The Yellowstone caldera is rising faster than ever.The supervolcano that is Yellowstone National Park is showing a certain amount of restlessness lately, rising about three inches a year for the last three years — more than three times the rate at any time since measurements began in 1923.
Most people don’t think about it when they visit the world’s first national park, but the same geologic activity that powers Old Faithful may someday explode in a catastrophic volcanic eruption.
Most of Yellowstone Park is located in a volcanic caldera that has had an exciting history over the last several million years. Over the past two million years, the caldera has erupted with enormous power every 640,000 years or so. And the last major eruption was – ulp! – 640,000 years ago.
Yellowstone’s explosions, when they occur, make contemporary volcanic eruptions like Mount Pinatubo and Mount St. Helens look like soap bubbles. The eruption that occurred two million years ago was 2,500 times more powerful than Mount St. Helens and covered North America with ash a couple of feet deep as far away as Iowa.
The eruption 1.3 million years ago was 280 times more powerful than Mount St. Helens, and the one 640,000 years ago was 1,000 times bigger. Over the lasts 16.5 million years, the caldera has erupted 140 times.
In a study published in this week’s Science, University of Utah scientists reported that the caldera has been unusually active over the past three years. But:
“There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That’s the bottom line,” said seismologist Robert B. Smith, lead author of the study and professor of geophysics at the University of Utah. “A lot of volcanoes worldwide go up and down over decades without erupting.”
Yellowstone is North America’s largest volcanic field, produced by a “hotspot,” a gigantic plume of hot and molten rock. This hotspot begins about 400 miles beneath the surface and rises to about 30 miles underground, where it is about 300 miles across.
Yellowstone itself sits atop a magma chamber that is a sponge-like body of molten and solid rock. This magma chamber begins about five miles below the surface and extends to at least 10 miles below the surface. Fractures in the Earth between the top of the chamber and the earth’s surface carry heat, which drive the park’s famous thermal features — geysers, hotpots, fumaroles and so on.
Every so often, a blob of molten rock breaks off from the lower hot spot and rises to resupply the magma chamber.
That’s apparently what’s happening with the current activity. The scientists said that a slab of molten rock about the size of Los Angeles has risen to about 6 miles below the surface, driving the caldera upwards at an unprecedented rate.
“Our best evidence is that the crustal magma chamber is filling with molten rock,” Smith says. “But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again,” he adds.
Scientists are unable to predict volcanic eruptions with any degree of certainty, even in well-studied volcanoes. This “breathing” of the Yellowstone caldera is not unusual, however. According to a release from the University of Utah:
Conventional surveying of Yellowstone began in 1923. Measurements showed the caldera floor rose 40 inches between 1923 and 1984, and then fell 8 inches from 1985 to 1995.
Global Positioning System data showed the Yellowstone caldera floor sank 4.4 inches from 1987 to 1995. From 1995 to 2000, the caldera rose again, but the uplift was greatest — 3 inches — at Norris Geyser Basin, just outside the caldera’s northwest rim.
Between 2000 and 2003, the northwest area rose another 1.4 inches, but the caldera floor itself sank about 1.1 inches. The trend continued during the first half of 2004. Then, in July 2004, the caldera floor began its rapid rate of uplift, followed three months later by sinking of the Norris area that continued until mid-2006.
Map of Yellowstone Park with outline of caldera. Courtesy National Park Service