万物简史-15

15 DANGEROUS BEAUTY IN THE 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geological Survey became puzzled about something that, oddly, had not troubled anyone before: he couldn’t find the park’s volcano. It had been known for a long time that Yellowstone was volcanic in nature—that’s what accounted for all its geysers and other steamy features—and the one thing about volcanoes is that they are generally pretty conspicuous. But Christiansen couldn’t find the Yellowstone volcano anywhere. In particular what he couldn’t find was a structure known as a caldera. Most of us, when we think of volcanoes, think of the classic cone shapes of a Fuji or Kilimanjaro, which are created when erupting magma accumulates in a symmetrical mound. These can form remarkably quickly. In 1943, at Parícutin in Mexico, a farmer was startled to see smoke rising from a patch on his land. In one week he was the bemused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across. Altogether there are some ten thousand of these intrusively visible volcanoes on Earth, all but a few hundred of them extinct. But there is a second, less celebrated type of volcano that doesn’t involve mountain building. These are volcanoes so explosive that they burst open in a single mighty rupture, leaving behind a vast subsided pit, the caldera (from a Latin word for cauldron). Yellowstone obviously was of this second type, but Christiansen couldn’t find the caldera anywhere. By coincidence just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone, copies of which some thoughtful official passed on to the park authorities on the assumption that they might make a nice blow-up for one of the visitors’ centers. As soon as Christiansen saw the photos he realized why he had failed to spot the caldera: virtually the whole park—2.2 million acres—was caldera. The explosion had left a crater more than forty miles across—much too huge to be perceived from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans. Yellowstone, it turns out, is a supervolcano. It sits on top of an enormous hot spot, a reservoir of molten rock that rises from at least 125 miles down in the Earth. The heat from the hot spot is what powers all of Yellowstone’s vents, geysers, hot springs, and popping mud pots. Beneath the surface is a magma chamber that is about forty-five miles across—roughly the same dimensions as the park—and about eight miles thick at its thickest point. Imagine a pile of TNT about the size of Rhode Island and reaching eight miles into the sky, to about the height of the highest cirrus clouds, and you have some idea of what visitors to Yellowstone are shuffling around on top of. The pressure that such a pool of magma exerts on the crust above has lifted Yellowstone and about three hundred miles of surrounding territory about 1,700 feet higher than they would otherwise be. If it blew, the cataclysm is pretty well beyond imagining. According to Professor Bill McGuire of University College London, “you wouldn’t be able to get within a thousand kilometers of it” while it was erupting. The consequences that followed would be even worse. Superplumes of the type on which Yellowstone sits are rather like martini glasses—thin on the way up, but spreading out as they near the surface to create vast bowls of unstable magma. Some of these bowls can be up to 1,200 miles across. According to theories, they don’t always erupt explosively but sometimes burst forth in a vast, continuous outpouring—a flood—of molten rock, such as with the Deccan Traps in India sixty-five million years ago. (Trap in this context comes from a Swedish word for a type of lava; Deccan is simply an area.) These covered an area of 200,000 square miles and probably contributed to the demise of the dinosaurs—they certainly didn’t help—with their noxious outgassings. Superplumes may also be responsible for the rifts that cause continents to break up. Such plumes are not all that rare. There are about thirty active ones on the Earth at the moment, and they are responsible for many of the world’s best-known islands and island chains—Iceland, Hawaii, the Azores, Canaries, and Galápagos archipelagos, little Pitcairn in the middle of the South Pacific, and many others—but apart from Yellowstone they are all oceanic. No one has the faintest idea how or why Yellowstone’s ended up beneath a continental plate. Only two things are certain: that the crust at Yellowstone is thin and that the world beneath it is hot. But whether the crust is thin because of the hot spot or whether the hot spot is there because the crust is thin is a matter of heated (as it were) debate. The continental nature of the crust makes a huge difference to its eruptions. Where the other supervolcanoes tend to bubble away steadily and in a comparatively benign fashion, Yellowstone blows explosively. It doesn’t happen often, but when it does you want to stand well back. Since its first known eruption 16.5 million years ago, it has blown up about a hundred times, but the most recent three eruptions are the ones that get written about. The last eruption was a thousand times greater than that of Mount St. Helens; the one before that was 280 times bigger, and the one before was so big that nobody knows exactly how big it was. It was at least twenty-five hundred times greater than St. Helens, but perhaps eight thousand times more monstrous. We have absolutely nothing to compare it to. The biggest blast in recent times was that of Krakatau in Indonesia in August 1883, which made a bang that reverberated around the world for nine days, and made water slosh as far away as the English Channel. But if you imagine the volume of ejected material from Krakatau as being about the size of a golf ball, then the biggest of the Yellowstone blasts would be the size of a sphere you could just about hide behind. On this scale, Mount St. Helens’s would be no more than a pea. The Yellowstone eruption of two million years ago put out enough ash to bury New York State to a depth of sixty-seven feet or California to a depth of twenty. This was the ash that made Mike Voorhies’s fossil beds in eastern Nebraska. That blast occurred in what is now Idaho, but over millions of years, at a rate of about one inch a year, the Earth’s crust has traveled over it, so that today it is directly under northwest Wyoming. (The hot spot itself stays in one place, like an acetylene torch aimed at a ceiling.) In its wake it leaves the sort of rich volcanic plains that are ideal for growing potatoes, as Idaho’s farmers long ago discovered. In another two million years, geologists like to joke, Yellowstone will be producing French fries for McDonald’s, and the people of Billings, Montana, will be stepping around geysers. The ash fall from the last Yellowstone eruption covered all or parts of nineteen western states (plus parts of Canada and Mexico)—nearly the whole of the United States west of the Mississippi. This, bear in mind, is the breadbasket of America, an area that produces roughly half the world’s cereals. And ash, it is worth remembering, is not like a big snowfall that will melt in the spring. If you wanted to grow crops again, you would have to find some place to put all the ash. It took thousands of workers eight months to clear 1.8 billion tons of debris from the sixteen acres of the World Trade Center site in New York. Imagine what it would take to clear Kansas. And that’s not even to consider the climatic consequences. The last supervolcano eruption on Earth was at Toba, in northern Sumatra, seventy-four thousand years ago. No one knows quite how big it was other than that it was a whopper. Greenland ice cores show that the Toba blast was followed by at least six years of “volcanic winter” and goodness knows how many poor growing seasons after that. The event, it is thought, may have carried humans right to the brink of extinction, reducing the global population to no more than a few thousand individuals. That means that all modern humans arose from a very small population base, which would explain our lack of genetic diversity. At all events, there is some evidence to suggest that for the next twenty thousand years the total number of people on Earth was never more than a few thousand at any time. That is, needless to say, a long time to recover from a single volcanic blast. All this was hypothetically interesting until 1973, when an odd occurrence made it suddenly momentous: water in Yellowstone Lake, in the heart of the park, began to run over the banks at the lake’s southern end, flooding a meadow, while at the opposite end of the lake the water mysteriously flowed away. Geologists did a hasty survey and discovered that a large area of the park had developed an ominous bulge. This was lifting up one end of the lake and causing the water to run out at the other, as would happen if you lifted one side of a child’s wading pool. By 1984, the whole central region of the park—several dozen square miles— was more than three feet higher than it had been in 1924, when the park was last formally surveyed. Then in 1985, the whole of the central part of the park subsided by eight inches. It now seems to be swelling again. The geologists realized that only one thing could cause this—a restless magma chamber. Yellowstone wasn’t the site of an ancient supervolcano; it was the site of an active one. It was also at about this time that they were able to work out that the cycle of Yellowstone’s eruptions averaged one massive blow every 600,000 years. The last one, interestingly enough, was 630,000 years ago. Yellowstone, it appears, is due. “It may not feel like it, but you’re standing on the largest active volcano in the world,” Paul Doss, Yellowstone National Park geologist, told me soon after climbing off an enormous Harley-Davidson motorcycle and shaking hands when we met at the park headquarters at Mammoth Hot Springs early on a lovely morning in June. A native of Indiana, Doss is an amiable, soft-spoken, extremely thoughtful man who looks nothing like a National Park Service employee. He has a graying beard and hair tied back in a long ponytail. A small sapphire stud graces one ear. A slight paunch strains against his crisp Park Service uniform. He looks more like a blues musician than a government employee. In fact, he is a blues musician (harmonica). But he sure knows and loves geology. “And I’ve got the best place in the world to do it,” he says as we set off in a bouncy, battered four-wheel-drive vehicle in the general direction of Old Faithful. He has agreed to let me accompany him for a day as he goes about doing whatever it is a park geologist does. The first assignment today is to give an introductory talk to a new crop of tour guides. Yellowstone, I hardly need point out, is sensationally beautiful, with plump, stately mountains, bison-specked meadows, tumbling streams, a sky-blue lake, wildlife beyond counting. “It really doesn’t get any better than this if you’re a geologist,” Doss says. “You’ve got rocks up at Beartooth Gap that are nearly three billion years old—three-quarters of the way back to Earth’s beginning—and then you’ve got mineral springs here”—he points at the sulfurous hot springs from which Mammoth takes its title—“where you can see rocks as they are being born. And in between there’s everything you could possibly imagine. I’ve never been any place where geology is more evident—or prettier.” “So you like it?” I say. “Oh, no, I love it,” he answers with profound sincerity. “I mean I really love it here. The winters are tough and the pay’s not too hot, but when it’s good, it’s just—” He interrupted himself to point out a distant gap in a range of mountains to the west, which had just come into view over a rise. The mountains, he told me, were known as the Gallatins. “That gap is sixty or maybe seventy miles across. For a long time nobody could understand why that gap was there, and then Bob Christiansen realized that it had to be because the mountains were just blown away. When you’ve got sixty miles of mountains just obliterated, you know you’re dealing with something pretty potent. It took Christiansen six years to figure it all out.” I asked him what caused Yellowstone to blow when it did. “Don’t know. Nobody knows. Volcanoes are strange things. We really don’t understand them at all. Vesuvius, in Italy, was active for three hundred years until an eruption in 1944 and then it just stopped. It’s been silent ever since. Some volcanologists think that it is recharging in a big way, which is a little worrying because two million people live on or around it. But nobody knows.” “And how much warning would you get if Yellowstone was going to go?” He shrugged. “Nobody was around the last time it blew, so nobody knows what the warning signs are. Probably you would have swarms of earthquakes and some surface uplift and possibly some changes in the patterns of behavior of the geysers and steam vents, but nobody really knows.” “So it could just blow without warning?” He nodded thoughtfully. The trouble, he explained, is that nearly all the things that would constitute warning signs already exist in some measure at Yellowstone. “Earthquakes are generally a precursor of volcanic eruptions, but the park already has lots of earthquakes— 1,260 of them last year. Most of them are too small to be felt, but they are earthquakes nonetheless.” A change in the pattern of geyser eruptions might also be taken as a clue, he said, but these too vary unpredictably. Once the most famous geyser in the park was Excelsior Geyser. It used to erupt regularly and spectacularly to heights of three hundred feet, but in 1888 it just stopped. Then in 1985 it erupted again, though only to a height of eighty feet. Steamboat Geyser is the biggest geyser in the world when it blows, shooting water four hundred feet into the air, but the intervals between its eruptions have ranged from as little as four days to almost fifty years. “If it blew today and again next week, that wouldn’t tell us anything at all about what it might do the following week or the week after or twenty years from now,” Doss says. “The whole park is so volatile that it’s essentially impossible to draw conclusions from almost anything that happens.” Evacuating Yellowstone would never be easy. The park gets some three million visitors a year, mostly in the three peak months of summer. The park’s roads are comparatively few and they are kept intentionally narrow, partly to slow traffic, partly to preserve an air of picturesqueness, and partly because of topographical constraints. At the height of summer, it can easily take half a day to cross the park and hours to get anywhere within it. “Whenever people see animals, they just stop, wherever they are,” Doss says. “We get bear jams. We get bison jams. We get wolf jams.” In the autumn of 2000, representatives from the U.S. Geological Survey and National Park Service, along with some academics, met and formed something called the Yellowstone Volcanic Observatory. Four such bodies were in existence already—in Hawaii, California, Alaska, and Washington—but oddly none in the largest volcanic zone in the world. The YVO is not actually a thing, but more an idea—an agreement to coordinate efforts at studying and analyzing the park’s diverse geology. One of their first tasks, Doss told me, was to draw up an “earthquake and volcano hazards plan”—a plan of action in the event of a crisis. “There isn’t one already?” I said. “No. Afraid not. But there will be soon.” “Isn’t that just a little tardy?” He smiled. “Well, let’s just say that it’s not any too soon.” Once it is in place, the idea is that three people—Christiansen in Menlo Park, California, Professor Robert B. Smith at the University of Utah, and Doss in the park—would assess the degree of danger of any potential cataclysm and advise the park superintendent. The superintendent would take the decision whether to evacuate the park. As for surrounding areas, there are no plans. If Yellowstone were going to blow in a really big way, you would be on your own once you left the park gates. Of course it may be tens of thousands of years before that day comes. Doss thinks such a day may not come at all. “Just because there was a pattern in the past doesn’t mean that it still holds true,” he says. “There is some evidence to suggest that the pattern may be a series of catastrophic explosions, then a long period of quiet. We may be in that now. The evidence now is that most of the magma chamber is cooling and crystallizing. It is releasing its volatiles; you need to trap volatiles for an explosive eruption.” In the meantime there are plenty of other dangers in and around Yellowstone, as was made devastatingly evident on the night of August 17, 1959, at a place called Hebgen Lake just outside the park. At twenty minutes to midnight on that date, Hebgen Lake suffered a catastrophic quake. It was magnitude 7.5, not vast as earthquakes go, but so abrupt and wrenching that it collapsed an entire mountainside. It was the height of the summer season, though fortunately not so many people went to Yellowstone in those days as now. Eighty million tons of rock, moving at more than one hundred miles an hour, just fell off the mountain, traveling with such force and momentum that the leading edge of the landslide ran four hundred feet up a mountain on the other side of the valley. Along its path lay part of the Rock Creek Campground. Twenty-eight campers were killed, nineteen of them buried too deep ever to be found again. The devastation was swift but heartbreakingly fickle. Three brothers, sleeping in one tent, were spared. Their parents, sleeping in another tent beside them, were swept away and never seen again. “A big earthquake—and I mean big—will happen sometime,” Doss told me. “You can count on that. This is a big fault zone for earthquakes.” Despite the Hebgen Lake quake and the other known risks, Yellowstone didn’t get permanent seismometers until the 1970s. If you needed a way to appreciate the grandeur and inexorable nature of geologic processes, you could do worse than to consider the Tetons, the sumptuously jagged range that stands just to the south of Yellowstone National Park. Nine million years ago, the Tetons didn’t exist. The land around Jackson Hole was just a high grassy plain. But then a forty-mile-long fault opened within the Earth, and since then, about once every nine hundred years, the Tetons experience a really big earthquake, enough to jerk them another six feet higher. It is these repeated jerks over eons that have raised them to their present majestic heights of seven thousand feet. That nine hundred years is an average—and a somewhat misleading one. According to Robert B. Smith and Lee J. Siegel in Windows into the Earth , a geological history of the region, the last major Teton quake was somewhere between about five and seven thousand years ago. The Tetons, in short, are about the most overdue earthquake zone on the planet. Hydrothermal explosions are also a significant risk. They can happen anytime, pretty much anywhere, and without any predictability. “You know, by design we funnel visitors into thermal basins,