by Bill Bryson
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. Whereas 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 as big as that of Mount St. Helens; the one before that was 280 times as big, and the one before that was so big nobody knows exactly how big it was. It was at least 2,500 times as big as St. Helens, but perhaps 8,000 times as 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 that from the biggest of the Yellowstone blasts would be the size of a sphere you could just about hide behind. On this scale, the Mount St. Helens eruption would be no more than a pea.
Indonesia’s Krakatau vents smoke and ash shortly before blowing apart spectacularly in August 1883. The explosion was the biggest in modern times, but would seem a passing tremor compared with a supervolcano blast. (credit 15.3)
The Yellowstone eruption of two million years ago put out enough ash to bury New York State to a depth of 20 metres or California to a depth of 6 metres. This was the ash that made Mike Voorhies’ fossil beds in eastern Nebraska. That blast occurred in what is now Idaho, but over millions of years, at a rate of about 2.5 centimetres a year, the Earth’s crust has travelled 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 tonnes of debris from the 6.5 hectares 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, 74,000 years ago. No-one knows quite how big it was, but 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 would mean 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 spend recovering 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 paddling pool. By 1984, the whole central region of the park—over 100 square kilometres—was more than a metre higher than it had been in 1924, when the park was last formally surveyed. Then, in 1985, the central part of the park subsided by 20 centimetres (about 8 inches). It now seems to be swelling again.
The picturesque oddities of the Yellowstone region have attracted sightseers since the opening of the American west. This photograph at Mammoth Hot Springs was taken in the 1870s at just the time Yellowstone became the world’s first national park. (credit 15.4)
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 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 greying 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.
Gorgeously tranquil, Yellowstone Lake at dawn gives no indication of the dangerous turmoil that lurks beneath it. A gigantic magma chamber could erupt at any time with a violence thousands of times greater than any witnessed by modern humans. (credit 15.5)
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 the Earth’s beginning—and then you’ve got mineral springs here”—he points at the sulphurous 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 m
ountains, 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 behaviour of the geysers and steam vents, but nobody really knows.”
“So it could just blow without warning?”
One of Yellowstone’s ten thousand geysers and steamy springs, a visible reminder of the landscape’s extreme instability. (credit 15.6)
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—twelve hundred and sixty of them last year. Most of them are too small to be felt, but they are earthquakes nonetheless.”
With its rich beauty and stunning wildlife, Yellowstone attracts three million tourists a year. But the narrow roads that preserve its timeless charm mean that evacuation in a crisis will always be a nightmare. (credit 15.7)
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 100 metres, but in 1888 it just stopped. Then in 1985 it erupted again, though only to a height of 25 metres. Steamboat Geyser is the biggest geyser in the world when it blows, shooting water 120 metres 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 US Geological Survey and National Park Service, along with some academics, met and formed something called the Yellowstone Volcanic Observatory. Four of these bodies were in existence already—in Hawaii, California, Alaska and Washington—but, oddly, there was none in the largest volcanic zone in the world. The YVO is not actually a thing so much as an idea—an agreement to co-ordinate efforts at studying and analysing the park’s diverse geology. One of its 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 make the decision whether to evacuate the park. As for surrounding areas, there are no plans. You would be on your own once you left the park gates—not much help if Yellowstone were going to blow in a really big way.
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 17 August 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 tonnes of rock, moving at more than 160 kilometres an hour, just fell off the mountain, travelling with such force and momentum that the leading edge of the landslide ran 120 metres 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 tattered lakeside highway testifies to the startling violence of an earthquake that ripped through a valley at Hebgen Lake, near Yellowstone, in 1959. Twenty-eight people were killed at one campsite. (credit 15.8)
“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 inexorability of geological 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 64-kilometre-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 2 metres higher. It is these repeated jerks over aeons that have raised them to their present majestic heights of 2,000 metres.
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 thousand 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 any time, pretty much anywhere and without any predictability. “You know, by design we funnel visitors into thermal basins,” Doss told me after we had watched Old Faithful blow. “It’s what they come to see. Did you know there are more geysers and hot springs at Yellowstone than in all the rest of the world combined?”
“I didn’t know that.”
He nodded. “Ten thousand of them, and nobody knows when a new vent m
ight open.”
We drove to a place called Duck Lake, a body of water a couple of hundred metres across. “It looks completely innocuous,” he said. “It’s just a big pond. But this big hole didn’t used to be here. At some time in the last fifteen thousand years this blew in a really big way. You’d have had several tens of millions of tons of earth and rock and superheated water blowing out at hypersonic speeds. You can imagine what it would be like if this happened under, say, the parking lot at Old Faithful or one of the visitor centres.” He made an unhappy face.
“Would there be any warning?”
“Probably not. The last significant explosion in the park was at a place called Pork Chop Geyser in 1989. That left a crater about five metres across—not huge by any means, but big enough if you happened to be standing there at the time. Fortunately, nobody was around so nobody was hurt, but that happened without warning. In the very ancient past there have been explosions that have made holes a mile across. And nobody can tell you where or when that might happen again. You just have to hope that you’re not standing there when it does.”
Big rockfalls are also a danger. There was a big one at Gardiner Canyon in 1999, but again fortunately no-one was hurt. Late in the afternoon, Doss and I stopped at a place where there was a rock overhang poised above a busy park road. Cracks were clearly visible. “It could go at any time,” Doss said thoughtfully.