Back in 2005 a docudrama called Supervolcano explored what might happen if the caldera beneath Yellowstone National Park erupted today. It was riveting television and, according to most experts, fairly accurate in its portrayal.
Now comes word that Yellowstone's volcanic basement has risen by roughly 7 inches in the past three years as a result of a massive intrusion of molten magma about 6 miles beneath the caldera. That's evidence of the park's underlying volcanics, but not indicative of an impending eruption, according to University of Utah scientists.
"There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That's the bottom line," says seismologist Robert B. Smith, a geophysics professor and lead author of the study published in the journal Science. "A lot of calderas [giant volcanic craters] worldwide go up and down over decades without erupting."
Just the same, the upward movement of the Yellowstone caldera floor is more than three times greater than ever observed since such measurements began in 1923, says the study published in the November 9th issue of Science by Smith, geophysics postdoctoral associate Wu-Lung Chang, and colleagues.
Conventional surveying of Yellowstone began in 1923. Measurements showed the caldera floor rose 40 inches between 1923-1984, and then fell 8 inches during 1985-1995. Now the caldera is once again on the rise.
"Our best evidence is that the crustal magma chamber is filling with molten rock," says Professor Smith. "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."
According to the university, the magma chamber beneath Yellowstone is a not a chamber of molten rock, but rather a sponge-like body with molten rock between areas of hot, solid rock. And it's really not that unusual for such chambers to expand and contract down through the years, as calderas such as Yellowstone, California's Long Valley (site of the Mammoth Lakes ski area) and Italy's Campi Flegrei (near Naples) rise and fall "repeatedly for decades to tens of thousands of years without catastrophic eruptions," according to the researchers.
Professor Smith and Chang conducted the study with University of Utah geophysics doctoral students Jamie M. Farrell and Christine Puskas, and with geophysicist Charles Wicks, of the U.S. Geological Survey in Menlo Park, Calif.
Yellowstone: A Gigantic Volcano Atop a Hotspot
According to Professor Smith, Yellowstone is North America's largest volcanic field. It was created by a gigantic plume of hot and molten rock that begins at least 400 miles beneath Earth's surface and rises to 30 miles underground, where it widens to about 300 miles across. There, blobs of magma or molten rock occasionally break off from the top of the plume, and rise farther, resupplying the magma chamber beneath the Yellowstone caldera.
Previous research indicates the magma chamber begins about 5 miles beneath Yellowstone and extends down to a depth of at least 10 miles. Without this heat source, Yellowstone's famous geysers, hot springs, and fumaroles would die off.
Now, according to Professor Smith, as Earth's crust moved southwest over the Yellowstone hotspot
during the past 16.5 million years, it produced more than 140 cataclysmic explosions known as caldera eruptions, the largest but rarest volcanic eruptions known. Remnants of ancient calderas reveal the
eruptions began at the Oregon-Idaho-Nevada border some 16.5 million years ago, then moved progressively northeast across what is now the Snake River Plain.
The hotspot arrived under the Yellowstone area sometime after about 4 million years ago, producing gargantuan eruptions there 2 million, 1.3 million and 642,000 years ago. These eruptions were 2,500,
280 and 1,000 times bigger, respectively, than the 1980 eruption of Mount St. Helens. The eruptions covered as much as half the continental United States with inches to feet of volcanic ash.
The most recent giant eruption created the 40-mile-by-25-mile oval-shaped Yellowstone caldera. The caldera walls have eroded away in many areas - although they remain visible in the northwest portion of
the park. Yellowstone Lake sits roughly half inside and half outside the eroded caldera. Many smaller volcanic eruptions occurred at Yellowstone between and since the three big blasts, most recently 70,000 years ago. Smaller steam and hot water explosions have been more frequent and more recent.
Measuring a Volcano Getting Pumped Up
In the latest study, the scientists measured uplift of the Yellowstone caldera from July 2004 through the end of 2006 with two techniques:
-- Twelve Global Positioning System ground stations that receive timed signals from satellites, making it possible to measure ground uplift precisely.
-- The European Space Agency's Envisat satellite, which bounces radar waves off the Yellowstone caldera's floor, another way to measure elevation change.
The measurements showed that from mid-2004 through 2006, the Yellowstone caldera floor rose as fast as 2.8 inches (7 centimeters) per year - and by a total of 7 inches (18 centimeters) during the 30-month
period, says Chang.
"The uplift is still going on today but at a little slower rate," says Professor Smith, adding there is no way to know when it will stop.
The professor says the fastest rate of uplift previously observed at Yellowstone was about 0.8 inch (2 centimeters) per year between 1976 and 1985. He says that Yellowstone's recent upward motion may seem small, but is twice as fast as the average rate of horizontal movement along California's San Andreas fault.
Computer simulations were used by the scientists to better determine what changes in shape of the underground magma chamber best explain the recent uplift. These simulations or "modeling" suggested the molten rock injected since mid-2004 is a nearly horizontal slab - known to geologists as a
sill. The slab sits within and near the top of the pre-existing magma chamber, which resembles two anvil-shaped blobs expanding upward from a common base.
Professor Smith describes the slab's computer-simulated shape as "kind of like a mattress" about 38 miles long and 12 miles wide, but only tens or hundreds of yards thick. In reality, he believes the slab resembles a large, spongy pancake formed as molten rock injected from below spread out near the top of the magma chamber.
While the scientists believe steam and hot water contribute to uplift of the Yellowstone caldera, particularly during some previous episodes, the latest evidence indicates molten rock is responsible for most of the current uplift.
Chang says that when rising molten rock reaches the top of the magma chamber, it starts to crystallize and solidify, releasing hot water and gases, pressuring the magma chamber. But gases and steam
compress more easily than molten rock, so much greater volumes would be required to explain the volcano's inflation, the researchers say.
The research was funded by the National Science Foundation, the U.S. Geological Survey and the Brinson Foundation.
(Much of the above text was prepared by Lee Siegel, from the University of Utah)
You can stay on top of Yellowstone's volcanic gyrations by visiting the Yellowstone Volcano Observatory.