If Yellowstone's volcanic system erupted, how could it impact the US?

While officials have said the hydrothermal explosion in Yellowstone earlier this month isn't a warning sign of impending doom, it's hard not to wonder what a 'supereruption' could look like.

DENVER (KDVR) — On Tuesday, visitors at Yellowstone National Park fled when a hydrothermal explosion occurred a few miles north of the famous Old Faithful geyser. No one was injured, but the event destroyed a nearby boardwalk and prompted officials to close the Biscuit Basin area of the park for the rest of the year.

As alarming as the video and subsequent damage seemed, the phenomenon is a normal, common occurrence at Yellowstone, happening at least once each year. Officials with the U.S. Geological Survey told Nexstar hydrothermal explosions like the one that occurred this week are "not a sign of impending volcanic eruptions."

Still, the explosion sparked conversation on the internet about what would happen if this were a sign of the "big one," and what would happen if the volcanic system at Yellowstone were to erupt.

The Yellowstone Volcano Observatory said a large eruption at Yellowstone "will not lead to the end of the human race," but it would have national and global impacts.

How would a large eruption at Yellowstone impact the US?

According to the USGS, a "supereruption" at Yellowstone would have regional effects, including falling ash and short-term changes to the global climate that would last years or even decades.

The states closest to Yellowstone, like Montana, Idaho and Wyoming, could be affected by destructive pyroclastic flows, which are a mix of lava blocks, pumice, ash and volcanic gas that flows around a volcano after an eruption.

Much of the rest of the country could be blanketed in falling volcanic ash — in some places, it could be more than three feet deep.

"Thick ash deposits would bury vast areas of the United States, and injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate," according to the USGS.

In research published roughly a decade ago, USGS scientists Larry Mastin and Jacob Lowenstern, as well as National Science Foundation researcher Alexa Van Eaton, analyzed where volcanic ash would fall if a supereruption were to happen in the present day.

In order to understand what would happen in the modern day, scientists modeled where ash has been found from previous big eruptions at Yellowstone. You can see those ash beds outlined in the map below.

The most recent eruption, which happened roughly 640,000 years ago, created the Mesa Falls ash bed, stretching east-southeast from the Yellowstone Plateau, scattering ash primarily in Colorado, Nebraska, Kansas, and South Dakota. It's furthest extents reached into New Mexico, Texas, and Oklahoma.

More expansive is the Huckleberry Ridge ash bed, sparked by an eruption some 2.1 million years ago. That reached into more than 15 present-day states, from California east to Texas, north through Missouri into Minnesota, and west into Idaho.

The largest of the ash beds, dubbed Lava Creek, covers all or part of 21 states as well as slivers of Canada and Mexico. Like Mesa Falls and Huckleberry Ridge, the Lava Creek ashbed impacted a triangle-like region with points in California, Louisiana, and North Dakota.

However, this map is based only on the ash deposits that have avoided erosion and rapid redistribution long enough for humans to find them. It doesn't take into account the thinner deposits that may have reached outside the areas marked on the map above.

Scientists have since used historical evidence and modern-day weather patterns to create a model of where the ash from a hypothetical Yellowstone supereruption could fall.

"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," a USGS article about the study said.

With the model, an example of which you can see below, USGS scientists learned that "supereruptions distribute ash in a fundamentally different pattern than smaller eruptions." Supereruptions create an umbrella cloud of ash and volcanic debris that are much less affected by winds than smaller eruptions.

As a result, thick ash would be dispersed in all directions away from the eruption site, gradually thinning out with distance.

Unsurprisingly, areas closest to the Yellowstone Plateau would likely see more ash. In Billings, Montana, for example, the model suggests ash could be more than 1,000 millimeters thick — that's more than three feet.

Moving further away from Yosemite, in places like Salt Lake City and Casper, Wyoming, the ash could reach a thickness of anywhere between one to three feet.

Even areas left unscatched by the aforementioned ash beds could see ash following a Yosemite supereruption. In much of Wisconsin, Illinois, Oregon, and Washington state — all outside the ash bed ranges — ash could accumulate up to an inch (though sections of Oregon and Washington that are closer to Yosemite could see much more).

Further still, most of the East Coast, from Florida to Maine, would likely see some ash.

However, the USGS notes that most "doomsday" scenarios portray far worse impacts than what scientists believe would happen. Even the researchers associated with the model above note that if Yellowstone erupts, it would "almost certainly not" cause the ash spread outline above.

"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," they explained.

The Yellowstone Volcano Observatory said Yellowstone is behaving normally, as it has over the past 140 years. Yellowstone has not erupted in some 70,000 years, and the odds of a large eruption in the coming centuries are very low.