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Yellowstone volcanic activity slowing

SALT LAKE CITY, July 15 (UPI) -- The Yellowstone caldera, an active volcano that has exploded with alarming force over the last 16 million years or so, appears to be slowing its activity, two University of Utah geologists report.

For the past 2 million years, the Yellowstone caldera has erupted fairly regularly, about every 600,000 years, with the last eruption -- about 600,000 years ago. The research, by geologists Michael Perkins and Barbara Nash, indicates the caldera may not be ready to blow quite yet, however.

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Perkins and Nash found Yellowstone's volcanic eruptions have become less frequent and the magma temperature associated with eruptions has been lower. In addition, the rate at which the North American tectonic plate is moving over the Yellowstone hotspot appears to have slowed.

Perkins, research assistant professor of geology and geophysics, told United Press International, "The number of explosive eruptions per million years was at its peak about 16 million years ago. There were 30 eruptions per million years."

Perkins said the eruptions were huge, spreading volcanic ash up to a meter thick over the Rockies and Great Plains, as well as some on the Pacific Coast -- an area of about 4 million square miles. The eruptions happened many times, he said. In fact, the Yellowstone hotspot probably had 142 of these huge eruptions over the 16-million-year period, many more than the 100 previously estimated. This raises the possibility of a potential future catastrophic eruption that could bury the western United States under ash.

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"They've been going on for 16 million years, so I don't think they've stopped," Perkins said. However, there would be significant seismic activity prior to any catastrophic eruption, he added.

The Yellowstone hotspot is named for the plateau volcanic field in what is now Yellowstone National Park. The hotspot is a magma source deep within Earth's mantle whose heat surges up to the surface, driving the famous thermal features of the park, its hot springs, geysers, mud pots and fumaroles. It also makes Yellowstone one of the most active earthquake zones in the world.

The hotspot poked up through the surface originally about 18 million years ago, just east of where the borders of Oregon, Nevada and California now intersect. Since then, it has been wandering northeastward at the leisurely average pace of about an inch and a half per year.

Yellowstone's seismic history and current activity are of more than simple academic interest, said park supervisory geologist Hank Heasler. "A lot of science is going into looking at the periodicity and lot of estimates are being refined on the amount of material that's been erupted," Heasler told UPI.

"Though time, we are getting improved resolution and precision, and in relation to the National Park Service, we are interested ... because it does relate to public safety," he said.

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Heasler also serves as coordinating scientist for the Yellowstone Volcano Observatory, which is monitoring seismic activity in the park. Yellowstone has two or three "mini-earthquakes" a day, and sometimes as many as 65. A flurry of the quakes could indicate growing seismic hazards, either from earthquakes or volcanic activity.

Perkins and Nash looked at the chemistry of the ashfalls left behind by the many eruptions of the hotspot to determine its life history. Perkins said the temperature of the magma in the eruptions has decreased from about 1,800 degrees Fahrenheit in the earliest period to about 1,475 degrees F more recently.

According to work by University of Utah geologist Robert Smith, the recent Yellowstone eruptions were gigantic compared with any modern volcano. For instance, the 1980 eruption of Mount St. Helens in Washington state ejected 0.07 cubic mile of rock, dirt and ash into the atmosphere. The Lava Creek Caldera eruption about 600,000 years ago ejected 240 cubic miles of material into the air -- more than 3,400 times as much. The Huckleberry Ridge explosion about 2 million years ago was two-and-a-half times larger than Lava Creek.

"These volumes make Yellowstone one the world's largest, if not the largest, known center of active silicic volcanism," Smith said.

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The work by Perkins and Nash shows the explosive frequency of the volcanic system appears to be slowing, and it may be that the rate of migration of the North American tectonic plate also is slowing. During the past 8.5 million years, the plate has moved at about 15.5 miles per million years. However, from roughly 16 million to 8.5 million years ago, it was racing along at twice that rate -- about 31 miles per million years. This relative rate of motion between the mantle and the plate may be important in determining the amount of discharge in eruptions, the scientists argue.

(Reported by Dan Whipple, UPI Environment News, in Broomfield, Colo.)

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