 |
A new University of Utah study in the journal
Science provides the first complete view of the plumbing system that
supplies hot and partly molten rock from the Yellowstone hotspot to the
Yellowstone supervolcano. The study revealed a gigantic magma reservoir
beneath the previously known magma chamber. This cross-section
illustration cutting southwest-northeast under Yelowstone depicts the
view revealed by seismic imaging. Seismologists say new techniques have
provided a better view of Yellowstone's plumbing system, and that it
hasn't grown larger or closer to erupting. They estimate the annual
chance of a Yellowstone supervolcano eruption is 1 in 700,000.
Credit: Hsin-Hua Huang, University of Utah
|
University
of Utah seismologists discovered and made images of a reservoir of hot, partly
molten rock 12 to 28 miles beneath the Yellowstone supervolcano, and it is 4.4
times larger than the shallower, long-known magma chamber.
The hot
rock in the newly discovered, deeper magma reservoir would fill the
1,000-cubic-mile Grand Canyon 11.2 times, while the previously known magma
chamber would fill the Grand Canyon 2.5 times, says postdoctoral researcher
Jamie Farrell, a co-author of the study published online today in the journal Science.
"For
the first time, we have imaged the continuous volcanic plumbing system under
Yellowstone," says first author Hsin-Hua Huang, also a postdoctoral
researcher in geology and geophysics. "That includes the upper crustal
magma chamber we have seen previously plus a lower crustal magma reservoir that
has never been imaged before and that connects the upper chamber to the
Yellowstone hotspot plume below."
Contrary
to popular perception, the magma chamber and magma reservoir are not full of
molten rock. Instead, the rock is hot, mostly solid and spongelike, with
pockets of molten rock within it. Huang says the new study indicates the upper
magma chamber averages about 9 percent molten rock -- consistent with earlier
estimates of 5 percent to 15 percent melt -- and the lower magma reservoir is
about 2 percent melt.
So there
is about one-quarter of a Grand Canyon worth of molten rock within the much
larger volumes of either the magma chamber or the magma reservoir, Farrell
says.
No
increase in the danger
The
researchers emphasize that Yellowstone's plumbing system is no larger -- nor
closer to erupting -- than before, only that they now have used advanced
techniques to make a complete image of the system that carries hot and partly
molten rock upward from the top of the Yellowstone hotspot plume -- about 40
miles beneath the surface -- to the magma reservoir and the magma chamber above
it.
"The
magma chamber and reservoir are not getting any bigger than they have been,
it's just that we can see them better now using new techniques," Farrell
says.
Study
co-author Fan-Chi Lin, an assistant professor of geology and geophysics, says:
"It gives us a better understanding the Yellowstone magmatic system. We
can now use these new models to better estimate the potential seismic and
volcanic hazards."
The
researchers point out that the previously known upper magma chamber was the
immediate source of three cataclysmic eruptions of the Yellowstone caldera 2
million, 1.2 million and 640,000 years ago, and that isn't changed by discovery
of the underlying magma reservoir that supplies the magma chamber.
"The
actual hazard is the same, but now we have a much better understanding of the
complete crustal magma system," says study co-author Robert B. Smith, a
research and emeritus professor of geology and geophysics at the University of
Utah.
The three
supervolcano eruptions at Yellowstone -- on the Wyoming-Idaho-Montana border --
covered much of North America in volcanic ash. A supervolcano eruption today
would be cataclysmic, but Smith says the annual chance is 1 in 700,000.
Before
the new discovery, researchers had envisioned partly molten rock moving upward
from the Yellowstone hotspot plume via a series of vertical and horizontal
cracks, known as dikes and sills, or as blobs. They still believe such cracks
move hot rock from the plume head to the magma reservoir and from there to the
shallow magma chamber.
Anatomy
of a supervolcano
The study
in Science is titled, "The Yellowstone magmatic system from the
mantle plume to the upper crust." Huang, Lin, Farrell and Smith conducted
the research with Brandon Schmandt at the University of New Mexico and Victor
Tsai at the California Institute of Technology. Funding came from the
University of Utah, National Science Foundation, Brinson Foundation and William
Carrico.
Yellowstone
is among the world's largest supervolcanoes, with frequent earthquakes and
Earth's most vigorous continental geothermal system.
The three
ancient Yellowstone supervolcano eruptions were only the latest in a series of
more than 140 as the North American plate of Earth's crust and upper mantle
moved southwest over the Yellowstone hotspot, starting 17 million years ago at
the Oregon-Idaho-Nevada border. The hotspot eruptions progressed northeast
before reaching Yellowstone 2 million years ago.
Here is
how the new study depicts the Yellowstone system, from bottom to top:
--
Previous research has shown the Yellowstone hotspot plume rises from a depth of
at least 440 miles in Earth's mantle. Some researchers suspect it originates
1,800 miles deep at Earth's core. The plume rises from the depths northwest of
Yellowstone. The plume conduit is roughly 50 miles wide as it rises through
Earth's mantle and then spreads out like a pancake as it hits the uppermost mantle
about 40 miles deep. Earlier Utah studies indicated the plume head was 300
miles wide. The new study suggests it may be smaller, but the data aren't good
enough to know for sure.
-- Hot
and partly molten rock rises in dikes from the top of the plume at 40 miles
depth up to the bottom of the 11,200-cubic mile magma reservoir, about 28 miles
deep. The top of this newly discovered blob-shaped magma reservoir is about 12
miles deep, Huang says. The reservoir measures 30 miles northwest to southeast
and 44 miles southwest to northeast. "Having this lower magma body
resolved the missing link of how the plume connects to the magma chamber in the
upper crust," Lin says.
-- The
2,500-cubic mile upper magma chamber sits beneath Yellowstone's 40-by-25-mile
caldera, or giant crater. Farrell says it is shaped like a gigantic frying pan
about 3 to 9 miles beneath the surface, with a "handle" rising to the
northeast. The chamber is about 19 miles from northwest to southeast and 55
miles southwest to northeast. The handle is the shallowest, long part of the
chamber that extends 10 miles northeast of the caldera.
Scientists
once thought the shallow magma chamber was 1,000 cubic miles. But at science
meetings and in a published paper this past year, Farrell and Smith showed the
chamber was 2.5 times bigger than once thought. That has not changed in the new
study.
Discovery
of the magma reservoir below the magma chamber solves a longstanding mystery:
Why Yellowstone's soil and geothermal features emit more carbon dioxide than
can be explained by gases from the magma chamber, Huang says. Farrell says a
deeper magma reservoir had been hypothesized because of the excess carbon
dioxide, which comes from molten and partly molten rock.
A better,
deeper look at Yellowstone
As with past
studies that made images of Yellowstone's volcanic plumbing, the new study used
seismic imaging, which is somewhat like a medical CT scan but uses earthquake
waves instead of X-rays to distinguish rock of various densities. Quake waves
go faster through cold rock, and slower through hot and molten rock.
For the
new study, Huang developed a technique to combine two kinds of seismic
information: Data from local quakes detected in Utah, Idaho, the Teton Range
and Yellowstone by the University of Utah Seismograph Stations and data from
more distant quakes detected by the National Science Foundation-funded
EarthScope array of seismometers, which was used to map the underground
structure of the lower 48 states.
The Utah
seismic network has closely spaced seismometers that are better at making
images of the shallower crust beneath Yellowstone, while EarthScope's
seismometers are better at making images of deeper structures.
"It's
a technique combining local and distant earthquake data better to look at this
lower crustal magma reservoir," Huang says.
Story
Source:
The above
story is based on materials provided by University
of Utah. Note: Materials may be edited for content and
length.
Journal
Reference:
Hsin-Hua Huang, Fan-Chi Lin, Brandon Schmandt,
Jamie Farrell, Robert B. Smith, Victor C. Tsai. The Yellowstone magmatic
system from the mantle plume to the upper crust. Science, 2015 Click Here to visit original site link for this Journal
0 comments:
Post a Comment