks rather spectacular eruptions of the Chilean volcano Chaitén (which is being well-covered by the Volcanism Blog; more cool photos are available here, and at NASA's Earth Observatory), a commentator has asked for some clarification on "how geologists classify volcanoes as active, dormant or extinct."
The simple answer to this question goes thusly:
- An active volcano is one that is presently erupting (or at least growling a lot, with lots of seismic and thermal activity).
- A dormant volcano is currently inactive, but could feasibly erupt in the future.
- An extinct volcano is one that is both inactive and unlikely to erupt again in the future.
In many ways, I'm not sure that this classification is actually very informative, because these categories turn out to be rather fuzzy and tricky to determine. Even what constitutes a 'presently active' volcano is a little problematic: a cycle of magma chamber recharge and eruption occurs over geological timescales, so it makes little sense to only include volcanoes that have erupted in the past week, month or year (or even the past decade or century). For this reason, places such as the Smithsonian's Global Volcanism Programme put any volcano that has erupted in recorded human history, or in the Holocene (the last 12,000 years or so), onto the active list.
The division into 'dormant' and 'extinct' is also tricky, because what you need is some idea of a volcano's eruptive history; not just the date of its last eruption, but the number and frequency of eruptions over a longer time. For example, lets say that you can date the last eruption of a particular volcano to 20,000 years ago, and dating of older lava flows shows that prior to that time there were eruptions every couple of thousand years or so:
In this case, several eruptive cycles have been and gone since the last eruption, and we might be justified in saying that this volcano is extinct. In contrast, we might find that the eruptions are much more infrequent, say every 15,000 years or so:
In this case, we are still within the timeframe required for one eruptive cycle, and the magma chamber could still be refilling as a prelude to the next explosion. More detailed examination - trying to seismically image the magma chamber, or looking for gas emissions or hot springs, might give us some clues about whether your friendly neighbourhood mountain is dead, or merely dozing.
The problem, of course, is that for many of the world's volcanoes this long-term information is just not available. If we look at Chaitén's eruptive history, there was only one known eruption prior to this week's pyrotechnics, which has been dated to around 10,000 years ago. The eruption has been described as a 'surprise', but in fact we don't know enough to know if we should be surprised or not.
Comments
Thank you. I had a feeling it was something like that. I can imagine an Abbot and Costello type conversation:
Geologist: This volcano has been extinct for 30,000 years.
Abbot: You know that. And I know that. But does the VOLCANO KNOW THAT!!!
[Resounding BOOM!]
Posted by: justawriter | May 8, 2008 12:53 PM
My 4-year-old, who travels by the volcanoes/cinder cones in northern AZ quite regularly (on the way to see dad, an engineering geologist), recently claimed that the SF Peaks were "...broken, and my dad can fix them."
Posted by: coconino | May 9, 2008 6:24 PM
Coconino: I spent '72 to '78 living at the Grand Canyon and fondly remember the hundreds (?) of cinder cones scattered around the San Francisco Peaks (I thought they had been renamed the Kachina Peaks? or did I get that one wrong (probably)). As a kid, I remember how exciting it was to have these remarkably recent eruptions (less than a thousand years ago) and thinking how exciting it would be to have one erupting 'now'.
Seems that (with Sunset Crater the most recent) those volcanoes can be deemed extinct. Though I remember (or seem to (it was a long time ago)) that there are still some pretty good hot spots up on the slopes of Mount Humphry.
Chris: thanks for the explanation regarding natural spacing of eruptions and the determination of 'dormant' and 'extinct.'
Posted by: (((Billy))) | May 10, 2008 3:22 PM
That is the best explanation of this classification scheme I've ever seen! My uni level geology text book (I'm a poli sci and education major) has such a long winded and in the end pointless rationalization of the system that misses the point...
I usually just stick to sedimentary geo, but have been reading up on New Zealand's geologic history, and the classification of the various volcanos around NZ's cities are all over the place book to book (Dunedin where I'm at these days is listed as all three in the 4 books I've been reading). I'll see now if I can find the histories of the ones I'm interested in
Cheers!
Posted by: Traumador the Tyrannosaur | May 11, 2008 7:36 PM