Thursday, March 02, 2006
Super volcano underneath Yellowstone
A new model helps explain the latest episode of rapid surface rise and increased geyser activity—from 1997 to 2003—in the volcanically active region in the western United States.
Much of Yellowstone National Park lies in the crater of a massive volcano, formed in a landscape-altering eruption 640,000 years ago. The crater, or caldera, measures some 28 miles wide by 47 miles long (45 by 75 kilometers).
Subsequent lava flows—most recently 70,000 years ago—filled in much of the blasted-out crater, disguising the area's volcanic identity.
Since the 1970s scientists have known that the Yellowstone volcano remains highly active.
But the precise relationship between volcanic activity deep underground and Yellowstone's well-known network of geysers and other geothermal features has long been a puzzle for geologists.
Now a study by scientists with the U. S. Geological Survey (USGS) and the Yellowstone Volcano Observatory attributes changes in both surface terrain and geyser behavior to flows of magma, or molten rock, 9 miles (15 kilometers) below the Earth's surface.
"We're not sure yet if this is a normal episode or not," said Charles Wicks, a geologist at the USGS Western Region headquarters in Menlo Park, California.
Wrinkles and Cracks
Using satellite-based radar, Wicks and his colleagues were able to map small changes in surface elevation continuously across a wide area.
The new, detailed view of the Yellowstone crater shows a surface in constant motion, rising and falling in different locations and over fairly short intervals of time. From earlier surveys, scientists know that the caldera floor raised about 7 inches (18 centimeters) from 1976 to 1984 and then settled back about 5.5 inches (14 centimeters) from 1985 to 1995.
Researchers later noted a vertical rise in the crater floor, beginning in 1995. The floor largely began sinking again by 1998.
The new report, to be published in tomorrow's edition of the journal Nature, focuses on an isolated area along the north rim of the crater that continued to rise while the crater floor was sinking.
This localized uplift raised the ground level about 5 inches (13 centimeters) from 1997 to 2003.
"This was something new," Wicks said. "We had never seen uplift under the caldera rim before."
At the same time thermal activity in and around the Norris Geyser Basin, near the uplifting-rim area, began moving into high gear.
Steamboat Geyser, the world's largest, broke a nine-year silence with a series of eruptions from 2000 to 2003.
Park officials had to close some hiking trails due to increasing ground temperatures, and in 2003 a line of new steam vents appeared, roaring like jet engines.
Wicks and his colleagues believe that a pulse of volcanic magma moving horizontally underground caused the complex rippling of the land surface and the unusual hydrothermal displays.
A New Model
The researchers' theory is based on a mathematical model that helps explain the pattern of lifting revealed by the radar imaging.
The land's rise and fall over time, they say, can be attributed to variation in what may be a continuous flow of molten basalt. Basalt is cooled, hardened magma.
Wicks believes that in the 1997-to-2003 episode, an unusually large pulse of magma rose from deep underground and spread outward just beneath the caldera surface.
"As it spreads it looks for a way out," Wicks said. "A system of faults under the north rim provides a way for the magma to exit the caldera."
Outward movement of the magma pulse would cause the caldera floor to rise and then fall back, exactly as observed.
The continuing uplift of the caldera rim can be explained by the restricted size of the magma's exit route.
Wicks thinks a sort of underground bottleneck caused part of the north rim to continue rising. Forced through a narrowing passage, he says, the magma exerted a pressure that caused the rim to rise.
Uplift in this relatively confined area may have opened new underground passages for steam and superheated water, causing the unusual geyser activity.
"It's like bending a slab of clay. You see cracks form on the upper surface," Wicks said.
Park geologist Henry Heasler said that, while the new study is compelling, further tests of the model are needed.
"What they have nailed is that an intrusion from the caldera up to the Norris area matches the ground deformation pattern," Heasler said. "It's a fascinating paper, but there are still competing hypotheses."
For example, the surface changes may have been driven by flows of hot water and gas rather than magma.
Heasler said one way to test this would be by taking precise temperature measurements at the land surface, which he and others plan to carry out.