Beneath Yellowstone National Park, a vast region of spectacular wilderness visited by around 3 million people annually (opens in a new tab)lurks one of the largest volcanoes in the world.
The Yellowstone caldera – the cauldron-like basin at the top of the volcano – is so colossal that it is often called a “supervolcano”, which, according to the Museum of Natural History (opens in a new tab) in London, meaning it has the capacity to “produce an eruption of magnitude eight on the Volcanic Explosivity Index, releasing more than 1,000 cubic kilometers [240 cubic miles] of material.”
To put that in perspective, the 1991 eruption of Pinatubo in the Philippines, arguably the most powerful volcanic eruption in living memory, ranked 6 on the Volcanic Explosivity Index, making it, according to the Natural History Museum, “about 100 times smaller than the benchmark for a supervolcano.”
So should we be worried? Will Yellowstone Erupt Anytime Soon?
Is Yellowstone “due” for an eruption?
Media reports have often claimed that Yellowstone is due for an eruption. They claim that because the last eruption of the supervolcano was 70,000 years ago (opens in a new tab), it’s sure to be windy soon. But it is not so volcanoes work.
“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 the scientist-in-charge at the Yellowstone Volcano Observatory, told LiveScience in an email. “They erupt when there is enough eruptible magma below the surface, and pressure to make that magma rise.
“Neither condition is in place at Yellowstone right now,” he added. “It’s about that magma supply. Cut it off and the volcano won’t erupt.”
Many volcanoes go through cycles of activity and inactivity, Poland said. More often than not, a volcano’s activity is a direct consequence of the magma supply. “Some volcanoes appear to have regular eruptions, but this is because the magma supply is relatively constant – think Kilauea in Hawaii or Stromboli in Italy,” Poland said.
Related: The 11 largest volcanic eruptions in history
So where does the idea that Yellowstone is “overdue” for an eruption come from, then?
“I suspect the ‘decay’ idea comes from the term earthquake,” Poland said. “Earthquakes happen as stress accumulates on faults, and in many places that stress accumulates at relatively constant rates due to things like plate motion. Then you can expect earthquakes to occur at somewhat regular intervals. Of course it is. , more complicated than that—there are many variables at play—but for that reason it makes more sense to say that a fault is ‘due’ for an earthquake.”
Poland also noted that “supervolcanoes” – a term he considers somewhat crude and sensational – are “no more or less temperamental” than other volcanoes. So how do experts monitor Yellowstone’s underground activity so that warnings can be issued in the event of a major volcanic eruption?
“Yellowstone is very well monitored by a variety of techniques,” Poland said. “It’s covered in terms of seismicity and ground deformation. We’re tracking temperatures of some thermal properties, although this is not an indicator of volcanic activity, but rather of the behavior of specific hydrothermal areas. We look at general thermal emissions from space, collect gas and water to consider Chemistry over time, and track current/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 remarkable, as long as it was not an aftershock sequence from a tectonic event,” Poland said. “This seismicity must be combined with really extreme ground deformation (tens of centimeters over the same short period), park-wide changes in geyser activity and thermal/gas emissions. The ground normally rises and falls by 2-3 cm [0.8 to 1.2 inches] each year, and there are typically ~2,000 earthquakes annually in the area, so it must rise well above these normal background levels.”
While Yellowstone is relatively stable right now and has not shown any unusual seismic activity recently, if it were to erupt, the consequences could be extreme. Volcanologists have suggested that the ramification they are most concerned about is ash from wind, which could end up coating a surrounding region 800 kilometers across in more than 10 centimeters of ash. This could, experts predict, lead to short-term destruction of Midwestern agriculture and will leave many waterways clogged. 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 elsewhere in the United States would be affected by falling ash.” Poland added that the effects would also be felt beyond US borders.
“If there was a very large explosive eruption, it could affect the global climate by releasing ash and gas into the stratosphere, which could block sunlight and lower 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 found that the Yellowstone caldera contains more liquid molten rock than previously estimated. Given that volcanoes tend to erupt only when a huge amount of magma is readily available, should this finding be a cause for concern?
“This [research] really just confirms what we already know about Yellowstone,” Poland said. “The 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 is closer to 16-20% molten. The take home message is that the magma chamber is mostly solid. And that means that there is far less likelihood of a subsequent outbreak. I find this result reassuring.”