3-D view of Long Valley, Calif., created by imaging radar on the space shuttle Endeavor. Courtesy of NASA/JPL |
Enormous volcanic eruptions with potential to end civilizations may have surprisingly short fuses, researchers have discovered.
These
eruptions are known as super-eruptions because they are more than 100
times the size of ordinary volcanic eruptions like Mount St. Helens.
They spew out tremendous flows of super-heated gas, ash and rock capable
of blanketing entire continents and inject enough particulate into the
stratosphere to throw the global climate into decade-long volcanic
winters. In fact, there is evidence that one super-eruption, which took
place in Indonesia 74,000 years ago, may have come remarkably close to
wiping out the entire human species.
Geologists
generally believe that a super-eruption is produced by a giant pool of
magma that forms a couple of miles below the surface and then simmers
for 100,000 to 200,000 years before erupting. But a new study suggests
that once they form, these giant magma bodies may only exist for a few
thousand years, perhaps only a few hundred years, before erupting.
“Our
study suggests that when these exceptionally large magma pools form
they are ephemeral and cannot exist very long without erupting,” said
Guilherme Gualda, the assistant professor of earth and environmental
sciences at Vanderbilt University who directed the study, which appears
in the May 30 issue of the journal Public Library of Science ONE.
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The
study was performed on the remnants of the Bishop Tuff, the Long Valley
super-eruption that occurred in east-central California 760,000 years
ago. Using the latest methods for dating the process of magma formation,
Gualda and his colleagues found several independent lines of evidence
that indicate the magma pool formed within a few thousand years, perhaps
within a few hundred years, before it erupted, covering half of the
North American continent with smoldering ash.
These
giant magma pools tend to be shaped like pancakes and are 10 to 25
miles in diameter and one half to three miles deep. In the beginning,
the molten rock in these pools is largely free from crystals and
bubbles. After they form, however, crystals and bubbles form gradually
and progressively change the magma.s physical and chemical properties, a
process that halts when an eruption takes place. As far as geologists
can tell, no such giant crystal-poor magma body currently exists that is
capable of producing a super-eruption. The research team believes this
may be because these magma bodies exist for a relatively short time
rather than persisting for hundreds of thousands of years as previously
thought.
According
to Gualda, the estimates for the 100,000 year-plus lifetimes of these
giant magma bodies appears to be an artifact of the method that
geologists have used to make them. The measurements have been made using
zircon crystals. Zircons are commonplace in volcanic rocks and they
contain small amounts of radioactive uranium and thorium, which decay
into lead at a set rate, allowing scientists to accurately determine
when the crystals formed. They are extremely useful for many purposes
because they can survive most geologic processes. However, the fact that
zircons can withstand the heat and the forces found in a magma chamber
means that they are not good at recording the lifetimes of crystal-poor
magma bodies.
X-ray tomography of small pieces of pumice yield a 3D model of the crystal and vesicle (bubble casts) size and distribution. These models can be used to determine the lifespan of the giant magma body that erupted to form the Bishop Tuff. Color legend: yellow = large vesicles; green = quartz; red = feldspar; blue = magnetite. Courtesy of Gualda Lab |
Gualda
and his colleagues took a different approach in his studies of the
Bishop Tuff. They determined crystallization rates of quartz—the most
abundant mineral in the deposits—to gather information about the
lifespan of these giant magma bodies. They developed four independent
lines of evidence that agreed that the formation process took less than
10,000 years and most likely between 500 to 3,000 years before the
eruption. They suggest that the zircon crystal measurements record the
extensive changes that take place in the crust required before the giant
magma bodies can begin forming as opposed to the formation itself.
.The
fact that the process of magma body formation occurs in historical
time, instead of geological time, completely changes the nature of the
problem,. said Gualda. Instead of concluding that there is virtually no
risk of another super-eruption for the foreseeable future because there
are no suitable magma bodies, geologists need to regularly monitor areas
where super-eruptions are likely, such as Yellowstone, to provide
advanced warning if such a magma body begins to form.
According
to a 2005 report by the Geological Society of London, .Even science
fiction cannot produce a credible mechanism for averting a
super-eruption. We can, however, work to better understand the
mechanisms involved in super-eruptions, with the goal of being able to
predict them ahead of time and provide a warning for society.
Preparedness is the key to mitigation of the disastrous effects of a
super-eruption.
Vanderbilt
doctoral student Ayla S. Pamukcu, Mark S. Ghiorso of OFM Research, and
Alfred T. Anderson Jr., Stephen R. Sutton and Mark L. Rivers from the
University of Chicago participated in the study, which was supported by
grants from the National Science Foundation.
Source: Vanderbilt University
