Yellowstone National Park, the world’s first and arguably most famous national park, is home to one of the planet’s largest and potentially most destructive volcanoes. The 50- by 70-kilometer Yellowstone caldera complex is so massive that it can really be appreciated only from the air. But although the caldera isn’t always visible on the ground, it’s certainly no secret: Copious thermal features like hot springs and geyser basins dot the landscape and have attracted people to the uniquely beautiful and ecologically rich area for at least 11,000 years.
As people seek to explain the area’s geology, Yellowstone’s unusually active landscape has inspired myths and legends, from Indigenous origin stories to misleading headlines about the future. Every season, recurring bouts of earthquake swarms trigger sensational stories that Yellowstone could be gearing up for another “big one.” But there’s no need to cancel your family vacation to see the park’s free-roaming bison and grizzly bears: The geologists who keep a very close eye on the Yellowstone caldera system say it’s not going to erupt again in our lifetimes.
The story of Yellowstone begins around 16.5 million years ago, when a plume of magma began fueling intense bouts of volcanism along the border of what is now Idaho, Nevada, and Oregon. This magma plume, like the one that formed the Hawaiian Islands, is stationary, but as the North American plate moves to the southwest over the hot spot, its surface expression migrates, creating a 750-kilometer-long trail of volcanism, including dozens of calderas, across southern Idaho and into northwest Wyoming.
Around 2.1 million years ago, when the hot spot was centered under the southwest corner of what is now Yellowstone National Park, the volcano’s magma reservoirs filled to bursting, resulting in one of the largest volcanic eruptions in the geologic record. The explosion spewed ash and debris all the way to the Mississippi River, ejecting more than 6,000 times the volume of material erupted during the 1980 Mount St. Helens eruption. As the magma chambers emptied, the overlying layers collapsed, forming a massive caldera.
This cycle of explosive eruptions repeated twice more, around 1.3 million and 630,000 years ago, resulting in three overlapping calderas. In between these events, slow-moving lava flows drastically altered the landscape but didn’t affect the region beyond the immediate area. The last of these lava flows, which formed the Pitchstone Plateau in the southwest corner of the park, erupted around 70,000 years ago, and the volcano has been relatively quiet ever since.
Nobody was around to witness Yellowstone’s last lava flow; humans were not yet living in North America 70,000 years ago. But people have been living in the area for at least 11,000 years, and thousands of artifacts and campsites have been found throughout the park, often concentrated around rivers, lakes, and obsidian sources.
Prime campsites on the shores of Yellowstone Lake were continuously occupied for 9,500 years, and obsidian mined from dozens of quarries around the park has been found as far away as Wisconsin, Michigan, and Ontario. “Yellowstone was a nexus for trade and culture and is crossed by ancient trails from every direction,” said Shane Doyle, a research scientist at Montana State University (MSU) in Bozeman and a member of the Apsaalooke (Crow) Nation.
When Yellowstone became the world’s first national park on 1 March 1872, Indigenous Peoples, including Bannock, Blackfeet, Crow, Flathead, Sheepeater, Shoshone, and Nez Perce, were still living in and migrating through the area. Tourism campaigns, however, touted Yellowstone as a pristine wilderness untouched by humanity. “The earliest intentions were to make people think that there were no Native Americans in the park and that they were never there,” Doyle said.
One of the myths perpetuated by the park’s second superintendent is that Native Peoples were afraid of the area’s thermal features and avoided the area. But in fact, the hot springs and geysers were revered and used in ceremonies and vision quests, as well as daily life for processing food and trade goods, Doyle said. “Native people believe that Yellowstone is a very powerful and sacred place. They weren’t afraid of it. They had great respect for it, and geology plays an important role in many tribal legends and origin stories.”
Such stories are only recently being shared with park visitors, Doyle said. “We’ve finally seen a breakthrough in the last year in efforts to educate visitors about Native history and culture. I look forward to seeing more signage and a more prominent Native presence throughout the park.”
The Supervolcano Myth
Yellowstone has an impressive volcanic resume—but don’t call it a supervolcano, a colloquial term with no scientific definition. Instead, geoscientists prefer the term Yellowstone caldera system or Yellowstone caldera complex. “I wish the word supervolcano could be banished from the record as it enforces the myth that Yellowstone only produces supereruptions,” said Michael Poland, the current scientist-in-charge of the Yellowstone Volcano Observatory (YVO), the research consortium that monitors the volcano.
In its 2.2-million-year history, the Yellowstone caldera system has erupted catastrophically only three times, while producing many localized lava flows. “Yellowstone is not going to erupt again anytime soon, and when it does, it’s much more likely to be a lava flow than an explosive event,” Poland said. “These lava flows are really impressive. They can be hundreds of feet thick. But they’re not particularly hazardous beyond the immediate area.”
The last supereruption (defined as an event greater than magnitude 8 on the volcano explosivity index) at Yellowstone took place 630,000 years ago. The last lava flow took place 70,000 years ago. But the relative quiescence since the last eruptions doesn’t mean the system is due for an eruption, Poland said.
“The most common misconception about Yellowstone is that it’s overdue for an eruption. But volcanoes don’t work like that,” he said. “They erupt when there is a sufficient supply of eruptable magma in the subsurface and enough pressure to get that magma to the surface, and right now, neither condition exists at Yellowstone.”
Currently, the two stacked magma chambers under Yellowstone are mostly stagnant. “People tend to picture a giant pool of molten magma down there just waiting to erupt, but that’s not the case,” said Jamie Farrell, a seismologist at the University of Utah who runs the seismic monitoring program at Yellowstone.
Seismic studies that image the interior of Earth indicate that the two magma reservoirs contain between only 5% and 15% molten material. “That tells us the volcanic system is nowhere near primed for an eruption,” Farrell said. “Typically, you need at least 50% melt for it to mobilize and begin moving toward the surface.”
The process of filling a magma chamber with molten material is not a quiet one. “We would expect to see increased seismicity, ground deformation, changes in thermal and gas emissions for decades and perhaps centuries in advance of an eruption,” Poland said. “We have a lot of confidence that if Yellowstone were gearing up for an eruptive event, we would know about it years in advance. It’s not going to take us by surprise.”
Next-Level Neighborhood Watch
Very little of what happens at Yellowstone above or below the ground goes unnoticed; the Yellowstone caldera is one of the best-monitored volcanoes on Earth. Satellites keep an eye on the seasonal cycles of ground deformation, while thermal and gas monitoring networks detect subtle changes in heat and gas outputs.
Dozens of permanent and hundreds of portable seismic stations spread throughout the park and around its borders keep tabs on Yellowstone’s near-constant quivering, including earthquake swarms, where hundreds of small earthquakes can occur over a period of days to months. These events often inspire sensational headlines that Yellowstone is awakening—but they are not harbingers of catastrophe, Farrell said, as they are usually triggered by water moving underground in the geothermal areas.
The most likely hazards to strike the park on human timescales are not magma related, Farrell said. “The most likely geologic hazard would probably be a hydrothermal explosion.” As mineral-rich groundwater moves through hot springs and geysers, deposits thicken on the walls of the underground passages. Clogs can cause pressure to build up until an explosion occurs, sometimes forming a crater at the surface. “Some of these explosions can be pretty large. They happen annually, mostly in the backcountry, but they have happened in the major geyser basins before.”
Explosions can also occur when groundwater rapidly flashes into steam. “In Yellowstone, there are a dozen or so decent-sized craters, a few hundreds of meters across, from hydrothermal explosion events,” Poland said “If that were to happen today in the front country, it could cause a lot of damage.”
The next most likely hazard to affect park visitors is a large earthquake, Poland said. On 17 August 1959, a magnitude 7.3 earthquake struck the Yellowstone area and kicked off a 73-million-metric-ton landslide that dammed the Madison River. The landslide and resulting flooding killed 28 people, most of whom were camping along the river, and drastically changed the landscape by creating a new lake, Quake Lake.
Today, another “strong earthquake could do a lot of damage to the park and impact visitors, but it’s not going to set off the volcano,” Poland said. “The system doesn’t work like that.”
However, a big earthquake could affect the hydrothermal systems and perhaps increase or decrease geyser activity, Farrell said. “The thermal areas are very dynamic. There are a lot of old, inactive hydrothermal areas in the park, and we’ve seen new ones form in the past few decades. Old Faithful could shut down tomorrow, which would be a big change to the Yellowstone experience.”
Farrell and his team are hoping to learn more about what factors drive changes to the park’s thermal features by deploying hundreds of battery-powered seismic instruments throughout the geyser basins. “We are hoping to develop hydrothermal monitoring systems, where we use seismometers, GPS stations, thermal and gas monitoring instruments to track changes on short timescales,” he said. The monitoring systems, which are on the YVO’s 10-year plan, may also provide some way of forewarning of impending hydrothermal explosions. “That’s a hazard we still don’t know much about,” Poland said.
What’s Scarier Than Lions and Bisons and Bears? Climate Change, Oh My!
In April, I backpacked through the Black Canyon of the Yellowstone, a 32-kilometer trek known for being the best early-season backpacking trip in the park. In the 3 days we spent on trail, we saw only two day hikers, dodged hundreds of bison and elk, and followed in the frighteningly fresh footsteps of both grizzly bears and mountain lions.
When hiking in bear country, I travel in groups, make noise (I skip the bells and use my voice), carry bear spray, and store all food and scented items away from camp. In hundreds of kilometers of hiking in the Greater Yellowstone Ecosystem, I’ve seen a few bears in distant, peaceful encounters, and I’m sure many more have seen or heard me coming and stepped off the trail to let me pass. Bears have a huge task in feeding themselves with a mostly plant based diet, and I firmly believe that humans are not on their menu—they don’t want to encounter us any more than we want to encounter them.
Keeping a clean camp and storing food properly high in a tree, up a bear pole, or in an approved bear canister are the best ways to keep bears from associating humans with food rewards. A famous park service saying is “a fed bear is a dead bear”: Sloppy people are far more dangerous to bears than bears are to people. Hiking, camping, and doing fieldwork in grizzly bear country can be stressful, agrees Madison Myers, a volcanologist at MSU in Bozeman, but with proper precautions, “I am honored to share space with them.”
Yellowstone is famous for its long, deep winters, and a few decades ago, I might have needed snowshoes to hike the Black Canyon in early April and may have also been less likely to cross paths with still-hibernating bears. But the spring thaw is coming weeks earlier to Yellowstone, affecting snowpack, streamflow, water availability, vegetation patterns, and bear sleep schedules and stoking landscape-scale wildfires.
In June, a team led by researchers at MSU released a new “Greater Yellowstone Climate Assessment” that found that average temperatures are the warmest they’ve been in the past 800,000 years, and carbon dioxide levels are the highest they’ve been in the past 3.3 million years. Since 1950, average temperatures have increased by 1.3℃, and the report predicts that without drastic measures to reduce carbon dioxide emissions, temperatures could soar by as much as 5.6℃ by the end of the 21st century.
Grizzly tracks are formidable, but the human footprint on Yellowstone is large and getting larger. In 2019, more than 4.2 million people visited the park, with visitation expected to soar even higher in 2021. Often portrayed as a vast wilderness, in reality the nearly 9,000-square-kilometer park is crisscrossed by more than 750 kilometers of roads that connect more than 1,500 buildings, including nine hotels and 11 visitor centers and museums.
“On human timescales, I don’t think people will see that much large-scale geologic change in Yellowstone,” said Carol Stein, a geophysicist at the University of Illinois at Chicago. “Yellowstone is a lovely place and will stay lovely for a long time, but climate change is happening before our eyes and quickly altering the landscape. In our lifetimes, I expect climate will be the dominating force of change in Yellowstone.”
Ask the average person to imagine the future of Yellowstone, and that person might picture a mushroom cloud looming over a smoking crater. “When people hear I study Yellowstone, they always ask, ‘When is it going to erupt?’ and when I tell them that the chance of an eruption in their lifetime is next to nothing, they’re almost disappointed,” Myers said. On longer timescales, “there is a chance of another eruption on million-year timescales, or it may never erupt again at all.”
On multimillion-year timescales, as the North American plate continues moving southwest over the Yellowstone hot spot, the plume will migrate to the northeast, toward the thicker crust of the Beartooth Plateau. “When the plume hits the Beartooth Mountains, we don’t know what will happen,” Myers said. “Can volcanism work its way up through the plateau? Or will it somehow flow around the sides? Or will it wait until it pops out the other side near Billings in another 5 million years or so?”
Could another Yellowstone arise in Montana’s largest city in a few million years? Will Billings even be on the map by then? Only geologic time will tell.
Mary Caperton Morton (@theblondecoyote), Science Writer
Living in Geologic Time is a series of personal accounts that highlight the past, present, and future of famous landmarks on geologic timescales.