Yellowstone Volcano Supereruption 2 Million Years Ago Lasted for Decades, Researchers Find

A supereruption at Yellowstone volcano 2.1 million years ago took place over the course of decades, rather than a single explosive event that lasted hours or days, researchers have found.

Volcanologist Colin Wilson, from the Victoria University of Wellington in New Zealand, has analyzed deposits from an ancient supereruption at Yellowstone to understand exactly how magma and ash was ejected from the volcano.

Wilson's work on the event, which created the Huckleberry Ridge Tuff, has been featured in the Caldera Chronicles—a weekly column published by the Yellowstone Volcano Observatory, which is part of the U.S. Geological Survey (USGS).

The eruption Wilson and colleagues have focused on saw ash plumes rising miles into the atmosphere, with pyroclastic flows—a mass of incredibly hot and fast-moving ash, gas and lava fragments—stretching up to 100 miles from the source.

"So much magma was evacuated that the ground surface around the eruption vents collapsed to form a 100 x 50 km (62 x 31 mile) caldera that is among the largest on Earth," Wilson wrote. "The fall deposits from the eruption plume can still be found over most of the western half of the coterminous [U.S.], and remnants of the ignimbrite extend from Big Sky, Montana, to Idaho Falls, Idaho."

By studying the deposit layers from this eruption, Wilson and colleagues have been able to piece together a timeline of events that took place at the volcano over two million years ago.

Their findings show that there were time breaks in between eruptive events. In one layer enough time had passed for snow to fall and weather systems to pick up ash fall and redeposit it.

They identify three main "ignimbrite units"—sheets of volcanic deposits each of which would have been deposited over the course of days. From these units, they were able to work out that the volcano erupted, stopped, cooled down and then erupted again. The time between the first two events was probably several months, Wilson said. It would be years if not decades before the next eruptive event took place, with the deposits indicating there was a far longer period of cooling before the next ignimbrite unit was formed.

He said that if humans had been present for this eruption, they would not have witnessed one massive eruption, but several smaller events over many years. "Even when the vast volumes of material were being erupted to generate the ignimbrite units and their associated widespread fall deposits, the eruption stopped twice, both times for periods that, although geologically negligible, would have been highly relevant to human interests, through impacts by repeated hazards and interruptions to recovery efforts," Wilson wrote.

Understanding how supereruptions at Yellowstone volcano occur is important for disaster preparedness. The last caldera-forming eruption took place over 640,000 years ago, while the most recent volcanic activity came in the form of rhyolitic lava flows around 70,000 years ago.

Earlier this year a team of researchers announced they had discovered the biggest eruption ever to take place at Yellowstone—an enormous, explosive event that took place around 8.7 million years ago and covered a New Jersey-sized area in volcanic glass. This same research showed that eruptions at Yellowstone have been getting smaller over time, potentially suggesting hotspot activity in the area "may be waning."

Wilson said their findings at Huckleberry Ridge Tuff have implications for future eruptions: "These findings change the way we think about the massive Yellowstone explosions—rather than single large events, they may be composed of multiple smaller events, and this would have significant implications for our understanding of these eruptions and their impact on the landscape," he wrote.