5 January 2010

The biggest bang for your buck

Posted by Jessica Ball

Happy New Year! Having finally beaten the latest installment of the common cold, I’m back to blogging as I get ready for the new semester.

Recently there’s been a bit of amusement among the geobloggers about an Paddy Power, an Irish betting website that’s will take wagers on which volcano will erupt next with a VEI 3 or greater magnitude eruption. That in itself is amusing (and it would be an interesting way for grad students to fund their research, although I’m not sure it’s legal). 

Doubtless, however, the folks running (and looking at) this website don’t know a whole lot about the VEI system that they’re basing the bets on. VEI stands for Volcanic Explosivity Index, and it’s a scale that was introduced by Christopher Newhall and Stephen Self in 1982. It was originally meant to be a way to estimate the explosive magnitude of past volcanic eruptions, and combines a number of criteria:

  • Volume of material erupted (also called ejecta or tephra)
  • Height of the eruptive column
  • Qualitative descriptions (i.e., “effusive”, “explosive”, “paroxysmal”)
  • Classification (Hawaiian, Strombolian, Vulcanian, Plinian, Ultraplinian)
  • Duration in hours
  • The most explosive type of activity observed
  • Tropospheric (up to ~10 km) and stratospheric (~10-50 km) injection of material (i.e., did the eruption column reach these atmospheric levels?

Including both quantitative and qualitative criteria allows volcanologists to evaluate eruptions that may not have been well-monitored or were only observed by non-scientists. The scale is logarithmic with respect to eruption volume; this means that a VEI 5 eruption (the size of the 1991 Pinatubo eruption, for instance) is 10 times stronger than a VEI 4 eruption (the size of the 1980 Mount St. Helens eruption). 

The VEI scale, however, is more a convenient way to describe the size of an eruption; it doesn’t tell you much about the specific hazards at a volcano, because these are dependent on lots of other variables. A VEI 3 eruption at an Alaskan volcano like Redoubt is going to be a lot less hazardous than one at Mount Vesuvius, simply because Redoubt is located in such an isolated, unpopulated area and Vesuvius sits over a city of 3 million people.

(The image above is taken from the USGS Volcano Hazards Program Photo Glossary.)

Eruption forecasting in general is a tricky business. Even volcanoes that have been monitored for decades can behave in unexpected ways; if you look at things on a geologic timescale, those decades are only a blip in the volcano’s history. The VEI is a great way to describe past eruptions, but the fact that a volcano has erupted with a certain magnitude doesn’t necessarily mean it will do the same thing in the future. 

Of course, you’re all wondering what I would place my bets on. I certainly wouldn’t rank Santa Maria along with Yellowstone; while the 1902 eruption was one of the largest in the 20th century, the only activity there now is at the Santiaguito lava dome complex, and it’s a little hard to compare some little Vulcanian puffs with a potential caldera eruption. I’d probably have to go with Galeras, considering the activity that’s been going on there lately, but don’t take my word for it. (And tempting as it would be to fund some research by betting on an eruption, I wouldn’t waste my money!)


USGS Volcano Hazards Program Photo Glossary
Ready, Steady, Blow! Volcano Betting Erupts! (Paddy Power website)
Newhall, C. and Self, S., 1982, The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism. Journal of Geophysical Research, v. 87, no. C2, p. 1231-1238.