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As national parks go, Yellowstone National Park is a big one.

The big one, in fact: It was the world’s first national park, signed into existence by President Ulysses S. Grant in 1872. At the time, it was the biggest in the world, and though a park in Denmark holds that title now, Yellowstone still has international name recognition for its beauty and history.

But what really makes Yellowstone unique, valuable, even sacred – the geysers, hot springs, the lava flows, and more – are also what makes it extremely dangerous.

The 3,472 square mile plot of land reserved for Yellowstone across three states is roughly square-shaped and includes parts of Wyoming, Idaho, and Montana. Yellowstone is a rocky scab, of sorts, a series of calderas — volcanic expanses left over after a volcano erupts so forcefully it collapses into itself. The park has four calderas inside its borders. They’ve been filled in by lava flows since erupting, but that only makes them flatter, not less potent.

Yellowstone’s calderas are part of a chain of them that make up the Snake River Plain, extending south and west of the park. They track along the movement of the North American tectonic plate, which is crawling roughly south and west as the hotspot stays in the same place. Anyone referring to the “Yellowstone Volcano” is talking about that hotspot, a massive cone of magma which currently sits under the park. It’s a relatively young caldera volcano, one of the most prominent on Earth.

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“There’s actually a tremendous amount of similarity with Hawaii,” says Mike Poland of the US Geological Survey’s Yellowstone Volcano Observatory (YVO). Poland, currently Scientist-in-Charge at the Yellowstone Volcano Observatory, also worked with the Hawaii Volcano Observatory, perched on the rim of the Kilauea volcano.

“In Hawaii, it’s a slightly different setting,” he said. “It’s not under a continent, it’s under an oceanic plate and gets to the surface easier and it’s a different composition of magma—but fundamentally it’s a very similar process happening in both places.”

While Yellowstone is the focus of his work, it’s just one visible part of a constantly changing American West—one that’s being pulled apart by tectonic forces.

The Yellowstone hotspot is fed by heat from the core of the Earth. That heat can produce a whole host of volcanic problems, including the big one: a continent-changing eruption event that would change life in the United States, if not the planet Earth. We’ll start there—and acknowledge that such giant eruptions are extremely rare. The three most recent caldera-forming eruptions have been 600,000 to 700,000 years apart. By timescale alone, we’re due for another one very soon.

The oldest of the three Yellowstone eruptions is believed to be one of the five largest volcanic eruptions ever, expelling more than 600 cubic miles of rock and ash to create the Island Park caldera, the largest of the three in the Yellowstone area, and the Huckleberry Ridge Tuff. The next eruption formed the Mesa Falls tuff and the Henry’s Fork caldera, 1.3 million years ago. The third was 640,000 years ago, leaving behind the 1750-square-mile Yellowstone Caldera that essentially created the park.

A new eruption on this scale would fire a plume of ash and pumice into the atmosphere. As it spreads, the cloud could rain ash on a massive area hundreds of miles in diameter—including some of the most fertile agricultural lands in the United States—as has happened in previous eruptions. Scientists have found ancient ash across most of the Central and Western United States, stretching from Iowa and Montana to California and Mexico. And depending where that miles-high column of ash and rock goes in the sky, it could get picked up by winds that would carry it around the world. That could even darken the sky in places.

A full eruption would also release deadly gases. Sulfur dioxide, a common volcanic gas, can block out the sun in the upper atmosphere and, at lower elevations, returns to Earth as acid rain; fluorine, chlorine and bromine halide gases found in magma do the same. Carbon dioxide (CO2), when concentrated, is fatal. Foul-smelling hydrogen sulfide is also lethal in high doses.

Fresh lava flows from the new volcano would creep across the Yellowstone landscape as well, threatening visitors and infrastructure at the park, like hundreds of campsites and miles of roads. It probably wouldn’t get beyond the park’s boundaries, though; visitors hike over cooled lava flows in the park now. Almost all of the most recent volcanic eruptions at the park have come with lava flows.

But these are the most theatrical possibilities. A much more likely hazard at Yellowstone is a hydrothermal explosion, caused by a sudden eruption of superheated steam and water—the same forces that cause the geysers concentrated in the park. Some of these explosions can be relatively small, but the largest hydrothermal explosions can leave craters hundreds of meters in diameter, according to the USGS, and throw rock and other material more than a mile.

“At least” 18 such eruptions have occurred in the last 16,000 years. Scientists estimate that the park is good for “at least” one rock-hurling explosion every two years. (This is a conservative estimate, too!)

Earthquakes and “earthquake swarms” are other hazards inherent in the volatile Yellowstone landscape. Past earthquakes—of 6.0 and higher—have been recorded inside and outside the park, some triggering fatal landslides. In the summer of 2017, Yellowstone was shaken by 500 small earthquakes in a single week. After a two-week period, that number swelled to 878 earthquakes.

Single earthquakes, or a storm of them, can identify a sustained collapse or sudden movement underground. The last “swarm” started June 12, 2017. While most were extremely minor—at or below a 1.0M on the Moment Magnitude scale—the strongest earthquake hit 4.5M.

RELATED: A Montana family was in for a grizzly surprise while driving to good ol’ Yellowstone National Park

Aside: The Richter scale we’re familiar with and the Moment Magnitude scale measure the same thing but are fundamentally different measurements, per USGS. While they’re both logarithmic—meaning a 4.0 is ten times the strength of a 3.0 and 100 times stronger than a 2.0—the Moment Magnitude scale contains an actual physical measurement that the Richter scale does not. The MM scale measures the area that slips and the distance it moves to determine the strength of a quake, compiling that with other shake indicators to provide a more thorough picture of how strong a quake really is.

On average, Yellowstone experiences thousands of small earthquakes every year. That’s the nature of the place. But Yellowstone and the surrounding area are so volatile that they’re subject to two varieties of earthquakes: those caused by volcanic activity at Yellowstone and those caused by the fault lines spread across the West. Even now, scientists aren’t always sure which one they’re seeing.

For the present, it seems like we’re safe from the worst possible eruption, at least measured in human lifetimes. Poland says the last Yellowstone eruption—some 631,000 years ago—cleared out much of the underground gas that’s trapped by the thick silica-rich magma under Yellowstone. And right now, the conditions on the ground don’t forecast a catastrophic eruption, according to the Yellowstone Volcano Observatory, which (thankfully) keeps watch.

They maintain a network of onsite and regional monitoring stations for changes in air and ground conditions, though Poland admits that monitoring a caldera of this size is difficult at best.

“The gas is coming out over a huge area, so it’s incredibly hard to measure it,” he says. “We can’t just put out an instrument and think that we’re going to measure all the gases coming out of Yellowstone. So we do surveys, from time to time, of the gases coming from fumerals, hot springs, and the like, and look at how that chemistry changes over time.”

“You have to think of it like an investment portfolio,” he concluded. “It’s got to be diverse… You have to have a lot of different tools in order to try to understand what’s happening beneath the ground.”

Right now, it seems like an eruption’s just not there. If conditions aren’t there, an eruption won’t happen—averages be damned. That’s what makes any estimating by average time between eruptions a big problem.

Calderas are dynamic, ever-changing features. They swell and bulge unevenly, and where a caldera grows and rises first is called a resurgent dome. Yellowstone’s two resurgent domes—the Sour Creek and Mallard Lake domes—have been steadily monitored since the 1970s, though scientists began monitoring them in the 1920s.

They’ve scared us in the past. Sudden rises and falls in the heights of these domes—and other parts of the caldera—are part of a dynamic volcanic zone. But they’re also the symptoms of a rising volcano; a resurgent dome of more than 450 feet formed on Mount St. Helens before it blew in 1980.

In Yellowstone around 2004, monitors noticed the ground was moving, and fast (at least in geologic terms). Parts of the caldera began suddenly gaining as much as 3 inches per year, prompting fears of an eruption. In some places, land rose by as much as ten inches in just a few years. In geological time measured in millions of years, that’s the blink of an eye. It would later slow as suddenly as it began. Scientists now believe the volcano was just taking a “breath”—one of several occurrences in the park’s monitored history.

When Yellowstone does begin to turn, the process will be much faster than originally believed. Last year, a team from Arizona State University dating fossilized ash in the park discovered that it took just decades for new magma to rise and erupt, not the millennia it was once believed to take. Hannah Shamloo, a graduate student who worked on the team that discovered the rapid timeline, said the findings stunned them.

“It’s shocking how little time is required to take a volcanic system from being quiet and sitting there to the edge of an eruption,” she told the New York Times.

Other volcanoes in the United States don’t even provide that much warning. Mount St. Helens was active for around six months before its 1980 eruption. And it showed signs of erupting for barely a week before it blew in 2004.

RELATED: Volcano under Yellowstone is way bigger than we thought

NASA scientists have some longshot plans to make sure this never happens. Not just keep us safe from starving in the darkness of a volcanic winter, but to harness the power of the volcano for electricity.

It’s something out of a sci-fi movie. Yellowstone’s hotspot follows an established and well-documented cycle—magma and gases accumulate in gigantic underground chambers, heat and pressure feeding each other. At some point, that becomes too much to hold, and it only has one place to go: up. But if that planet-altering heat and the pressure could be tapped and ventilated before anything happened, the eruptions would never happen, right? Scientists think so. NASA estimates if they could bleed 35 percent more heat from that ecosystem than currently escapes just through hydrothermal activity, it would calm Yellowstone, maybe for good.

That’s led to an ambitious plan estimated at $3.46 billion last year: just drill into the volcano.

NASA has proposed tunneling miles into the caldera and pumping high-pressure water into it. That water would return to the surface at a scalding 662 degrees Farenheit. By exchanging heat at the source, it would effectively cool the caldera. That’s good for more than keeping us from suffocating in a cloud of ash. It would also be a good source of renewable electrical power.

There are a few concerns, of course. A prominent fear is that such an operation could accidentally set off the eruption it’s supposed to stop.

NASA proposes to alleviate some of that risk by digging into the bottom of the cone of magma under Yellowstone, not the top. Since heat rises, this could counteract some of the risk of triggering a cataclysmic eruption; in theory, starting at the bottom wouldn’t give the gas and magma anywhere to go.

It’s a risky plan, and an expensive one, despite the fact that the cost could be slowly recuperated from generating power. But when it comes to a volcano of this size and power, it may be better than watching and waiting.

That’s not to say there’s no plan for the inevitable sorts of “volcanic unrest” at the site. The Yellowstone Volcano Observatory—a collective of agencies and groups, not a single building—has a geologic hazards response plan that outlines how the agency would identify and respond to a geographic threat of some sort.

The plan, linked here, names formal leads for specific areas of study and concern in the park: geophysics, seismology, geology, and more. As Scientist-In-Charge, Poland is also in charge of an emergency response at the park and says the Yellowstone Volcano Observatory has plans and processes to establish incident command centers onsite in the event of an emergency.

Their plan appears to bear that out, with processes and duties organized and defined for declaring an incident and alerting authorities of changing conditions at the volcano.

Though Poland says the Observatory doesn’t have a formal relationship with FEMA, any kind of an alert from the YVO would be routed through FEMA’s National Incident Management System, alerting authorities of something rumbling at the site.

Ideally—and most likely—that something isn’t The Big One.

RELATED: This is what happens when your front yard is Yellowstone National Park

At one of America’s greatest national parks, scientists watch for signs of a fiery and violent past Michael Keator/NPS via Getty Images
Patrick is a content editor for Rare.
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