Lava flows

This map shows post-caldera lava flows from the Yellowstone volcano. The flow dated at 72,000 years erupted as a single event. Others were likely formed from multiple eruptions. 

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and scientist-in-charge of the Yellowstone Volcano Observatory.

It seems like you can't read an article or watch a documentary about a natural hazard without hearing that some event is "overdue." A flood on a certain river is overdue, or a hurricane in a given place is overdue, or an earthquake of some magnitude is overdue, or a volcanic eruption is overdue. It's basically a cliché, and no documentary or article can be complete without it.

We've heard many statements that Yellowstone is overdue — that it has a major eruption every 600,000 years on average, and since the last eruption was 631,000 years ago ... well ... you can see where this is going. Is this true? In a word, no. In two words, no way. In three words, not even close. Yellowstone doesn't work that way.

Let's take two different approaches to the question — first based on the timeline of eruptions, and second based on how volcanoes work.

It's easy to understand why someone might think a volcano could be overdue. After all, we tend to think of big earthquakes that way. Earthquakes occur when enough stress builds up on a fault that the fault snaps. The stress accumulates because of consistent motion of the rocks on either side of the fault. The rate of this motion is generally constant over thousands to millions of years, so the earthquakes that result from the motion can have fairly regular timing (as long as the conditions along the fault, like the stress rate and the types of rock, stay consistent). This is why it is possible to calculate the long-term probabilities of earthquakes in different areas.

Ash fall

Areas of the United States that once were covered by volcanic ash from Yellowstone's giant eruptions 2 million and 630,000 years ago, compared with ashfall from the 760,000-year-old Long Valley caldera eruptions at Mammoth Lakes, California, and the 1980 eruption of Mount St. Helens, Washington. Used with permission from "Windows into the Earth, The Geologic Story of Yellowstone and Grand Teton National Park", Robert B. Smith and Lee J. Siegel, Oxford University Press, 2000.

By this logic, we should be able to look at the ages of past Yellowstone eruptions and calculate an average recurrence interval (assuming Yellowstone eruptions occurred on a regular schedule). In terms of large explosions, Yellowstone has experienced three — at 2.08, 1.3, and 0.631 million years ago. This comes out to an average of about 725,000 years between eruptions. That being the case, we still have about 100,000 years to go, but this number is based on very little data and so is basically meaningless. (Would you base any conclusion on the average of just two numbers?) The point, however, is that if someone, or some article or documentary, says that Yellowstone erupts every 600,000 years, you know right off the bat that they are full of baloney.

Volcanoes, however, are not like faults. With rare exceptions, volcanoes do not accumulate magma at a constant rate (in the few cases where that does happen eruptions can be somewhat regular). Instead, volcanoes erupt when there is a sufficient supply of liquid magma in the subsurface and sufficient pressure to cause that magma to ascend to the surface. This does not generally happen on a schedule.

Take the occurrence of Yellowstone lava flows as an example. Lava flows are the most common form of magmatic eruption at Yellowstone, with several dozen having occurred since the last big explosion (the most recent lava flow was 70,000 years ago). But these lava flows did not erupt regularly through time. Instead, they erupted in tight clusters, with several eruptions happening within the space of a few thousand years, separated by up to hundreds of thousands of years with no eruptions. This is because the Yellowstone magma reservoir system receives new magma only in discontinuous batches, causing several eruptions in a short period of time with long periods of quiet in between these episodes.

So the next time you hear that Yellowstone is "overdue," feel free to roll your eyes. Now you know that's not the way Yellowstone works (and even if it was, the timing is off anyway). No matter how you slice it, Yellowstone is not overdue. No. No way. Not even close. But we can't say the same about the oil change for your car, so you might want to check on that.

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