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Modeling the Ash Distribution of a Yellowstone Supereruption (2014)

The topic of Yellowstone supereruptions (ones producing greater than one thousand cubic kilometers of volcanic debris) generates much interest, but also occasional confusion.

August 27, 2014 

Map of the known ash-fall boundaries for several U.S. eruptions
Fact Sheet 2005-3024Map of the known ash-fall boundaries for major eruptions from Long Valley Caldera, Mount St. Helens and Yellowstone. (Public domain.)

The topic of Yellowstone supereruptions (ones producing greater than one thousand cubic kilometers of volcanic debris) generates much interest, but also occasional confusion. New computer models can help clarify the reality of eruption impacts, and provide insight on past eruptions. In August 2014, USGS scientists Larry Mastin and Jacob Lowenstern, and National Science Foundation researcher Alexa Van Eaton published research on where volcanic ash would fall if a Yellowstone supereruption were to occur today (with present-day weather patterns); the research models an eruption similar to the caldera-forming event that occurred 640,000 years ago. Yellowstone is an obvious target for ash distribution studies, as it provides an opportunity to understand very large eruptions that generate umbrella clouds, which spread radially in the atmosphere (see last image below). The distribution of deposits from ancient Yellowstone eruptions can be compared with the output from the computer models.

For additional information, read the entire article published in the scientific journal Geochemistry, Geophysics, Geosystems. The following FAQ adds background and context to this research study.

Why did you write this paper?

At YVO, we are constantly asked questions like "how much ash would I get if Yellowstone had another supereruption." Instead of stating "we don't know" or "it depends on the winds," we decided to take advantage of an ash-dispersion modeling program, co-developed by Larry Mastin who works at the USGS Cascades Volcano Observatory. He spent many years working with colleagues to develop computer models that predict ash fallout from volcanic eruptions, and he decided to try applying the new model to Yellowstone.

What about the maps showing Yellowstone deposits across the U.S.? Don't they indicate where ash will go?

No. They show an outline of the area where ashhas been found from the previous big eruptions at Yellowstone (640,000, 1,300,000, and 2,100,000 years ago). However, they do not provide information on the original ash thickness at any particular location. One reason we don't know the thicknesses from the previous eruptions is because the ash deposits are eroded and rapidly re-distributed by rain, rivers, and wind in the years following the original eruptions. The very thin deposits far away from the volcanic source are not preserved at all in the geologic record.

Lack of reliable information left the door open to speculation and fanciful depictions of the effects of supereruptions, which are easily found on the Internet. Results of the new study show that ash accumulation, while widespread and substantial, is far less than in most of these "doomsday" scenarios.

What is new and significant about this study?

Ashfall model output for Yellowstone supereruption
Example model output of possible ash distribution from a month-long Yellowstone supereruption. Results vary depending on wind and eruption conditions. Historical winds for January 2001 used here. (Public domain.)

Models have been used for decades to forecast ashfall during eruptions. But only in recent years have tephra models like Ash3d been developed that use a 3-D, time-changing wind field, enabling us to model eruptions that last weeks and spread ash across an entire continent. These features, plus the development of a method for calculating growth of an umbrella cloud, have made it possible to simulate eruptions of this scale.

Did you learn anything new of scientific interest through this modeling?

Yes, we learned that supereruptions distribute ashin a fundamentally different pattern than smaller eruptions by creating an umbrella cloud that can push ash more than a thousand kilometers upwind. The mapped pattern of ash deposition from weaker eruptions looks roughly like a fan, spreading downwind from the volcano; while that from a supereruption looks more like a bull's eye, centered on the volcano. A powerfully spreading umbrella cloud means that ash dispersal is much less affected by atmospheric winds.

Plume shape illustrations for different types of eruptions
Plume shape illustrations for different types of eruptions; small eruptions produce weak plumes, very large eruptions produce strong plumes with major umbrella clouds. Supereruptions make the latter. (Public domain.)

What's happening geologically at Yellowstone now?

Seismicity and ground deformation are within historical norms. The caldera started moving up this year after about four years of slow subsidence. Earthquakes were more abundant early in 2014 than in mid 2014, especially in the area near the Norris Geyser Basin.

Is there any evidence that Yellowstone will erupt soon?

No. Yellowstone is behaving as it has for the past 140 years. And geological evidence indicates that similar or higher rates of earthquakes, ground uplift and steam explosions were experienced at Yellowstone over much of the past ~10,000 years. Odds are very high that Yellowstone will be eruption- free for the coming centuries.

If Yellowstone erupts, will it be the "big one" modeled in this recent article?

Almost certainly not. The past 20 eruptions at Yellowstone have been lava flows with no significant amounts of ash fall outside of Yellowstone. The past 60-80 eruptions would have had little regional (or continental) impact.

How will you know if an eruption is beginning?

Yellowstone hasn't erupted for 70,000 years, so it's going to take some impressive earthquakes and ground uplift to get things started. Besides intense earthquake swarms (with many earthquakes above M4 or M5) we expect rapid and notable uplift around the caldera (possibly tens of inches per year). Finally, rising magma will cause explosions from the boiling-temperature geothermal reservoirs. Even with explosions, earthquakes, and notable ground uplift, the most likely volcanic eruptions would be the type that would have minimal affect outside the park itself.

Who is YVO?

Besides the USGS, YVO includes staff from two universities (Utah and Wyoming), three state geological surveys (Wyoming, Montana and Idaho), Yellowstone National Park, and UNAVCO, a non-profit that installs and maintains geophysical equipment funded primarily through the National Science Foundation. Of course, we also have the resources of all other US volcano observatories available to help (Alaska, Hawaiian, Cascades and California volcano observatories). We are in frequent contact with other scientists throughout the US and all over the world. Our data is distributed globally, so anyone with resources and experience can assist in monitoring.

Should I prepare for an eruption?

Regardless of where you live, it's always a good idea to be prepared for emergencies, including earthquakes, tornadoes, chemical spills, and other random events. Emergency experts recommend keeping extra supplies on hand and creating a family emergency communication plan. Beyond that, there is no reason to specifically prepare for an eruptive event at Yellowstone, which remains very unlikely.