The last huge eruption was 640,000 years ago. Since then, a series of smaller ones have filled in the caldera "like tubes of toothpaste squeezing out all over the place," Smith said. The 3,000-foot-thick (one kilometer-thick) glaciers of the last Ice Age erased edges of the caldera, which is now a broad, undulating plateau rimmed by mountains.
The Earth has always shaken periodically around Yellowstone. But without the proper monitoring equipment in place, no one knew how often it happened or why. Smith, who has been investigating here for more than 30 years, set up seismometers and found earthquakes by the hundreds.
The Basin and Range country that extends from California to Montana is one of the most seismically active regions east of California's San Andreas Fault. It is being stretched apart as tectonic plates beneath it move.
But the earthquakes Smith started tracking three decades ago15,000 between 1973 and 1998, often in swarmsdidn't altogether fit conventional notions of seismicity. There were quakes where you would expect them to occur, along north/south fault lines perpendicular to the stretching. But there were also some along parallel fault linesactivity that seemed to have no relation to the stretching.
Smith started thinking about the quakes in combination with Yellowstone's famously unstable plumbing. Was it possible that both the quakes and the geysers were products of volcanic action, of underground magma flows?
Atop a volcano, mountains are pushed up by swelling magma; the subsequent explosion then destroys them and engulfs their remains.
In 1965 a team led by Robert Christiansen of the U.S. Geological Survey mapped the massive caldera and various lava flows in detail while NASA tried out a new remote-sensing technology in the region.
"It was not a surprise it was a young volcano," Christiansen recalled. "It was a surprise it was as young as it is."
He turned to Smith, whose seismic data would reveal whether the volcano was still rumbling. Together, the two men were able to see the system for what it was: a very active and large volcano that had sculpted much of the Northwest.
Smith and Christiansen saw evidence that a huge plume of magma rose from deep within the Earth and bore through the continental plate. As the plate moved southwest, the "hot spot" left a series of what Smith terms "ancient Yellowstones" across a 500-mile (800-kilometer) swath of southern Idaho from Oregon to Montana.
The hot-spot theory was dismissed when it was introduced by Smith in 1973. Accepted wisdom said volcanoes were found at the edges of tectonic plates and that hot spots occur mainly on the seafloor. "It took people a while to catch on," Smith said.
The evidence, ultimately, was incontrovertible.
There was the blasted topography, the layers of lava flows, the misaligned earthquake faults and Yellowstone's superheated, effervescent plumbing. Only one force was big enough to account for it all: a massive volcano. What Smith still didn't know was whether it was asleep.
In the mid-1970s, while surveying an old benchmark put into place when the first roads were cut through Yellowstone in 1923, Smith found that the ground had risen three feet (one meter) in five decades.
There could be only one explanation. The volcano was bulging upward. Smith and his students spent two years confirming the observation. By 1979, when he published the findings in the journal Science, even skeptics were becoming convinced that Yellowstone was an active volcano.
The caldera rose an inch a year until 1985. Then a swarm of earthquakes occurred nearby. By 1987 measurements showed that the caldera was falling an inch (2.5 centimeters) a year. In 1995 it started rising again. The caldera is now bulging again, toward the southwest.
Confirmation that the volcano was active was one of the most important factors in getting a new observatory established here. The movement of the volcano also suggests a controversial new idea forcing many geologists to rethink the very definition of hot spots and how they work.
Will It Blow Again?
Until Smith came along, most scientists believed that hot spots originate 1,800 miles (2,900 kilometers) down, at the boundary between the Earth's core and mantle. The newly revealed geology of Yellowstone suggests that this hot spot might be very shallow, born of the vagaries of heat and changing pressures or some other process yet unknown.
As far as it goes, the scientists work has yet to answer the most important question of all: Will the volcano blow its top again?
New studies by a research team at the University of Wisconsin that analyzed tiny crystals within hardened lava suggest a "dying, but still potent, cycle of volcanism."
Some people believe that the hot spot is moving under the Rocky Mountains, a much thicker and colder part of the continent, and that it will be effectively capped. Others contend that the cap won't stop the fury of the hot spot.
Smith and Christiansen can't say for sure, but they know the volcano is not dead. There is no reason, they say, it won't blow again.
Christiansen doubts the likelihood of another cataclysmic eruption any time soon, but he doesn't rule out something smaller. Earthquakes, rock slides, and steam explosions from geyser basins are all possible. A blowout on the scale of Mount St. Helens is conceivable, he said, adding: "We need to be prepared."
Copyright 2001 Ogden Publishing Corporation (for StandardNet)