by Bill Bryson
Tokyo has already suffered one of the most devastating earthquakes in modern times. On September 1, 1923, just before noon, the city was hit by what is known as the Great Kanto quake--an event more than ten times more powerful than Kobe's earthquake. Two hundred thousand people were killed. Since that time, Tokyo has been eerily quiet, so the strain beneath the surface has been building for eighty years. Eventually it is bound to snap. In 1923, Tokyo had a population of about three million. Today it is approaching thirty million. Nobody cares to guess how many people might die, but the potential economic cost has been put as high as $7 trillion.
Even more unnerving, because they are less well understood and capable of occurring anywhere at any time, are the rarer type of shakings known as intraplate quakes. These happen away from plate boundaries, which makes them wholly unpredictable. And because they come from a much greater depth, they tend to propagate over much wider areas. The most notorious such quakes ever to hit the United States were a series of three in New Madrid, Missouri, in the winter of 1811-12. The adventure started just after midnight on December 16 when people were awakened first by the noise of panicking farm animals (the restiveness of animals before quakes is not an old wives' tale, but is in fact well established, though not at all understood) and then by an almighty rupturing noise from deep within the Earth. Emerging from their houses, locals found the land rolling in waves up to three feet high and opening up in fissures several feet deep. A strong smell of sulfur filled the air. The shaking lasted for four minutes with the usual devastating effects to property. Among the witnesses was the artist John James Audubon, who happened to be in the area. The quake radiated outward with such force that it knocked down chimneys in Cincinnati four hundred miles away and, according to at least one account, "wrecked boats in East Coast harbors and . . . even collapsed scaffolding erected around the Capitol Building in Washington, D.C." On January 23 and February 4 further quakes of similar magnitude followed. New Madrid has been silent ever since--but not surprisingly, since such episodes have never been known to happen in the same place twice. As far as we know, they are as random as lightning. The next one could be under Chicago or Paris or Kinshasa. No one can even begin to guess. And what causes these massive intraplate rupturings? Something deep within the Earth. More than that we don't know.
By the 1960s scientists had grown sufficiently frustrated by how little they understood of the Earth's interior that they decided to try to do something about it. Specifically, they got the idea to drill through the ocean floor (the continental crust was too thick) to the Moho discontinuity and to extract a piece of the Earth's mantle for examination at leisure. The thinking was that if they could understand the nature of the rocks inside the Earth, they might begin to understand how they interacted, and thus possibly be able to predict earthquakes and other unwelcome events.
The project became known, all but inevitably, as the Mohole and it was pretty well disastrous. The hope was to lower a drill through 14,000 feet of Pacific Ocean water off the coast of Mexico and drill some 17,000 feet through relatively thin crustal rock. Drilling from a ship in open waters is, in the words of one oceanographer, "like trying to drill a hole in the sidewalks of New York from atop the Empire State Building using a strand of spaghetti." Every attempt ended in failure. The deepest they penetrated was only about 600 feet. The Mohole became known as the No Hole. In 1966, exasperated with ever-rising costs and no results, Congress killed the project.
Four years later, Soviet scientists decided to try their luck on dry land. They chose a spot on Russia's Kola Peninsula, near the Finnish border, and set to work with the hope of drilling to a depth of fifteen kilometers. The work proved harder than expected, but the Soviets were commendably persistent. When at last they gave up, nineteen years later, they had drilled to a depth of 12,262 meters, or about 7.6 miles. Bearing in mind that the crust of the Earth represents only about 0.3 percent of the planet's volume and that the Kola hole had not cut even one-third of the way through the crust, we can hardly claim to have conquered the interior.
Interestingly, even though the hole was modest, nearly everything about it was surprising. Seismic wave studies had led the scientists to predict, and pretty confidently, that they would encounter sedimentary rock to a depth of 4,700 meters, followed by granite for the next 2,300 meters and basalt from there on down. In the event, the sedimentary layer was 50 percent deeper than expected and the basaltic layer was never found at all. Moreover, the world down there was far warmer than anyone had expected, with a temperature at 10,000 meters of 180 degrees centigrade, nearly twice the forecasted level. Most surprising of all was that the rock at that depth was saturated with water--something that had not been thought possible.
Because we can't see into the Earth, we have to use other techniques, which mostly involve reading waves as they travel through the interior. We also know a little bit about the mantle from what are known as kimberlite pipes, where diamonds are formed. What happens is that deep in the Earth there is an explosion that fires, in effect, a cannonball of magma to the surface at supersonic speeds. It is a totally random event. A kimberlite pipe could explode in your backyard as you read this. Because they come up from such depths--up to 120 miles down--kimberlite pipes bring up all kinds of things not normally found on or near the surface: a rock called peridotite, crystals of olivine, and--just occasionally, in about one pipe in a hundred--diamonds. Lots of carbon comes up with kimberlite ejecta, but most is vaporized or turns to graphite. Only occasionally does a hunk of it shoot up at just the right speed and cool down with the necessary swiftness to become a diamond. It was such a pipe that made Johannesburg the most productive diamond mining city in the world, but there may be others even bigger that we don't know about. Geologists know that somewhere in the vicinity of northeastern Indiana there is evidence of a pipe or group of pipes that may be truly colossal. Diamonds up to twenty carats or more have been found at scattered sites throughout the region. But no one has ever found the source. As John McPhee notes, it may be buried under glacially deposited soil, like the Manson crater in Iowa, or under the Great Lakes.
So how much do we know about what's inside the Earth? Very little. Scientists are generally agreed that the world beneath us is composed of four layers--rocky outer crust, a mantle of hot, viscous rock, a liquid outer core, and a solid inner core. * 28 We know that the surface is dominated by silicates, which are relatively light and not heavy enough to account for the planet's overall density. Therefore there must be heavier stuff inside. We know that to generate our magnetic field somewhere in the interior there must be a concentrated belt of metallic elements in a liquid state. That much is universally agreed upon. Almost everything beyond that--how the layers interact, what causes them to behave in the way they do, what they will do at any time in the future--is a matter of at least some uncertainty, and generally quite a lot of uncertainty.
Even the one part of it we can see, the crust, is a matter of some fairly strident debate. Nearly all geology texts tell you that continental crust is three to six miles thick under the oceans, about twenty-five miles thick under the continents, and forty to sixty miles thick under big mountain chains, but there are many puzzling variabilities within these generalizations. The crust beneath the Sierra Nevada Mountains, for instance, is only about nineteen to twenty-five miles thick, and no one knows why. By all the laws of geophysics the Sierra Nevadas should be sinking, as if into quicksand. (Some people think they may be.)
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 w
ith 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 Greek words 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 rocks are viscous, but only in the same way that glass is. It may not look it, but all the glass on Earth is flowing downward under the relentless drag of gravity. Remove a pane of really old glass from the window of a European cathedral and it will be noticeably thicker at the bottom than at the top. That is the sort of "flow" we are talking about. 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 as well, 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 four hundred miles down, others two thousand miles below us. The problem, as Donald 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 Earth'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.
So all that can be said is that at some slightly indeterminate point as we head toward the center of Earth we leave the asthenosphere and plunge into pure mantle. Considering that it accounts for 82 percent of the Earth's volume and 65 percent 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 a hundred miles the mantle consists predominantly of a type of rock known as peridotite, but what fills the space beyond 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 center 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 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 200°F. No one knows exactly how hot the Earth's core is, but estimates range from something over 7,000°F to 13,000°F--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 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 now. We also know that it reverses itself every 500,000 years or so on average, though that average hides a huge degree of unpredictability. The last reversal was about 750,000 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 20,000 years. Altogether in the last 100 million years it has reversed itself about two hundred times, and we don't have any real idea why. It 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 percent 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 that in the absence of magnetic protection would tear through our bodies, leaving much of our DNA in useless tatters. 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, interestingly enough, is that traditionally there has been little effort to coordinate what's happening on top of the Earth with what's going on inside. 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 acts 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 in 1980.
At that time, the lower forty-eight United States had not seen a volcanic eruption for over sixty-five years. Therefore the government volcanologists called in to monitor and forecast St. Helens's behavior primarily had seen only Hawaiian volcanoes in action, and they, it turned out, were not the same thing at all.
St. Helens started its ominous rumblings on March 20. 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 eight miles. 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 seen climbing over the mountain. On one day, more than seventy copters and light aircraft circled the summit. 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 April 19 the northern flank of the mountain began to bulge conspicuously. Remarkably, no one in a position of responsibility saw that this strongly signaled a lateral blast. The seismologists resolutely based their conclusions on the behavior 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, May 18, the north side of the volcano collapsed, sending an enormous avalanche of dirt and rock rushing down the mountain slope at 150 miles an hour. It was the biggest landslide in human history and carried enough material to bury the whole of Manhattan to a depth of four hundred feet. 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 650 miles an hour--much too fast, clearly, for anyone nearby to outrace. 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 except that it was a Sunday. Had it been a weekday many lumber workers would have been working within the death zone. As it was, people were killed eighteen miles away.
The luckiest person on that day was a graduate student named Harry Glicken. He had been manning an observation post 5.7 miles from the mountain, but he had a college placement interview on May 18 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.