- MADISON - Reading the geochemical
fine print found in tiny crystals of the minerals zircon and quartz, scientists
are forming a new picture of the life history - and a geologic timetable
- of a type of volcano in the western United States capable of dramatically
altering climate sometime within the next 100,000 years.
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- With insight gained from new analytical techniques to
study crystals of zircon and quartz, minerals that serve as veritable time
capsules of geologic events, a group of scientists from the University
of Wisconsin-Madison has proposed a new model for the origin of volcanism
in young calderas.
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- These are volcanoes that occur over "hot spots"
in the Earth and they erupt every few hundred thousand years in catastrophic
explosions, sending hundreds to thousands of cubic kilometers of ash into
the atmosphere and wreaking climatic havoc on a global scale.
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- In a series of papers, UW-Madison geologists Ilya N.
Bindeman and John W. Valley present a life history of the hot spot volcanism
that has occurred in the Yellowstone basin of the western United States
over the past 2 million years. Their findings suggest a dying, but still
potent cycle of volcanism, and a high probability of a future catastrophic
eruption sometime within the next million years, and possibly within the
next hundred thousand years.
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- Today's Yellowstone landscape represents the last in
a sequence of calderas - the broad crater-like basins created when volcanoes
explode and their characteristic cones collapse - that formed in regular
progression over the past 2 million years. The near-clockwork timing of
eruptions there - 2 million years ago, 1.3 million years ago and 600,000
years ago - suggests a pattern that may foreshadow an eruption of catastrophic
proportions, said Bindeman and Valley.
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- Beneath Yellowstone and its spectacular landscape of
hot springs and geysers is a hot spot, an upwelling plume of melted rock
from the Earth's mantle. As the plume of hot, liquid rock rises in the
Earth, it melts the Earth's crust and creates large magma chambers.
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- "These magmas usually erupt in a very catastrophic
way," said Bindeman. "By comparison, the eruption of Mount St.
Helens sent about two cubic kilometers of ash into the atmosphere. These
catastrophic types of eruptions send thousands of cubic kilometers of ash
skyward."
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- The hot spot deep beneath Yellowstone acts like a burner,
said Bindeman. "It's a constant source of heat that acts on the upper
crust and forms magma chambers that contain tens of thousands of cubic
kilometers" of molten rock.
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- One of the massive plates that helps make up the crust
of the Earth, the North American plate, is slowly moving over the hot spot,
said Bindeman. "The plate has been moving across the heat source which
makes it seem like the volcanoes are moving across the continent. Moreover,
we have a progression of explosive eruptions which seem to have some periodicity."
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- Bindeman and Valley studied rocks that span the entire
2-million-year long eruptive sequence at Yellowstone with a special emphasis
on lavas that erupted the last time one of the massive volcanoes popped
off creating what geologists call the Yellowstone Caldera. Their conclusion
is that the volcanic cycle is waning, but that there is still a very real
potential of an eruption of massive proportions sometime in the near geologic
future.
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- Such an eruption would disrupt global climate by injecting
millions of tons of ash into the atmosphere. Some of the ash would remain
in the atmosphere for years, reflect sunlight back into space and cool
the planet, significantly affecting life. In addition, a blanket of ash
over a meter thick would be deposited in nearby regions and effectively
smother life there.
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- The most recent caldera is 600,000 years old and encompasses
an area of more than 2,000 square kilometers. When it erupted, it blasted
1,000 cubic kilometers of volcanic rock into the atmosphere and it settled
as ash over more than half of the United States.
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- After that last major eruption, volcanism in Yellowstone
continued in a quieter fashion with another, much smaller eruption occurring
70,000 years ago.
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- Today's spectacular geysers and hot springs at Yellowstone
are the most visible part of the volcanic system there. They contain heated
snow and rainwater which leave a geochemical record that provides insight
into the region's geologic activity. Prior to the last catastrophic eruption
at Yellowstone 600,000 years ago, an even more spectacular geothermal landscape
existed there, said Bindeman.
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- "The unique thing about Yellowstone is that the
volcanic rocks that erupted following the collapse of the big calderas
contain up to 50 percent oxygen which was ultimately derived from rain
waters," Bindeman said. "The zircon and quartz tell us that rocks
near the surface were altered by heated snow and rainwater. These rocks
were then remelted to become magmas."
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- This scenario changes the view of magmatism at Yellowstone
and other calderas as representing new magma coming from deep in the Earth.
On the contrary, Bindeman and Valley make a case for the total remelting
and recycling of previously erupted surface rocks.
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- Their findings have been published in a series of papers,
the first in the August 2000 edition of the journal Geology. Another paper
is to appear this month (July) in Earth and Planetary Science Letters,
and another is scheduled for publication next month (August) in the Journal
of Petrology.
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