National Geographic : 2009 Aug
• was planning to use the same boat dock as on the earlier trip. But the dock was also inundated. What was going on? Intrigued, Smith set out to resurvey bench- marks that park workers had placed on various roads throughout the park beginning in 1923. His survey revealed that the Hayden Valley, which sits atop the caldera to the north of the lake, had risen by some 30 inches over the inter- vening decades. But the lower end of the lake hadn t risen at all. In e ect, the north end of the lake had risen and tipped water down into the southern end. e ground was doming. e volcano was alive. Smith published his results in 1979, refer- ring in interviews to Yellowstone as "the living, breathing caldera." en in 1985, heralded by a "swarm" of mostly tiny earthquakes, the terrain subsided again. Smith modi ed his metaphor: Yellowstone was now the "living, breathing, shaking caldera." In the years since, Smith and his colleagues have used every trick they can devise to "see" beneath the park. Gradually, the proportions and potential of the subterranean volcanic system have emerged. At the shallowest level, surface water percolates several miles into the crust, is heated, and boils back up, supplying the geysers and fumaroles. About ve to seven miles deep is the top of the magma chamber, a res- ervoir of partially melted rock roughly 30 miles wide. Basaltic magma is trapped inside the cham- ber by denser, overlying rhyolitic magma, which oats on top of the liquid basalt like cream on milk. By looking at the way sound waves created by earthquakes propagate through subsurface rock of varying densities, the scientists have discovered that the magma chamber is fed by a gigantic plume of hot rock, rising from the Earth s upper mantle, tilted downward to the northwest by 60 degrees, its base perhaps 400 miles below the surface. When the plume pumps more heat into the chamber, the land heaves upward. Small earthquakes allow hydro- thermal fluids to escape to the surface, eas- ing the pressure inside the chamber, which causes the ground to subside again. A er the 1985 earthquake swarm, Yellowstone fell eight inches over the course of a decade or so. en it rose again, faster this time. Since 2004, por- tions of the caldera have surged upward at a rate of nearly three inches a year, much faster than any upli since close observations began in the 1970s. e surface continues to rise despite an 11-day earthquake swarm that began late in 2008, causing a urry of apocalyptic rumors on the Internet. "We call this a caldera at unrest," Smith says. " e net e ect over many cycles is to nally get enough magma to erupt. And we don t know what those cycles are." So, the colossal question: Is it going to blow again? Some kind of eruption---perhaps a mod- est one like Mount Pinatubo s in the Philippines, which killed 800 people in 1991---is highly likely at some point. e odds of a full, caldera-forming eruption---a cataclysm that could kill untold thousands of people and plunge the Earth into a volcanic winter---are anyone s guess; it could happen in our lifetimes, or 100,000 years or more from now, or perhaps never. Bob Christian- sen, now retired, suspects the supervol- cano may be safely bottled up. For most of its history, the Yellowstone hot spot has formed calderas in the thin crust of the Basin and Range area of the Ameri- can West. Now the hot spot is lodged beneath a much thicker crust at the crest of the Rockies. "I think that the system has more or less equilibrated itself," says Christian- sen. en he quickly adds, "But that s an interpretation that would not stand up in court." j Hayden Valley, at the north end of the lake, had risen some 30 inches. The ground was doming. The volcano was alive.