The volcano below Yellowstone National Park is among the largest on the planet and has a history of generating huge eruptions. There have long been fears — many exaggerated — that it is only a matter of time before it blows, taking much of North America with it.
But new research, published this month in the journal Geology, suggests that this volcanic menace may be losing its strength, and that you can be a bit less alarmed the next time you read a viral headline about that supervolcano out west.
Yellowstone’s volcano is the result of a hot spot, or a superheated area just below Earth’s crust. These regions burn through tectonic plates that glide over them.
This geological phenomenon is part of what gives Yellowstone its character, providing the steady flow of heat that warms groundwater and generates simmering, prismatic pools, caldrons of boiling mud and geysers found throughout the national park.
Unfortunately, ancient hot-spot eruptions on continents are much more difficult to study than similar eruptions that took place out at sea, because they are more explosive. While researchers can see the path that Yellowstone burned as the hot spot migrated from Oregon across Idaho and into Wyoming, discerning one eruption from another has been a chore as most volcanic deposits are scattered across vast landscapes in a chaotic jumble.
“My predecessors thought these messy deposits might be related to one another, but nobody was sure,” said Thomas Knott, the study’s lead author and a geochemist at the University of Leicester in England. He set about the grueling task of fingerprinting volcanic samples from 50 sites in Idaho.
Because each Yellowstone eruption would have involved different portions of the continent being melted, Dr. Knott reasoned that each eruption ought to be subtly different in its chemical profile.
To gain further resolution beyond chemical analysis, his team looked at paleomagnetics. Because the iron from the hot spot was liquid when ejected, it oriented itself toward magnetic north upon eruption, and then got locked into place when it cooled. Because magnetic north has moved throughout Earth’s history, Dr. Knott’s team was able to determine when this iron was erupted.
What they found transformed the timeline of some of the supervolcano’s eruptions. Instead of a series of small eruptions that geologists have long thought took place as the hot spot migrated across Idaho, there had actually been two very big outbursts.
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.
“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.
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