The Yellowstone caldera has been classified a high threat for volcanic eruption, according to a report from the U.S. Geological Survey.

Yellowstone ranks 21st most dangerous of the 169 volcano centers in the United States, according to the Geological Survey's first-ever comprehensive review of the nation's volcanoes.

Kilauea in Hawaii received the highest overall threat score followed by Mount St. Helens and Mount Rainer in Washington, Mount Hood in Oregon and Mount Shasta in California.

Kilauea has been erupting since 1983. Mount St. Helens, which erupted catastrophically in 1980, began venting again in 2004.

Those volcanoes fall within the very high threat group, which includes 18 systems. Yellowstone is classified with 36 others as high threat.

Recurring earthquake swarms, swelling and falling ground, and changes in hydrothermal features are cited in the report as evidence of unrest at Yellowstone.

The report calls for better monitoring of the 55 volcanoes in the very high and high threat categories to track seismic activity, ground bulging, gas emissions and hydrologic changes.

University of Utah geology professor Robert Smith, who monitors earthquakes and volcanic activity in Yellowstone, said more real-time monitoring should be helpful.

"We've really been stressing over the last couple of years that the USGS should consider hazards as a very high priority in their future," he said. "We need to get the public's confidence and the perception that we're doing it right."

The university has joined the Geological Survey and Yellowstone National Park in creating the Yellowstone Volcano Observatory, which uses ground-based instruments throughout the region and satellite data to monitor volcanic and earthquake unrest in the world's first national park.

The USGS report recognizes Yellowstone as an unusual hazard because of the millions of people who visit the park and walk amid features created by North America's largest volcanic system, Smith said, a status he has been advocating for years.

Smith does not paint the devastating picture portrayed in a recent TV docudrama but said smaller threats exist. For example, a lower-scale hydrothermal blast could scald tourists strolling along boardwalks.

Emissions of toxic gases from the park's geothermal features also pose a threat. Five bison dropped dead last year after inhaling poisonous gases trapped near the ground due to cold, calm weather near Norris Geyser Basin.

Stepped up monitoring and a new 24-hour watch office could lead to more timely warnings and help avoid human catastrophes at Yellowstone and nationally, according to the USGS.

Forty-five eruptions, including 15 cases of notable volcanic unrest, have been documented at 33 volcanoes in the U.S. since 1980, according to the report, released April 29.

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On the Net:

U.S. Geological Survey: http://www.usgs.gov

Volcano Threat Report: http://pubs.usgs.gov/of/2005/1164

Yellowstone Volcano Observatory: http://volcanoes.usgs.gov/yvo/index.html