A super-eruption at Yellowstone volcano 2.1 million years ago occurred over decades, and not a single explosive event lasting hours or days, as researchers have found out.
Volcanologist Colin Wilson of Victoria University of Wellington in New Zealand has analyzed the deposits of an ancient super-eruption at Yellowstone to understand exactly how magma and ash were ejected from the volcano.
Wilson’s work on the event that created the Huckleberry Ridge Tuff was published in the Caldera Chronicles, a weekly column published by the Yellowstone Volcano Observatory, part of the U.S. Geological Survey (USGS).
During the eruption, Wilson and colleagues primarily saw ash clouds rising into the atmosphere for miles and causing pyroclastic flows – a mass of incredibly hot and rapidly moving ash, gas and lava fragments that extended up to 100 miles from the source.
“So much magma was evacuated that the ground surface around the eruption vents collapsed, forming a 100 x 50 km (62 x 31 miles) caldera, one of the largest on Earth,” Wilson wrote. “The plume of the eruption plume is still found over most of the western half of the feces [USA], and remains of the ignimbrite extend from Big Sky, Montana, to the Idaho Falls, Idaho.
By studying the sedimentary layers of this eruption, Wilson and his colleagues were able to compile a timeline of events that occurred at the volcano over two million years ago.
Their results show that there were time breaks between the eruption events. In one shift, enough time had elapsed for snow to fall and for the weather systems to collect and deposit the ash.
They identify three main “ignimbrite units” – layers of volcanic deposits that would have been deposited over a period of days. Using these units, they were able to determine that the volcano erupted, stopped, cooled and then erupted again. The time between the first two events was probably several months, Wilson said. It would take years, if not decades, for the next eruption event to occur, with the deposits indicating that there was a much longer cooling period before the next ignimbrite unit formed.
He said that if people had been present during this eruption, they would not have witnessed a massive eruption but several smaller events over many years. “Even as the huge amounts of material that formed the ignimbrite units and the extensive fall deposits associated with them erupted, the eruption stopped twice, both times for periods of time that, although geologically negligible, would have been highly relevant to human interests, due to the effects of repeated hazards and interruptions in the recovery efforts,” Wilson wrote.
It is important for disaster preparedness to understand how over-eruptions occur at Yellowstone Volcano. The last caldera-forming eruption took place over 640,000 years ago, while the most recent volcanic activity in the form of rhyolitic lava flows occurred about 70,000 years ago.
Earlier this year, a team of researchers announced that they had discovered the largest eruption ever to occur in Yellowstone – a huge explosive event that occurred about 8.7 million years ago and covered an area the size of New Jersey with volcanic glass. The same research showed that eruptions at Yellowstone have become smaller over time, possibly indicating that hotspot activity in the area “may be decreasing.
Wilson said their findings at Huckleberry Ridge Tuff have implications for future eruptions: “These finds change the way we think about the massive Yellowstone explosions – they may be composed of several smaller events, and this would have a significant impact on our understanding of these eruptions and their impact on the landscape,” he wrote.