First off, I wanted to thank everyone for making my last post on what might happen if you fell into lava one of my most read posts ever. I've gotten some great suggestions in the comments and by emails, so I'll have a follow-up post tackling some of that in the near future. Thursday and Friday of last week I was in San Francisco for the AGU 2011 meeting. It was a pleasure meeting a number of you Eruptions readers at the meeting! As usual, it was jam-packed with great science but I wanted to highlight a couple things I heard while I was there. Timing the duration of ancient eruptionsEach year, a prominent petrologist/volcanologist is chosen to give an hour long lecture - the Daly Lecture. It is kind of a "lifetime achievement" award for the field and this year Colin Wilson from Victoria in Wellington, New Zealand gave the lecture. Dr. Wilson is spent much of his career studying the largest explosive eruptions in the geologic record - he has published seminal work on the Oruanui eruption from Lake Taupo, the Bishop Tuff from Long Valley and the various caldera-forming eruptions from Yellowstone (amongst others).
Much of Dr. Wilson's talk was centered on the problems we have in understanding exactly how these large rhyolitic eruptions occur. Part of the problem is just how rare they are - the historical record is skewed towards eruptions that are much smaller and rhyolite eruptions are rare, with only three in the past century (Novarupta in Alaska, Tulaman and Chaiten). Only the Novarupta event in 1912 comes close to some of the large-scale VEI 7+ eruptions that strike fear into the heart's of documentary makers. In that case, we think that ~16 km^3 of material was eruption is ~60 hours. If we move back in the geologic record to a large event like the Bishop Tuff that erupted 760,000 years ago, it appears that the 600 km^3 of that eruption might have been deposited in as little as 6 days. However, by carefully examining these deposits for the telltale signs of gaps in time - such as areas where there is evidence for puddle formation, for animal activity, for wind-blown sorting of tephra - you can begin to see that some of these large deposits might have had breaks for hours, days, possibly even years (or more). So, large eruptions like the Oruanui in New Zealand 26,500 years ago, the last >500 km^3 eruption on the planet, might have been emplaced in pulses over many months to years. The implication of this lies in how we might mitigate against such an eruption in the future. If the dreaded Yellowstone caldera decides to have another major eruption, how we deal with such an event will vary greatly if the eruption is spread out over decades rather than days. Most public planners who have to deal with the problems of evacuation, resettlement and all the societal impacts tend to think only 5 years in the future, so an extended eruption could have serious repercussions on planning. This is the difference, as Dr. Wilson put it, between the human and geologist perspective. Other tidbits from AGU 2011:
Also from Dr. Wilson's talk: Does Yellowstone have two magma chambers instead of one? What would the implication of that be on predicting its behavior? The same can be said for the many calderas along the Taupo Volcanic Zone in New Zealand - it looks like there are multiple magmatic systems all closely spaced that may erupt simultaneously.
From a talk by Alison Koleszar: Although they are next door, Mt. Hood and Mt. St. Helens are very different the geometry of their respective magmatic systems, where Hood is a much simpler system where magma is staged at two depths and mixed while St. Helens is a much more complex mix of >2 magmas at depth (all seen through the composition of amphibole crystals in the magma).
From a talk by Olivier Bachmann: Southwestern Nevada saw two very large eruptions from Timber Mountain in close succession. The Ammonia Tanks Tuff was 900 km^3 of erupted materials and it erupted only 150,000 years after the Rainier Mesa Tuff that came in at 1200 km^3.
That is my report from AGU this year - the meeting is always fun, even if I was only on the ground in SF for 48 hours.