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December 26, 2009
National Parks Traveler
Detailed mapping shows the "hot spot" that fuels Yellowstone National Park's geothermal features is more than 400 miles deep, and might have been responsible for volcanic activity in Oregon, Washington, and Idaho 17 million years ago.
Image: A new study shows that the "hot spot" that fuels Yellowstone's geothermal features extends more than 400 miles beneath the park's landscape. University of Utah graphic.
The mapping contradicts previous theories that Yellowstone's geothermal features were powered by a relatively shallow -- perhaps 250 miles deep -- hot spot.
Additionally, University of Utah researchers say the "banana-shaped magma chamber" they mapped runs at an upward angle from below the park's northwestern corner and is 20 percent larger than previously thought. As a result, they say, "a future cataclysmic eruption could be even larger than thought."
Those are among the findings produced by four National Science Foundation-funded studies that are reported on in the current issue of the Journal of Volcanology and Geothermal Research. The studies were overseen by Robert Smith, a research professor and professor emeritus of geophysics at the University of Utah and the coordinating scientist for the Yellowstone Volcano Observatory.
“We have a clear image, using seismic waves from earthquakes, showing a mantle plume that extends from beneath Yellowstone,’’ the geophysicist said.
The plume angles downward 150 miles to the west-northwest of Yellowstone and reaches a depth of at least 410 miles, Professor Smith said in a release the university issued Monday. The study estimates the plume is mostly hot rock, with 1 percent to 2 percent molten rock in "sponge-like voids" within the hot rock.
According to the university, some 17 million years ago or so the Yellowstone hotspot was located beneath the Oregon-Idaho-Nevada border region, where it fed a plume of hot and molten rock that produced “caldera” eruptions the biggest kind of volcanic eruption on Earth. "As North America slid southwest over the hotspot, the plume generated more than 140 huge eruptions that produced a chain of giant craters calderas extending from the Oregon-Idaho-Nevada border northeast to the current site of Yellowstone National Park, where huge caldera eruptions happened 2.05 million, 1.3 million and 642,000 years ago," the university researchers said.
"These eruptions were 2,500, 280 and 1,000 times bigger, respectively, than the 1980 eruption of Mount St. Helens," they added. "The eruptions covered as much as half the continental United States with inches to feet of volcanic ash. TheYellowstone caldera, 40 miles by 25 miles, is the remnant of that last giant eruption."
According to the university, the latest studies reinforce the view that the hot and partly molten rock feeding volcanic and geothermal activity at Yellowstone isn’t vertical, but has three components:
* The 45-mile-wide plume that rises through Earth’s upper mantle from at least 410 miles beneath the surface. The plume angles upward to the east-southeast until it reaches the colder rock of the North American crustal plate, and flattens out like a 300-mile-wide pancake about 50 miles beneath Yellowstone. The plume includes several wider “blobs” at depths of 355 miles, 310 miles and 265 miles.
“This conduit is not one tube of constant thickness,” explained Professor Smith. “It varies in width at various depths, and we call those things blobs.”
* A little-understood zone, between 50 miles and 10 miles deep, in which blobs of hot and partly molten rock break off of the flattened top of the plume and slowly rise to feed the magma reservoir directly beneath Yellowstone.
* A magma reservoir 3.7 miles to 10 miles beneath the Yellowstone caldera. The reservoir is mostly sponge-like hot rock with spaces filled with molten rock.
“It looks like it’s up to 8 percent or 15 percent melt,” said the professor. “That’s a lot.”
According to Professor Smith, researchers previously believed the magma chamber measured roughly 6 to 15 miles from southeast to northwest, and 20 or 25 miles from southwest to northeast, but new measurements indicate the reservoir extends at least another 13 miles outside the caldera’s northeast boundary. He said the gravity and other data show the magma body “is an elongated structure that looks like a banana with the ends up. It is a lot larger than we thought I would say about 20 percent [by volume]. This would argue there might be a larger magma source available for a future eruption.”
The researchers produced the images of the magma reservoir based on the strength of Earth’s gravity at various points in Yellowstone. Hot and molten rock is less dense than cold rock, so the tug of gravity is measurably lower above magma reservoirs.
The Yellowstone caldera, like other calderas on Earth, huffs upward and puffs downward repeatedly over the ages, usually without erupting, the university said. Since 2004, the caldera floor has risen 3 inches per year, suggesting recharge of the magma body beneath it.
The study, the Yellowstone Geodynamics Project, was conducted during 1999-2005. It used an average of 160 temporary and permanent seismic stations and as many as 200 to detect waves from some 800 earthquakes, with the stations spaced 10 miles to 22 miles apart closer than other networks and better able to “see” underground, the university release said. Some 160 Global Positioning System stations measured crustal movements. By integrating seismic and GPS data, “it’s like a lens that made the upper 125 miles much clearer and allowed us to see deeper, down to 410 miles,” Professor Smith said.
The study also shows warm rock not as hot as the plume stretching from Yellowstone southwest under the Snake River Plain, at depths of 20 miles to 60 miles. The rock is still warm from eruptions before the hotspot reached Yellowstone.
While the imaging shows that the plume is about 410 miles beneath the town of Wisdom, Mont., which is 150 miles west-northwest of Yellowstone, Professor Smith said "it wouldn’t surprise me” if the plume extends even deeper, perhaps originating from the core-mantle boundary some 1,800 miles deep.
Scientists have debated for years whether Yellowstone’s volcanism is fed by a plume rising from deep in the Earth or by shallow churning in the upper mantle caused by movements of the overlying crust. Professor Smith said the new study has produced the most detailed image of the Yellowstone plume yet published. The notion that a deep plume feeds Yellowstone got more support from a study published this month indicating that the Hawaiian hotspot which created the Hawaiian Islands is fed by a plume that extends downward at least 930 miles, tilting southeast, the university noted.
Based on how the Yellowstone plume slants now, Professor Smith and colleagues projected on a map where the plume might have originated at depth when the hotspot was erupting at the Oregon-Idaho-Nevada border area from 17 million to almost 12 million years ago. They saw overlap, between the zones within the Earth where eruptions originated near the Oregon-Idaho-Nevada border and where the famed Columbia River Basalt eruptions originated when they were most vigorous 17 million to 14 million years ago. Their conclusion: the Yellowstone hotspot plume might have fed those gigantic lava eruptions, which covered much of eastern Oregon and eastern Washington state.
Professor Smith conducted the seismic study with six University of Utah present or former geophysicists former postdoctoral researchers Michael Jordan, of SINTEF Petroleum Research in Norway, and Stephan Husen, of the Swiss Federal Institute of Technology; postdoc Christine Puskas; Ph.D. student Jamie Farrell; and former Ph.D. students Gregory Waite, now at Michigan Technological University, and Wu-Lung Chang, of National Central University in Taiwan. Other co-authors were Bernhard Steinberger of the Geological Survey of Norway and Richard O’Connell of Harvard University.
The professor conducted the gravity study with former University of Utah graduate student Katrina DeNosaquo and Tony Lowry of Utah State University in Logan.