One took place 8.72 million years ago. Based upon the magma that it erupted, it scored 8.8 on the volcanic explosivity index created by the U.S. Geological Survey. The other, which took place 8.99 million years ago, scored 8.6.
“Given that the index does not go higher than 9.0 and anything above 8.0 ranks as ‘mega-colossal,’ it is safe to say that these qualify as super-eruptions,” Dr. Knott said.
What his study means for Yellowstone’s future has set off considerable debate.
Dr. Knott suggests that these newly identified super-eruptions paint a picture of the hot spot’s activity waning over time. Between 6 and 11 million years ago, giant eruptions once took place rather frequently, roughly every half million years. But his findings show that, since that time, such eruptions have become less frequent, occurring about every 1.5 million years.
Kari Cooper, a geochemist at University of California, Davis, is skeptical.
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“We don’t have a lot of data about what makes a magmatic flare-up happen, especially in a hot spot. Whatever caused the flare-up 9 million years ago could happen again” she said.
Yet, for others, Dr. Knott’s proposal seems logical.
“It makes sense that Yellowstone would weaken as it leaves the relatively thin western crust and travels toward the thicker center of the continent,” said Michael Poland at the Yellowstone Volcano Observatory.
But other researchers say this interpretation only works if you look at the biggest of the big eruptions, or “mega-colossal ones,” as Kenneth Verosub, also at the University of California, Davis, puts it.
“If you also include supercolossal, which, let’s face, it would still bring devastation to a number of states, you suddenly get three big eruptions in the past two million years, and can then argue that the caldera was quiescent between 6 and 2 million years ago and is now just waking up again,” he said.
As for who is right, a few million more years of monitoring should prove most insightful.