Earth Changes: A Surprise From the Supervolcano Under Yellowstone
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Beneath
Yellowstone National Park lies a supervolcano, a behemoth far more
powerful than your average volcano. It has the ability to expel more
than 1,000 cubic kilometers of rock and ash at once — 250,000 times more
material than erupted from Mount St. Helens in 1980, which killed 57
people. That could blanket most of the United States in a thick layer of
ash and even plunge the Earth into a volcanic winter.
Yellowstone’s
last supereruption occurred 631,000 years ago. And it’s not the
planet’s only buried supervolcano. Scientists suspect that a
supereruption scars the planet every 100,000 years, causing many to ask
when we can next expect such an explosive planet-changing event.
To
answer that question, scientists are seeking lessons from Yellowstone’s
past. And the results have been surprising. They show that the forces
that drive these rare and violent events can move much more rapidly than
volcanologists previously anticipated.
The early evidence, presented at a recent volcanology conference,
shows that Yellowstone’s most recent supereruption was sparked when new
magma moved into the system only decades before the eruption. Previous
estimates assumed that the geological process that led to the event took
millenniums to occur.
To
reach that conclusion, Hannah Shamloo, a graduate student at Arizona
State University, and her colleagues spent weeks at Yellowstone’s Lava
Creek Tuff — a fossilized ash deposit from its last supereruption.
There, they hauled rocks under the heat of the sun to gather samples,
occasionally suspending their work when a bison or a bear roamed nearby.
Ms.
Shamloo later analyzed trace crystals in the volcanic leftovers,
allowing her to pin down changes before the supervolcano’s eruption.
Each crystal once resided within the vast, seething ocean of magma deep
underground. As the crystals grew outward, layer upon layer, they
recorded changes in temperature, pressure and water content beneath the
volcano, much like a set of tree rings.
“We
expected that there might be processes happening over thousands of
years preceding the eruption,” said Christy Till, a geologist at Arizona
State, and Ms. Shamloo’s dissertation adviser. Instead, the outer rims
of the crystals revealed a clear uptick in temperature and a change in
composition that occurred on a rapid time scale. That could mean the
supereruption transpired only decades after an injection of fresh magma
beneath the volcano.
The time scale is the blink of an eye, geologically speaking. It’s even shorter than a previous study
that found that another ancient supervolcano beneath California’s Long
Valley caldera awoke hundreds of years before its eruption. As such,
scientists are just now starting to realize that the conditions that
lead to supereruptions might emerge within a human lifetime.
“It’s
shocking how little time is required to take a volcanic system from
being quiet and sitting there to the edge of an eruption,” said Ms.
Shamloo, though she warned that there’s more work to do before
scientists can verify a precise time scale.
Dr.
Kari Cooper, a geochemist at the University of California, Davis who
was not involved in the research, said Ms. Shamloo and Dr. Till’s
research offered more insights into the time frames of supereruptions,
although she is not yet convinced that scientists can pin down the
precise trigger of the last Yellowstone event. Geologists must now
figure out what kick-starts the rapid movements leading up to
supereruptions.
“It’s
one thing to think about this slow gradual buildup — it’s another thing
to think about how you mobilize 1,000 cubic kilometers of magma in a
decade,” she said.
As
the research advances, scientists hope they will be able to spot future
supereruptions in the making. The odds of Yellowstone, or any other
supervolcano, erupting anytime soon are small. But understanding the
largest eruptions can only help scientists better understand, and
therefore forecast, the entire spectrum of volcanic eruptions —
something that Dr. Cooper thinks will be possible in a matter of
decades.
