by Bill Bryson
How and when the Earth got its crust are questions that divide geologists into two broad camps—those who think it happened abruptly, early in the Earth’s history, and those who think it happened gradually and rather later. Strength of feeling runs deep on such matters. Richard Armstrong of Yale proposed an early-burst theory in the 1960s, then spent the rest of his career fighting those who did not agree with him. He died of cancer in 1991, but shortly before his death he “lashed out at his critics in a polemic in an Australian earth science journal that charged them with perpetuating myths,” according to a report in Earth magazine in 1998. “He died a bitter man,” reported a colleague.
The crust and part of the outer mantle together are called the lithosphere (from the Greek lithos, meaning stone), which in turn floats on top of a layer of softer rock called the asthenosphere (from a Greek word meaning “without strength,”) but such terms are never entirely satisfactory. To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy buoyancy that isn’t quite right. Similarly, it is misleading to think of the rocks as flowing in anything like the way we think of materials flowing on the surface. The hour hand on a clock moves about ten thousand times faster than the “flowing” rocks of the mantle.
The movements occur not just laterally, as the Earth’s plates move across the surface, but up and down too, as rocks rise and fall under the churning process known as convection. Convection as a process was first deduced by the eccentric Count von Rumford at the end of the eighteenth century. Sixty years later an English vicar named Osmond Fisher presciently suggested that the Earth’s interior might well be fluid enough for the contents to move about, but that idea took a very long time to gain support.
In about 1970, when geophysicists realized just how much turmoil was going on down there, it came as a considerable shock. As Shawna Vogel put it in the book Naked Earth: The New Geophysics: “It was as if scientists had spent decades figuring out the layers of the Earth’s atmosphere—troposphere, stratosphere and so forth—and then had suddenly found out about wind.”
How deep the convection process goes has been a matter of controversy ever since. Some say it begins 650 kilometres down, others more than 3,000 kilometres below us. The problem, as James Trefil has observed, is that “there are two sets of data, from two different disciplines, that cannot be reconciled.” Geochemists say that certain elements on the planet’s surface cannot have come from the upper mantle, but must have come from deeper within the Earth. Therefore, the materials in the upper and lower mantle must at least occasionally mix. Seismologists insist that there is no evidence to support such a thesis.
Computer model showing the direction and speed of convection currents within and above the Earth’s mantle. Scientists are still not agreed on how deep the convection process goes. (credit 14.7)
So all that can be said is that at some slightly indeterminate point as we head towards the centre of the Earth we leave the asthenosphere and plunge into pure mantle. Considering that it accounts for 82 per cent of the Earth’s volume and 65 per cent of its mass, the mantle doesn’t attract a great deal of attention, largely because the things that interest earth scientists and general readers alike happen either deeper down (as with magnetism) or nearer the surface (as with earthquakes). We know that to a depth of about 150 kilometres the mantle consists predominantly of a type of rock known as peridotite, but what fills the next 2,650 kilometres is uncertain. According to a Nature report, it seems not to be peridotite. More than this we do not know.
Beneath the mantle are the two cores, a solid inner core and a liquid outer one. Needless to say, our understanding of the nature of these cores is indirect, but scientists can make some reasonable assumptions. They know that the pressures at the centre of the Earth are sufficiently high—something over three million times those found at the surface—to turn any rock there solid. They also know from the Earth’s history (among other clues) that the inner core is very good at retaining its heat. Although it is little more than a guess, it is thought that in over four billion years the temperature at the core has fallen by no more than 110 degrees Celsius. No one knows exactly how hot the Earth’s core is, but estimates range from something over 4,000 degrees to over 7,000 degrees Celsius—about as hot as the surface of the Sun.
The outer core is in many ways even less well understood, though everyone is in agreement that it is fluid and that it is the seat of magnetism. The theory was put forward by E. C. Bullard of Cambridge University in 1949 that this fluid part of the Earth’s core revolves in a way that makes it, in effect, an electrical motor, creating the Earth’s magnetic field. The assumption is that the convecting fluids in the Earth act somehow like the currents in wires. Exactly what happens isn’t known, but it is felt pretty certain that it is connected with the core spinning and with its being liquid. Bodies that don’t have a liquid core—the Moon and Mars, for instance—don’t have magnetism.
We know that the Earth’s magnetic field changes in power from time to time: during the age of the dinosaurs, it was up to three times as strong as it is now. We also know that it reverses itself every five hundred thousand years or so on average, though that average hides a huge degree of unpredictability. The last reversal was about seven hundred and fifty thousand years ago. Sometimes it stays put for millions of years—37 million years appears to be the longest stretch—and at other times it has reversed after as little as twenty thousand years. Altogether in the last hundred million years it has reversed itself about two hundred times, and we don’t have any real idea why. This has been called “the greatest unanswered question in the geological sciences.”
We may be going through a reversal now. The Earth’s magnetic field has diminished by perhaps as much as 6 per cent in the last century alone. Any diminution in magnetism is likely to be bad news, because magnetism, apart from holding notes to refrigerators and keeping our compasses pointing the right way, plays a vital role in keeping us alive. Space is full of dangerous cosmic rays which, in the absence of magnetic protection, would tear through our bodies, leaving much of our DNA in useless shreds. When the magnetic field is working, these rays are safely herded away from the Earth’s surface and into two zones in near space called the Van Allen belts. They also interact with particles in the upper atmosphere to create the bewitching veils of light known as the auroras.
A big part of the reason for our ignorance is that traditionally there has been little effort to co-ordinate what’s happening on top of the Earth with what’s going on inside it. According to Shawna Vogel: “Geologists and geophysicists rarely go to the same meetings or collaborate on the same problems.”
Perhaps nothing better demonstrates our inadequate grasp of the dynamics of the Earth’s interior than how badly we are caught out when it plays up, and it would be hard to come up with a more salutary reminder of the limitations of our understanding than the eruption of Mount St. Helens in Washington state in 1980.
At that time, the lower forty-eight states of the Union had not seen a volcanic eruption for over sixty-five years. Therefore, most of the government volcanologists called in to monitor and forecast St. Helens’ behaviour had seen only Hawaiian volcanoes in action, and they, it turned out, were not the same thing at all.
Mount St. Helens, in Washington state, disgorges ash and smoke in the days before its spectacular eruption on 18 May 1980. Scientists tragically misread the volcano’s warning signs, allowing some witnesses to get too close. The blast killed fifty-seven of them. (credit 14.8)
St. Helens started its ominous rumblings on 20 March. Within a week it was erupting magma, albeit in modest amounts, up to a hundred times a day, and being constantly shaken with earthquakes. People were evacuated to what was assumed to be a safe distance of 13 kilometres. As the mountain’s rumblings grew, St. Helens became a tourist attraction for the world. Newspapers gave daily reports on the best places to get a view. Television crews repeatedly flew in helicopters to the summit and people were even s
een climbing over the mountain. On one day, more than seventy copters and light aircraft circled the peak. But as the days passed and the rumblings failed to develop into anything dramatic, people grew restless and the view became general that the volcano wasn’t going to blow after all.
On 19 April the northern flank of the mountain began to bulge conspicuously. Remarkably, no-one in a position of responsibility saw that this strongly signalled a lateral blast. The seismologists resolutely based their conclusions on the behaviour of Hawaiian volcanoes, which don’t blow out sideways. Almost the only person who believed that something really bad might happen was Jack Hyde, a geology professor at a community college in Tacoma. He pointed out that St. Helens didn’t have an open vent, as Hawaiian volcanoes have, so any pressure building up inside was bound to be released dramatically and probably catastrophically. However, Hyde was not part of the official team and his observations attracted little notice.
We all know what happened next. At 8.32 a.m. on a Sunday morning, 18 May, the north side of the volcano collapsed, sending an enormous avalanche of dirt and rock rushing down the mountain slope at nearly 250 kilometres an hour. It was the biggest landslide in human history and carried enough material to bury the whole of Manhattan to a depth of 120 metres. A minute later, its flank severely weakened, St. Helens exploded with the force of five hundred Hiroshima-sized atomic bombs, shooting out a murderous hot cloud at up to 1,050 kilometres an hour—much too fast, clearly, for anyone nearby to outrace it. Many people who were thought to be in safe areas, often far out of sight of the volcano, were overtaken. Fifty-seven people were killed. Twenty-three of the bodies were never found. The toll would have been much higher had it not been a Sunday. On any weekday, many lumber workers would have been working within the death zone. As it was, people were killed 30 kilometres away.
The luckiest person on that day was a graduate student named Harry Glicken. He had been manning an observation post 9 kilometres from the mountain, but he had a college placement interview on 18 May in California, and so had left the site the day before the eruption. His place was taken by David Johnston. Johnston was the first to report the volcano exploding; moments later he was dead. His body was never found. Glicken’s luck, alas, was temporary. Eleven years later he was one of forty-three scientists and journalists fatally caught up in a lethal outpouring of superheated ash, gases and molten rock—what is known as a pyroclastic flow—at Mount Unzen in Japan when yet another volcano was catastrophically misread.
Volcanologists may or may not be the worst scientists in the world at making predictions, but they are without question the worst in the world at realizing how bad their predictions are. Less than two years after the Unzen catastrophe another group of volcano-watchers, led by Stanley Williams of the University of Arizona, descended into the rim of an active volcano called Galeras in Colombia. Despite the deaths of recent years, only two of the sixteen members of Williams’s party wore safety helmets or other protective gear. The volcano erupted, killing six of the scientists, along with three tourists who had followed them, and seriously injuring several others, including Williams himself.
In an extraordinarily unselfcritical book called Surviving Galeras, Williams said he could “only shake my head in wonder” when he learned afterwards that his colleagues in the world of volcanology had suggested that he had overlooked or disregarded important seismic signals and behaved recklessly. “How easy it is to snipe after the fact, to apply the knowledge we have now to the events of 1993,” he wrote. He was guilty of nothing worse, he believed, than unlucky timing when Galeras “behaved capriciously, as natural forces are wont to do. I was fooled, and for that I will take responsibility. But I do not feel guilty about the deaths of my colleagues. There is no guilt. There was only an eruption.”
But to return to Washington. Mount St. Helens lost 400 metres of peak, and 600 square kilometres of forest were devastated. Enough trees to build 150,000 homes (or 300,000 according to some reports) were blown away. The damage was placed at $2.7 billion. A giant column of smoke and ash rose to a height of 18,000 metres in less than ten minutes. An airliner some 48 kilometres away reported being pelted with rocks.
Ninety minutes after the blast, ash began to rain down on Yakima, Washington, a community of fifty thousand people about 130 kilometres away. As you would expect, the ash turned day to night and got into everything, clogging motors, generators and electrical switching equipment, choking pedestrians, blocking filtration systems and generally bringing things to a halt. The airport shut down and highways in and out of the city were closed.
All this was happening, you will note, just downwind of a volcano that had been rumbling menacingly for two months. Yet Yakima had no volcano emergency procedures. The city’s emergency broadcast system, which was supposed to swing into action during a crisis, did not go on the air because “the Sunday-morning staff did not know how to operate the equipment.” For three days, Yakima was paralysed and cut off from the world, its airport closed, its approach roads impassable. Altogether the city received just over 1.5 centimetres of ash after the eruption of Mount St. Helens. Now bear that in mind, please, as we consider what a Yellowstone blast would do.
David Johnston, a USGS geologist, photographed at an observation post 9 kilometres from Mount St. Helens on the last afternoon of his life. The following morning Johnston became the first person to report the volcano exploding—and the first to be killed by it. (credit 14.9)
1 For those who crave a more detailed picture of the Earth’s interior, here are the dimensions of the various layers, using average figures: From 0 to 40 kilometres is the crust. From 40 to 400 kilometres is the upper mantle. From 400 to 650 kilometres is a transition zone between the upper and lower mantle. From 650 to 2,700 kilometres is the lower mantle. From 2,700 to 2,890 kilometres is the “D” layer. From 2,890 to 5,150 kilometres is the outer core, and from 5,150 to 6,370 kilometres is the inner core.
Japan’s Mount Fuji, shown here in a nineteenth-century woodblock print, is the classic cone-shaped mound that people call to mind when imagining a volcano, but in fact the most colossal volcanoes are often hidden from view. (credit 15.1)
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 shape of a Fuji or a Kilimanjaro, which is created when erupting magma accumulates in a symmetrical mound. These can form remarkably quickly. In 1943 at Paricutín 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 152 metres high. Within two years it had topped out at almost 430 metres and was more than 800 metres 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 a thoughtful official passed on to the park authorities on the assumption that they might make a nice display for one of the visitor centres. As soon as Christiansen saw the photos he realized why he had failed to spot the caldera: virtually the whole park�
�9,000 square kilometres—was caldera. The explosion had left a crater nearly 65 kilometres 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 begins at least 200 kilometres down in the Earth and rises to near the surface, forming what is known as a superplume. 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 72 kilometres across—roughly the same dimensions as the park—and about 13 kilometres thick at its thickest point. Imagine a pile of TNT about the size of an English county and reaching 13 kilometres 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 its surrounding territory about half a kilometre 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 kilometres of it” while it was erupting. The consequences that followed would be even worse.
Lava flowing into the sea, Hawaii Volcanoes National Park, June 2001. (credit 15.2)
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 1,900 kilometres across. According to current theories, they don’t always erupt explosively, but sometimes burst forth in a vast, continuous outpouring—a flood—of molten rock, as happened with the Deccan Traps in India 65 million years ago. These covered an area of over 500,000 square kilometres and probably contributed to the demise of the dinosaurs—they certainly didn’t help—with their noxious outpourings of gases. Superplumes may also be responsible for the rifts that cause continents to break up.
