Before the hotspot that created Yellowstone National Park’s incredible geysers, fumaroles and mud pots arrived in what is now northwestern Wyoming, it made a rowdy passage through southern Idaho.
Recent research has looked at that ancient geological history in greater detail and identified 12 major eruptions in southern Idaho, one of which was a super eruption similar in scale to Yellowstone’s most recent caldera-forming explosion about 640,000 years ago.
“Initially, people thought there were a lot more eruptions that were smaller,” said David Finn, a graduate student at the University of California Santa Cruz who took part in the study. “We’ve shown there were fewer and they were bigger — some of the largest in North America.”
The research is outlined in a recently published paper in the Geological Society of America Bulletin, of which Finn was a co-author. The lead author was Thomas Knott of the University of Leicester.
Over the course of its 17-million-year lifespan, the Yellowstone hotspot has migrated across the Snake River Plain in southern Idaho and northern Nevada to its present location.
“The Great Basin, formed 20 to 30 million years ago, was in existence before Yellowstone was formed and it was already fractured,” explained Bob Smith, a University of Utah scientist who has studied the geodynamics of Yellowstone for 60 years.
About 20 million years ago, the Great Basin began stretching to what is now twice its original width, Smith said, fracturing the Earth’s crust and upper mantle in the region. Those fractures and thinning of the lithosphere made it easier for the Yellowstone magma plume to push to the surface and create a series of about 150 volcanic eruptions.
“What we showed were the volcanic eruptions were most intense in early years, then got smaller as they came across the Snake River Plain,” Smith said of some of his earlier research.
Finn said the recent research he’s been undertaking in southern Idaho shows how big some of those eruptions were. Using a drill to bore into the earth more than 6,000 feet deep, the scientists couldn’t find the bottom of the area’s eruptive flow, Finn explained.
What was revealed is “one of the most complete successions of mid-Miocene Snake River volcanism in the region,” the paper noted. The mid-Miocene would have been roughly 11 to 16 million years ago.
One of those eruptions, called Castleford Crossing, took place about 8.1 million years ago. Mapping showed its outflow covered at least 8,700 cubic square miles in rock composed of volcanic ash. That rock extended to a depth of more than 4,400 feet thick.
Based on their calculations, that would have meant that the Castleford Crossing eruption was about 8.6 in magnitude on the volcanic explosivity index. Eruptions of VEI 5 or higher are considered very large explosive events, according to the U.S. Geological Survey, which would have included Mount St. Helens’ 1980 eruption. Yellowstone’s last super eruption is considered a VEI 8.
More to come
Given the dramatic explosions that took place, people may wonder why the Snake River Plain isn’t pockmarked with volcanic calderas like Yellowstone’s. The simple answer is that they’ve been filled in by other lava flows and sediments that have gathered in the low-lying topography of the Snake River Plain.
Finn said the paper on Idaho’s Cassia Hills is the first in a series examining the region in greater geological detail. He’s been working with other scientists for six years as he completes his doctorate concentrating on documenting the magnetic fields in the volcanic flows to help establish their age within 100 to 300 years.
The Yellowstone hotspot was also the basis of recent research by University of Illinois geology professor Lijun Liu who used a supercomputer to crunch data on the origins of the supervolcano.
“The main goal of the study was to examine whether the initiation and subsequent development of the Yellowstone volcanic system was driven by a mantle plume,” according to a press release by the University of Illinois. “The simulated data showed that the plume was blocked from traveling upward toward the surface by ancient tectonic plates, meaning that the plume could not have played a significant role in forming Yellowstone, Liu said.”
The University of Utah’s Bob Smith said he and fellow scientists had come to the same conclusion without creating a computer model many years ago.
“It’s a different approach than we took to come to a similar conclusion,” Smith said.
“Our physical model is more sophisticated and realistic than previous studies, because we simultaneously consider many more relevant dynamic processes,” Liu said in the press release. His research was published in the Geophysical Research Letters.
Speculation about whether the Yellowstone supervolcano is overdue for another massive explosion is downplayed by Smith. He said the hotspot has run into the colder, thicker North American Plate, meaning it will take a lot more energy and heat to blow through than when the hotspot was located in the thin and fractured Snake River Plain.
“People are always joking that Billings is going to be wiped out,” Smith said, “but there may not be enough energy for the hotspot to burn through the lithosphere. We may argue that Yellowstone may not have a super eruption again.”
But he said that doesn’t mean that Yellowstone’s volcanism — its tourist-drawing geysers and hot springs — will stop anytime soon.