National Geographic : 2009 Aug
may pay witness to such a catastrophe around 74,000 years ago, when a supervolcano called Toba erupted in Indonesia. e ensuing volca- nic winter may have contributed to a period of global cooling that reduced the entire human population to a few thousand individuals--- a close shave for the human race. , the supervolcanoes have le little behind beyond a faintly percep- tible sense of absence. e Yellowstone caldera has been eroded, lled in with lava ows and ash from smaller eruptions (the most recent was 70,000 years ago) and smoothed by glaciers. Peaceful forests cover any lingering scars. e combined e ect makes it almost impossible to detect, unless you ve got a good eye, like Doane had, or a geologist whispering in your ear. "You re seeing two-thirds of the entire cal- dera," says Bob Smith. " e size is so immense that people don t appreciate it." Smith is a Uni- versity of Utah geophysicist and a prominent expert on the supervolcano at Yellowstone. We re standing atop Lake Butte, an overlook at the east end of Yellowstone Lake, one of the best places to see the caldera. But I don t see it. I can see the lake spread out for miles beneath us and a few little hills to the north---old lava domes. But I can t follow the caldera rim visu- ally because much of it is beneath the lake and because of the sheer scale of the thing---roughly 45 miles across. Like Doane atop Mount Washburn, I see only distant mountains on the horizon on either side and between them, to the west, the "unmountains," the emptiness where the land swallowed itself in the course of a few days. e e ects of the past eruptions are nevertheless profoundly felt in the pres- ent. e lodgepole pines that dominate the park s forests are adapted to growing in nutrient-poor soils, like those in the Yellowstone caldera. So too are the whitebark pines, whose nuts sustain grizzlies and black bears. And of course, the land to this day is literally boiling over. e trout that riot in the rivers would not be so abundant without the warming e ects of the hydrothermal springs at the bottom of frigid Yellowstone Lake. The park roils with geysers, fumaroles, mud volca- noes, and other hydrothermal activity. Half the geysers on the planet are in Yellowstone. e hydrothermal features change constantly in temperature and behavior, with new ones pop- ping up in the forests, spewing clouds of steam visible from airplanes, exuding vapors that have been known to kill bison on the spot. In spite of this "most violent gaseous ebulli- tion," as one early explorer put it, the volcano beneath Yellowstone was long thought to be extinct, as Doane believed, or at least in its dying days. Indeed, a er federal surveys in the late 19th century, the volcanic nature of Yellow- stone received little scienti c scrutiny for dec- ades. en in the late 1950s, a young Harvard graduate student, Francis "Joe" Boyd, became intrigued by the presence of a welded tu ---a thick layer of heated and compacted ash, which he realized was a sign of pyroclastic ows from an explosive, geologically recent eruption. In 1965 Bob Christiansen found a second distinct welded tu ; the next year he and his colleagues identi ed a third. Using potassium- argon dating, they deter- mined that the three tu s were the result of three dis- tinct eruptions. Each cre- ated a giant caldera, with the most recent eruption largely burying signs of the previous two. Then one day in 1973, Bob Smith and a colleague were doing some work on Peale Island, in the South Arm of Yellowstone Lake, when Smith noticed some- thing odd: Some trees along the shoreline were partially submerged and dying. He had worked in the area back in 1956 and If the magma rises quickly, the gases can't escape fast enough. It's like opening a Coke bottle after you've shaken it.