RIVERTON n There's an inverse relationship between the frequency and destructiveness of hazardous geophysical events in Yellowstone National Park, a park geologist said Wednesday.

"Smaller events are more likely, and the catastrophic events are rare," said Hank Heasler, outlining a hierarchy of potential hazards associated with the world's most active volcanic system n Yellowstone.

The most catastrophic events n supervolcanoes n are thankfully the most rare, given their destructive power, he said. The 1980 eruption of Mount St. Helens was utterly dwarfed by the Yellowstone eruption of 2.1 million years ago, which was nearly 6,000 times greater. The other two supervolcanic eruptions in Yellowstone took place 1.3 million and 640,000 years ago.

In a presentation to the local emergency planning committee, Heasler showed a map that detailed the overlapping calderas, or basins of a volcano left by the three eruptions, encompassing the central and southwest parts of the park, reaching westward into Idaho.

Some people look at the timing of those eruptions and wonder if Yellowstone is "overdue" for yet another supervolcano, because they have happened roughly every 640,000 years.

Scientists don't know enough to say anything with certainty, he said, but the probability of a super eruption within the next few thousand years is exceedingly low.

More likely is a large lava flow, which could occur within a human lifetime. These flows, the last of which covered 125,000 acres, take place about every 10,000 years, he said. The character of that flow could range from the relatively slow-moving lava flows of the Hawaiian Islands to an explosive release like Mount St. Helens in Washington state.

Still more common are large hydrothermal explosions, of which there have been about 10 in the last 14,000 years or once in a thousand years, he said. Mary Bay, on the northern edge of Yellowstone, is the world's largest hydrothermal-explosion crater.

Heasler said earthquakes are constantly occurring in Yellowstone, with large quakes and resultant landslides about once a century. An average year can see 1,000 to 3,000 quakes, most too small to be felt.

The most significant earthquake in Yellowstone's recent history was in 1959, near Hebgen Lake, just west of the park. The 7.5 magnitude quake killed 28 people.

Asked about the concentration of earthquake activity centered on the Norris Geyser Basin and west toward Hebgen Lake, Heasler said it was connected to a magma plug under the Norris Basin.

Most common of all, aside from minor earthquakes, are the small hydrothermal explosions, Heasler said. Hot rock and water find ways to interact, creating hot springs and geysers, but when the connections change thanks to quakes or the movement of rock, new features can emerge slowly or explosively.

Prediction

In order to predict these hazards, scientists look at three things n seismic activity, ground deformation and chemical/temperature changes in gases and water emerging from geysers and springs.

"We have 27 stations scattered around the park," Heasler said, with data recorded and shared with the world via the Internet, updated every 10 minutes.

Volcanic activity deep below Yellowstone can cause the rock above to rise, fall or shift horizontally, Heasler said. "We have 11 GPS stations measuring movements of the earth around the clock."

Scientists have learned to provide solid, concrete support for their GPS stations, said Heasler n not because of earthquakes, but because itchy bison like to use the stations as scratching posts.

"We got a 3.5-inch uplift within one year, but there wasn't any seismic activity associated with it," he added, giving geologists yet another mystery to ponder.

All the data is posted by the Yellowstone Volcano Observatory on the Internet at {M7http://volcanoes.usgs.gov/yvo, he said, including fascinating seismograms.

"We're hoping some brilliant 8-year-old will see a pattern and tell us," Heasler said. As it is, he gets excited phone calls from scientists all over the world, because the really interesting events often happen in the middle of the night.

Chemicals in hydrothermal waters and temperatures are sensed and recorded by a series of stations in and around the park.

The U.S. Geological Survey is preparing a geologic hazard assessment for Yellowstone, which is due to be presented to Yellowstone staff on Sept. 30. That assessment should be available to the public the following week, and that's when the hard work begins as federal, state, county and municipal emergency planners develop interlocking plans of what to do in response to Yellowstone's volcanic activity, large or small.

Brodie Farquhar is a freelance writer based in Lander. He can be reached at brodiefarquhar@hotmail.com.

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