Beneath Yellowstone National Park, a vast region of spectacular nature visited by around 3 million people a year (opens in a new tab)stalks one of the largest volcanoes in the world.
The Yellowstone Caldera, the cauldron-shaped basin at the top of the volcano, is so colossal that it is often called a “supervolcano,” which, according to the Natural History Museum (opens in a new tab) in London, means it has the capacity to “produce an eruption of magnitude eight on the Volcanic Explosivity Index, discharging more than 1,000 cubic kilometers [240 cubic miles] of material.”
To put that in perspective, the 1991 eruption of Pinatubo in the Philippines, possibly the most powerful volcanic eruption in living memory, was rated a 6 on the Volcanic Explosivity Index, making it, according to the Museum of Natural History, ” about 100 times smaller.” than the benchmark for a supervolcano.”
So should we be worried? Will Yellowstone erupt soon?
Is Yellowstone “Prepared” for an Eruption?
Media reports have often claimed that Yellowstone is about to erupt. They claim that because the last eruption of the supervolcano was 70,000 years ago (opens in a new tab), it is likely to explode soon. But that’s not how volcanoes works.
“This is perhaps the most common misconception about Yellowstone and about volcanoes in general. Volcanoes don’t work on timelines.” michael poland (opens in a new tab), a geophysicist and scientist in charge of the Yellowstone Volcano Observatory, told Live Science in an email. “They hatch when there is enough eruptible magma below the surface and enough pressure to cause that magma to rise.
“No conditions are in effect at Yellowstone at this time,” he added. “It’s all about that magma supply. Cut it off and the volcano won’t erupt.”
Many volcanoes go through cycles of activity and inactivity, Polonia said. Most of the time, the activity of a volcano is a direct consequence of the magma supply. “Some volcanoes appear to erupt regularly, but this is because the magma supply is relatively constant – think Kilauea in Hawaii or Stromboli in Italy,” Polonia said.
Related: The 11 Largest Volcanic Eruptions in History
So where does the idea of Yellowstone being “overdue” for an eruption come from?
“I suspect that the ‘backward’ idea comes from the concept of earthquakes,” Polonia said. “Earthquakes occur when stress builds up on faults, and in many places, this stress builds up at relatively constant rates due to, for example, plate motion. That being the case, you would expect earthquakes to occur at somewhat regular intervals. It’s, of course, more complicated than that, there are many variables at play, but for that reason, it makes more sense to say that a fault is ‘behind’ for an earthquake.”
Poland also noted that “supervolcanoes,” a term it considers somewhat crude and sensational, are “no more and no less temperamental” than other volcanoes. So how do experts keep an eye on Yellowstone’s underground activity so that, in the event of a major volcanic eruption, warnings can be given?
“Yellowstone is very well monitored by a variety of techniques,” Polonia said. “It’s covered in terms of seismicity and ground deformation. We track the temperatures of some thermal characteristics, although this is not an indicator of volcanic activity, but of the behavior of specific hydrothermal areas. We look at general thermal emissions from space, collect gas and water to evaluate chemistry over time, and track stream/river flow and chemistry.”
So what could indicate that a massive eruption is imminent?
“Having thousands of earthquakes in a short period of time (a few weeks), with many felt events, would be noteworthy, as long as it was not an aftershock sequence of a tectonic event,” Polonia said. “That seismicity would have to be accompanied by really extreme ground deformation (tens of centimeters over the same short period), park-wide changes in geyser activity, and thermal/gas emissions. The ground rises and falls typically within 2 and 3 cm. [0.8 to 1.2 inches] every year, and there are typically about 2,000 earthquakes a year in the area, so it would have to rise well beyond these normal background levels.”
While Yellowstone is relatively stable at the moment and hasn’t shown any unusual seismic activity of late, if it were to erupt, the consequences could be extreme. Volcanologists have suggested that the branch they are most concerned about is windblown ash, which could end up blanketing a surrounding 500-mile (800-kilometer)-wide region with more than 4 inches (10 centimeters) of ash. This could, experts predict, result in the short-term destruction of Midwestern agriculture and clog many waterways. According to the US Department of the Interior. (opens in a new tab), “the surrounding states of Montana, Idaho, and Wyoming that are closest to Yellowstone would be affected by pyroclastic flows, while other locations in the United States would be affected by ashfall.” Poland added that the effects would also be felt beyond the borders of the United States.
“If there were a very large explosive eruption, it could affect the global climate by emitting ash and gas into the stratosphere, which could block sunlight and reduce global temperatures by a few degrees for a few years,” Poland explained.
Research published in the journal Science (opens in a new tab) in December 2022 he discovered that the Yellowstone caldera contains more liquid molten rock than previously estimated. Since volcanoes tend to erupt only when a large amount of magma is available, should this finding be cause for concern?
“This [research] it really just confirms what we already know about Yellowstone,” Poland said. “Initial findings were that the magma chamber beneath Yellowstone was only 5-15% molten. The new research, using more advanced techniques but the same data, suggests it’s closer to 16-20% melt. The bottom line is that the magma chamber is mostly solid. And that means there’s much less chance of a consequent eruption. I find this result reassuring.”