Geologic Puzzle: Part of Yellowstone Park Is On the Rise

Parts of Yellowstone National Park have risen by more than three feet since the 1920s as something, perhaps molten lava, moves into the rock beneath the Wyoming preserve.

But scientists say that there are no signs yet that the park, which was the site of several titanic volcanic explosions in prehistoric times, is getting ready to erupt again.

“I think it’s ‘breathing,’ ” said Robert B. Smith, a University of Utah geologist. He noted that terrain rising near the center of the park began to subside somewhat in 1984.

Yellowstone has not had a volcanic eruption for 70,000 years, Smith said. If one were on the way, “I would guess it would certainly have a lot of small earthquakes and uplift” of the ground before it erupted.

Smith, with U.S. Air Force Geophysics Laboratory scientists Don W. Vasco and Charles L. Taylor, reported earlier this month to the spring meeting of the American Geophysical Union in Baltimore on their investigations into the possible cause of the Yellowstone uplifting.

Comparison Surveys Cited

Taylor said that comparisons of surveys conducted during road building in the park in 1923 with new ones in the mid-1970s uncovered evidence of two major uplift “domes.”

One, in the vicinity of the Old Faithful geyser, is called the Mallard Lake Dome. The other, near the Grand Canyon of the Yellowstone River, is called the Sour Creek Dome. Together, the uplifted terrain covers an area more than 43 miles long and 25 miles wide.

The areas had risen more than 2.3 feet between 1923 and the 1970s, Taylor said. They had risen another 0.8 feet when new measurements were made in 1984.

Seismic studies by the U.S. Geological Survey suggested that “something unusual was happening” under the domes, Smith said. When earthquakes rattled through the geologically active Yellowstone area, “something beneath the caldera was slowing things (the seismic waves) down.”

That suggested something liquid was under there.

Created by ‘Intrusion’

Calculations by Vasco, Taylor and Smith now suggest that the domes have been created by the “intrusion” of 0.23 cubic kilometers of something, two to four miles below the surface.

“We have got to be careful not to say we know” what that “something” is, Smith said. But the “geological evidence” suggests either molten rock or some sort of “unusual hydrothermal activity”--hot water.

Geologists studying changes in the level of the shoreline of Yellowstone Lake, which has been tilted by the uplift, suggest that it “probably began a few thousand years ago,” Smith said.

Yellowstone, of course, is famous for its geysers and hot springs, phenomena driven by the interaction of ground water with hot rock and volcanic magma that lies close to the surface in northwestern Wyoming.

The entire 2.2 million-acre park lies within what geologists call the “Yellowstone caldera,” the collapsed remains of an ancient but active volcano. Three times in the recent geological past--2 million years ago, 1.2 million years ago and 600,000 years ago--the Yellowstone volcano has erupted in cataclysmic explosions. A small eruption took place 70,000 years ago, and the region continues to experience thousands of small earthquakes every year, more than any other site in the Rocky Mountains.

The most recent cataclysmic explosion, 600,000 years ago, threw an estimated 3,500 cubic kilometers of debris into the air, Smith said. That is about 3,000 times as much debris as was moved by the explosive 1980 eruption of Mt. St. Helens in Washington state.

Debris from that Yellowstone eruption has been found in deposits as far away as Kansas, Texas and the Gulf of Mexico, Smith said.

No one knows when the next big Yellowstone eruption is likely. The timing of the last three explosive eruptions suggests that they come roughly every 600,000 years, the last one 600,000 years ago.

“You can subtract 600,000 from 600,000 and see where we are now,” Smith joked.