We haven’t seen a large eruption since Tambora, 200 years ago. That is a good thing: the world has had enough troubles in that time, and a major volcanic disaster was really not needed. There has never been a major eruption in a highly developed area, and we don’t know how resilient a country living in a Just-In-Time economy is to such an eruption. If no one keeps stockpiles because transport is so fast it is easier to order from a distant factory, what happens when flights are stopped, roads are impassable and engines are killed by volcanic ash? What do we do when crops fail in years without summer and half the world can’t afford food? A VEI7 is a different ball game from the run-of-the-mill VEI6 eruptions. Tambora was 50 times larger than St Helens and 3 times Krakatoa – and Tambora wasn’t even the largest of the past 1000 year. We have no recent experience with such eruptions. (Toba was 25 times Tambora but luckily an eruption that size is very rare, with only a 1% chance of it happening in the next 1000 year.)
This blog has tried to identify high-risk volcanoes and I think came up with very valid suggestions. But that did not focus on VEI7 eruptions, but rather on volcanoes with local dangers. Which ones did we miss? Which ones are biding their times, living quietly while building up to a big explosion? Which ones erupt often but little, until one day the edifice collapses and it self destructs? This brief post is meant to stimulate a discussion on this.
There is a tendency to look at what volcanoes have done in the past, when predicting risks for the future. But it takes a very long time to recover from a VEI7 eruption and many volcanoes never will. At a typical recharge rate of 0.01 cubic km per year, it takes 10,000 years to get 100 cubic kilometer of new magma, which is what a VEI7 erupts. Recent history may therefore be a poor guide. There are other things to look for: large volcanoes near big calderas may indicate potential danger. For instance, San Salvador volcano is close to Llopango, the caldera responsible for the AD536 disaster. Thus, the area is capable of destruction and it has a volcano. But that is a tad general for basing predictions on.
There is a reason to discount regions with recent large eruptions. Volcanoes occur close together, but super eruptions avoid each other. Once an area has had a super eruption, it will not have another for a long period of time. This is shown in the figure below, where the filled circles show super eruptions (VEI8) of the past million years. A super eruption depletes a large region of magma, or perhaps damages the strata too much. To what degree is that also true for VEI7 eruptions? We may not expect an VEI7 from near Tambora, Kuwae, Rinjani, Llopango for the next few thousand years. Tambora and Rinjani were a bit over 100 km apart. Perhaps we can draw a 100-km exclusion zone around recent eruptions.
After the eruption the main feature will be the caldera, often a huge lake. How about before the eruption? Tambora is the only VEI7 where we know what the volcano had looked like. It was a stratovolcano over 4km high, and was among the highest peaks of Indonesia. One volcano is a small sample for statistics, but perhaps we can take this as an indication that we should look at tall volcanoes as candidates. Tambora was only mildly active, with the last measurable (i.e. significant) eruption before the big one dated to 740AD (+-150 years). Perhaps Cotopaxi was a bit like Tambora in this respect? It is a huge stratovolcano with numerous but small eruptions.
Over the next 1000 years, one may expect 3-5 low VEI7 to high VEI6 eruptions (measurement uncertainties makes the dividing line a bit fuzzy). Which do you think will be the new calderas of the year 3000?
Rulers of Earth
Honour the volcanic Rulers of Earth
Yellowstone, Toba, ancients by birth
Deeds are recorded in rocks everywhere
Depicting destruction, and death, and despair
Memories remain of their mighty explosions
But their faces are scarred now by age and erosion
Ash-buried continents recovered in time
The magma has cooled in their age of decline
Legacies fade as the ancients grow old
Rulers by fear but the fear has gone cold
Lands once wiped clean again simmer with life
refreshed from the fury of cleansing and strife
Beneath their calderas the Rulers still dream
Of a world in their power which gave them esteem
Once they were shakers that everyone knew
But times have moved on and their threats are now few
The Rulers have aged but the play must go on
To an unchanging game new players have come
Names still unknown, not yet spoken in fear
Waiting in shadows but their times will appear
El Chicon, Rainier, Sinabung, or Mayon
Supplanting the giants that to their slumbers have gone
not yet recognized but times once will be
When their smallest of shivers will cause us to flee
But the wheels turn again. Eruptions don’t last.
Beneath their calderas they too once will dream of their past
In a new place new magma will promise new birth
Forever the cycle of Rulers of Earth