Power Of Yellowstone Shapes The Land Between Eruptions
Pastor Dale Morgan
Power Of Yellowstone Shapes The Land Between Eruptions*
Wyoming's Teton Range looms behind a Global Positioning System (GPS)
antenna in Jackson Hole that was part of a 17-year University of Utah
study in which GPS devices were used to measure gradual movements of
Earth's crust in Grand Teton National Park, Yellowstone National Park
and surrounding areas. The study found the Teton Range and Jackson Hole
are moving in unexpected directions, complicating efforts to forecast
the likelihood of major earthquakes on the Teton fault.
by Staff Writers
Salt Lake City UT (SPX) Mar 05, 2007
A 17-year University of Utah study of ground movements shows that the
power of the huge volcanic hotspot beneath Yellowstone National Park is
much greater than previously thought during times when the giant volcano
is slumbering. The $2.3 million study, which used Global Positioning
System (GPS) satellites to measure horizontal and vertical motions of
Earth's crust from 1987 to 2004, found that the gigantic underground
plume of molten rock known as the Yellowstone hotspot exerts itself
forcefully even when it isn't triggering eruptions and earthquakes:
+ As it bulges upward, the hotspot expends 10 times more energy by
gradually deforming Earth's crust at Yellowstone than by producing
earthquakes.
+ The subterranean volcanic plume, 300 miles wide at its top end, may
explain why ground along the Teton fault moves in directions just the
opposite of those expected, a perplexing discovery that complicates
efforts to predict when the fault might generate disastrous 7.5
earthquakes near the ski resorts of Jackson Hole.
+ Molten rock and hot water generated by the hotspot continue to make
the 45-by-30-mile Yellowstone caldera - a giant crater - huff upward and
puff downward without producing eruptions. Measurements since the newly
published study ended in 2004 show the caldera rising upward at a faster
rate than ever observed before.
"The Yellowstone hotspot has had a much bigger effect over a larger area
with more energy than ever expected," says University of Utah geophysics
Professor Robert B. Smith, who led the study, which was scheduled for
publication Friday, March 2, 2007, in the Journal of Geophysical
Research - Solid Earth.
"We're seeing large-scale deformation of the Earth's crust in the
western United States because of the effects of the Yellowstone
hotspot," says Christine Puskas, a University of Utah geophysics
doctoral student and the study's first author.
The study was conducted by Puskas, Smith, University of Utah
postdoctoral fellow Wu-Lung Chang and former Utah researcher Chuck
Meertens, now at UNAVCO, a consortium that studies deformation of
Earth's crust. Measurements also were made by the National Park Service,
U.S. Geological Survey, Idaho National Laboratory, Brigham Young
University, MIT and the National Geodetic Survey. Smith estimated the
participants spent about $2.3 million for the 17 years of measurements.
The new study is based on measurements made from 1987 until 2004. The
measurements included 17 one- to three-month-long "campaigns," during
which portable GPS receivers were placed for two to 10 days each at 90
to 140 benchmarks in the Yellowstone-Teton-eastern Snake River Plain
region of Wyoming, Montana and Idaho. More data came from 15 permanent
GPS receiver sites installed during 1996-2003.
GPS allows precise surveying of ground movements because two dozen
Navstar GPS satellites orbiting Earth broadcast time signals. GPS
receivers on the ground record the signals, making it possible to
triangulate each receiver's location to within a few tenths of an inch
horizontally and vertically.
Restless Volcanic Caldera Huffs and Puffs
The study summarizes the movements of the floor of the 45-by-30-mile
Yellowstone caldera - a gigantic volcanic crater formed by a
catastrophic eruption 642,000 years ago that spread volcanic ash over
half of North America and was 1,000 times bigger than the 1980 eruption
of Mount St. Helens in Washington state. Other huge Yellowstone
eruptions happened 1.3 million and 2 million years ago, with 30 much
smaller but still large eruptions between and since the three
cataclysmic blasts.
Due to underground movement of molten rock and hot water, calderas often
huff upward and puff downward for tens of thousands of years without
catastrophic eruptions.
Earlier research found that a flattened chamber of partly molten rock
extends from about 5 miles beneath the Yellowstone caldera down to a
depth of at least 10 miles, heated by the much larger, underlying
hotspot. The 50-mile-wide hotspot plume of molten rock originates at
least 410 miles underground in Earth's mantle and rises to 75 miles
below the caldera, where it hits cooler rock and spreads out to a width
of 300 miles.
Conventional surveying of Yellowstone began in 1923. Measurements showed
the caldera floor rose 40 inches during 1923-1984, and then fell 8
inches during 1985-1995.
The GPS data show the Yellowstone caldera floor sank 4.4 inches during
1987-1995. 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. During 2000-2003, the northwest area rose another 1.4
inches, but the caldera floor itself sank about 1.1 inches.
Smith and Puskas believe the caldera sank when hot water, steam and
gases migrated northwest out of the caldera and to the Norris area,
making that area rise.
While not part of the new published study, Smith reported at an American
Geophysical Union meeting last December that 2004-2006 GPS measurements
show the northwest caldera area sank by 3.2 inches, but the central
caldera floor rose faster than ever recorded: about 6.7 inches since
mid-2004.
"The rate is unprecedented, at least in terms of what scientists have
been able to observe in Yellowstone," Smith says. "We think it's a
combination of magma [molten rock] being intruded under the caldera and
hot water released from the magma being pressurized because it's
trapped. I don't believe this is evidence for an impending volcanic
eruption. But it would be prudent to keep monitoring the volcano."
Puskas adds: "This episode may represent a period like those observed at
other large volcanic systems such as Long Valley caldera, California,
where episodes of unrest have lasted one to four years, with uplift
rates as high as 4 inches a year."
The fact some GPS receivers now are permanently installed in Yellowstone
and provide data daily made researchers realize that ups-and-downs of
the caldera occur not just in decades or years, but in months,
reflecting underground movements of molten rock and hot water that "also
happen over months," Puskas says.
Hotspot's Bulge May Explain a New Mystery on the Teton Fault
The Yellowstone hotspot made overlying region bulge upward by one-third
mile during the past 2 million years, lifting the Yellowstone Plateau to
8,000 feet elevation.
Smith and Puskas say 1987-2004 GPS measurements show the southwest part
of the Yellowstone Plateau is slowly sliding downhill to the southwest
at about one-sixth inch per year. That may explain the study's biggest
surprise: ground along the Teton fault moves opposite the expected
direction.
The fault runs 40 miles north-south along the eastern base of the Teton
Range. It is a "normal" fault, which means that during large quakes, the
mountains rise upward and move westward, while the valley of Jackson
Hole - just east of the mountains - drops downward and moves eastward.
That is because the Teton region is part of the Basin and Range Province
of the West, where Earth's crust is pulled apart east-west between the
Sierra Nevada in California and the Rocky Mountains in Utah, Wyoming and
Montana.
The vertical movement totals about 3 to 6 feet during a single
magnitude-7 quake. Previous evidence indicates there were thousands of
magnitude-7 to 7.5 quakes during the past 13 million years, lifting the
tallest Teton peaks to altitudes of more than 13,000 feet - almost 7,000
feet above Jackson Hole. Such a quake today would be a major disaster
for Jackson, Wyo., and nearby towns.
Yet 17 years of GPS measurements show "the textbook model for a normal
fault is not what's happening at the Teton fault," Smith says. "The
mountains are going down relative to the valley going up. That's a total
surprise."
The results - Jackson Hole moved west one-quarter inch and upward 1.7
inches from 1987 to 2004 - mean the Teton Range and Jackson Hole valley
are being squeezed together rather than stretched apart, as was expected.
Smith believes the bulging Yellowstone hotspot, located north of the
Tetons, may be "pushing Jackson Hole into the Teton Range." This
unexpected pressure may explain why there have been no magnitude-7
quakes on the fault since the last one about 4,800 years ago, yet also
may allow pressure to build for the next disastrous quake.
"The question is how do you store the stress that is leading to the next
big earthquake?" Smith says. "It might include episodes of compression
and stretching, and eventually the dominant Basin and Range stretching
prevails" to trigger a big quake.
Slow Deformation Overpowers Quick Earthquakes
During the new study, Puskas converted GPS-measured ground movements and
records of historic earthquake magnitudes into a quantity known as a
"moment," which measures the energy expended to deform the landscape.
That allowed her to compare total ground deformation measured by GPS
with ground deformation caused by quakes.
She found the ground-moving energy of Yellowstone's frequent, mostly
small quakes was overpowered 10-to-1 by the energy generated from other
ground-moving forces. Those include the Yellowstone hotspot's bulging
uplift of the landscape, smaller volcanic ups and downs within the
caldera, relaxation of the ground after quakes, and Basin-and-Range
stretching of Earth's crust that ultimately will generate future quakes.
Smith says the fact non-seismic forces to overwhelm quake energies by
10-to-1 "means there is much more energy related to active volcanic
processes of uplift and extension of the Earth's surface," he says.
An example: The southwestern Yellowstone Plateau is moving southwest at
one-sixth inch per year, twice as fast as the next block of land to the
southwest: the eastern Snake River Plain. Land from Island Park, Idaho,
southwest to Idaho Falls is being squeezed by the collision. Smith says
that represents a lot of energy in a small area because the entire
600-mile-wide Basin and Range Province between the Sierra and the
Rockies is stretching apart only three times faster, or about a half
inch per year.