If the Yellowstone supervolcano were to erupt – and there’s no indication that will happen any time soon – Missoula residents would find themselves with 2 to 19 inches of ash to sweep and shovel.
They’d be the lucky ones.
Billings residents could be up to their chins in ash.
Based on a recent study by the U.S. Geological Survey using a model called Ash3D, a Yellowstone eruption could dump from 40 to 70 inches of ash on Billings – more than 3 to 5 feet.
“It’s a crazy thing to think about because none of us have ever seen an eruption like Yellowstone,” said Larry Mastin, the lead author of the study and a USGS hydrologist who helped develop the Ash3D model. “It would be two or three orders of magnitude more ash than we’ve been able to observe.”
Casper, Wyoming, would fair better than Billings, since it is farther from Yellowstone. Casper would only see ash pile up 13 to 33 inches deep – not unlike snowfall from a bad winter blizzard.
And while Missoula is somewhat closer to the Yellowstone supervolcano than Casper, it is north rather than south of the eruption zone. And prevailing winds in Montana are more likely to move north to south and from west to east. So the USGS model shows Missoula receiving about 2 to 19 inches of ash.
But “that wasn’t the point of this, to talk about how much ash a place would get,” said Jacob Lowenstern, the scientist in charge of the Yellowstone Volcano Observatory and a co-author of the research.
Instead, the work has been targeted at aviation safety, especially in the wake of Iceland’s 2010 volcanic eruption, which shut down air traffic across northern Europe for weeks – the largest air traffic shutdown since World War II – stranding millions of travelers.
The concern was that the abrasive particles in the moving ash cloud would damage aircraft engines. Because of those fears, air traffic was halted in a wide swath around the ash.
“Now there’s an increasing demand for modelers to estimate the ash concentration in ash clouds,” Mastin said. That way airlines can determine how far out from abrasive ash clouds it is safe to fly their jets.
“This is a problem Jacob and I have been talking about for at least five years,” Mastin said. “We started developing this Ash3D model in 2010 and it is now used pretty routinely in Alaska, South and Central America, and Indonesia.”
“Our ability to model these things is pretty much brand new,” Lowenstern said.
Ash from a volcanic eruption is formed when magma blasts to the surface and shatters into fragments of abrasive glass, pumice and minerals. Since the ash is hot, it is less dense than the air and rises. A large eruption that generates lots of heat will carry the ash high into the atmosphere. The Mount Pinatubo eruption in the Philippines in 1991 shot ash up into the air 114,000 to 130,000 feet high. Commercial jets fly at about 30,000 feet.
Only about one-third of what’s ejected when a volcano erupts becomes ash. Another third falls back into the crater and the final third is composed of ground-hugging gas clouds and ignimbrites – a mixture of ash, pumice and fragments of rock – that may only travel 50 to 100 miles from the crater.
By using Yellowstone’s supervolcano as a basis, Mastin and colleague Alexa Van Eaton were able to plug in numbers that would give them a greater understanding of how the heat, mass and particles from a big eruption could generate its own wind by rising up to the edge of the stratosphere, miles above Earth, counteracting the predominant jet stream winds. Such a plume rising up from an eruption creates an umbrella cloud.
The plumes from smaller eruptions tend to look like a fan, spreading downwind from the volcano – like smoke from a smokestack.
Mastin’s model is based on the assumption that a Yellowstone eruption would be more likely to create a huge umbrella cloud that would spread ash in all directions around the supervolcano, not unlike the mushroom cloud of a nuclear explosion.
After a researcher in Italy created a model using the 1991 eruption of Mount Pinatubo – the second-largest eruption in the 20th century – Mastin adapted his model.
The initial simulation with no umbrella cloud put no ash on the West Coast, where traces of ash from previous Yellowstone eruptions have been found.
With the umbrella cloud, millimeters of ash would settle in Oregon and Washington and even farther offshore. Near the center of the eruption, there would be less ash fall, since more ash would be pushed upward and outward.
The modeling showed little change when Mastin tried wind flows for all four seasons and for the duration of the eruption.
“The wind field didn’t really alter the results hugely,” he said. “At closer distances it looks like a bull’s-eye at Yellowstone.”
“It pretty much doesn’t matter where the winds are from, the deposition remains the same,” Lowenstern said. “I don’t think people fully appreciated that before this.”
In addition to helping with modern air traffic, the research also helps scientists paint a more complete picture of prior Yellowstone eruptions, the most recent of which was 70,000 years ago. That eruption was relatively mild – a lava flow in the southern portion of the park’s Pitchstone Plateau – compared to the massive eruptions 640,000, 1.3 million and 2.1 million years ago. Although evidence of ash deposition from these eruptions can be found in the geologic record, some of that ash was likely moved beforehand, either by wind or water.