In about 1970, when geophysicists realized just how much turmoil was going on downthere, it came as a considerable shock. As Shawna Vogel put it in the book Naked Earth: TheNew Geophysics: “It was as if scientists had spent decades figuring out the layers of theEarth’s atmosphere—troposphere, stratosphere, and so forth—and then had suddenly foundout about wind.”
It’s not that there are more bones there. It’s just that you have some chance of spotting them.
At that time, the lower forty-eight United States had not seen a volcanic eruption for oversixty-five years. Therefore the government volcanologists called in to monitor and forecast St.
So how much do we know about what’s inside the Earth? Very little. Scientists aregenerally agreed that the world beneath us is composed of four layers—rocky outer crust, amantle of hot, viscous rock, a liquid outer core, and a solid inner core.
A few yards beyond, it turned out, was one of the most extraordinary fossil beds everdiscovered in North America, a dried-up water hole that had served as a mass grave for scoresof animals—rhinoceroses, zebra-like horses, saber-toothed deer, camels, turtles. All had diedfrom some mysterious cataclysm just under twelve million years ago in the time known togeology as the Miocene. In those days Nebraska stood on a vast, hot plain very like theSerengeti of Africa today. The animals had been found buried under volcanic ash up to tenfeet deep. The puzzle of it was that there were not, and never had been, any volcanoes inNebraska.
Earthquakes are fairly common. Every day on average somewhere in the world there aretwo of magnitude 2.0 or greater—that’s enough to give anyone nearby a pretty good jolt.
Eleven years later he was one of forty-three scientists and journalists fatally caught up in alethal outpouring of superheated ash, gases, and molten rock—what is known as a pyroclasticflow—at Mount Unzen in Japan when yet another volcano was catastrophically misread.
For three days, Yakima was paralyzed and cut off from the world, its airport closed, itsapproach roads impassable. Altogether the city received just five-eighths of an inch of ashafter the eruption of Mount St. Helens. Now bear that in mind, please, as we consider what aYellowstone blast would do.
The outer core is in many ways even less well understood, though everyone is in agreementthat it is fluid and that it is the seat of magnetism. The theory was put forward by E. C.
Seismic wave studies had led the scientists to predict, and pretty confidently, that they wouldencounter sedimentary rock to a depth of 4,700 meters, followed by granite for the next 2,300meters and basalt from there on down. In the event, the sedimentary layer was 50 percentdeeper than expected and the basaltic layer was never found at all. Moreover, the world downthere was far warmer than anyone had expected, with a temperature at 10,000 meters of 180degrees centigrade, nearly twice the forecasted level. Most surprising of all was that the rockat that depth was saturated with water—something that had not been thought possible.
The most common types of earthquakes are those where two plates meet, as in Californiaalong the San Andreas Fault. As the plates push against each other, pressures build up untilone or the other gives way. In general, the longer the interval between quakes, the greater thepent-up pressure and thus the greater the scope for a really big jolt. This is a particular worryfor Tokyo, which Bill McGuire, a hazards specialist at University College London, describesas “the city waiting to die” (not a motto you will find on many tourism leaflets). Tokyo standson the boundary of three tectonic plates in a country already well known for its seismicinstability. In 1995, as you will remember, the city of Kobe, three hundred miles to the west,was struck by a magnitude 7.2 quake, which killed 6,394 people. The damage was estimatedat $99 billion. But that was as nothing—well, as comparatively little—compared with whatmay await Tokyo.
All this was happening, you will note, just downwind of a volcano that had been rumblingmenacingly for two months. Yet Yakima had no volcano emergency procedures. The city’semergency broadcast system, which was supposed to swing into action during a crisis, did notgo on the air because “the Sunday-morning staff did not know how to operate the equipment.”
In an extraordinarily unself-critical book called Surviving Galeras, Williams said he could“only shake my head in wonder” when he learned afterward that his colleagues in the worldof volcanology had suggested that he had overlooked or disregarded important seismic signalsand behaved recklessly. “How easy it is to snipe after the fact, to apply the knowledge wehave now to the events of 1993,” he wrote. He was guilty of nothing worse, he believed, thanunlucky timing when Galeras “behaved capriciously, as natural forces are wont to do. I wasfooled, and for that I will take responsibility. But I do not feel guilty about the deaths of mycolleagues. There is no guilt. There was only an eruption.”
The crust and part of the outer mantle together are called the lithosphere (from the Greeklithos, meaning “stone”), which in turn floats on top of a layer of softer rock called theasthenosphere (from Greek words meaning “without strength”), but such terms are neverentirely satisfactory. To say that the lithosphere floats on top of the asthenosphere suggests adegree of easy buoyancy that isn’t quite right. Similarly it is misleading to think of the rocksas flowing in anything like the way we think of materials flowing on the surface. The rocksare viscous, but only in the same way that glass is. It may not look it, but all the glass on Earthis flowing downward under the relentless drag of gravity. Remove a pane of really old glassfrom the window of a European cathedral and it will be noticeably thicker at the bottom thanat the top. That is the sort of “flow” we are talking about. The hour hand on a clock movesabout ten thousand times faster than the “flowing” rocks of the mantle.
How deep the convection process goes has been a matter of controversy ever since. Somesay it begins four hundred miles down, others two thousand miles below us. The problem, asDonald 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 cannothave come from the upper mantle, but must have come from deeper within the Earth.
“Strange as it may seem,” wrote Richard Feynman, “we understand the distribution of matterin the interior of the Sun far better than we understand the interior of the Earth.”
The project became known, all but inevitably, as the Mohole and it was pretty welldisastrous. The hope was to lower a drill through 14,000 feet of Pacific Ocean water off thecoast of Mexico and drill some 17,000 feet through relatively thin crustal rock. Drilling froma ship in open waters is, in the words of one oceanographer, “like trying to drill a hole in thesidewalks of New York from atop the Empire State Building using a strand of spaghetti.”
Because we can’t see into the Earth, we have to use other techniques, which mostly involvereading waves as they travel through the interior. We also know a little bit about the mantlefrom what are known as kimberlite pipes, where diamonds are formed. What happens is thatdeep in the Earth there is an explosion that fires, in effect, a cannonball of magma to thesurface at supersonic speeds. It is a totally random event. A kimberlite pipe could explode inyour backyard as you read this. Because they come up from such depths—up to 120 milesdown—kimberlite pipes bring up all kinds of things not normally found on or near thesurface: a rock called peridotite, crystals of olivine, and—just occasionally, in about one pipein a hundred—diamonds. Lots of carbon comes up with kimberlite ejecta, but most isvaporized or turns to graphite. Only occasionally does a hunk of it shoot up at just the rightspeed and cool down with the necessary swiftness to become a diamond. It was such a pipethat made Johannesburg the most productive diamond mining city in the world, but there maybe others even bigger that we don’t know about. Geologists know that somewhere in thevicinity of northeastern Indiana there is evidence of a pipe or group of pipes that may be trulycolossal. Diamonds up to twenty carats or more have been found at scattered sites throughoutthe region. But no one has ever found the source. As John McPhee notes, it may be buriedunder glacially deposited soil, like the Manson crater in Iowa, or under the Great Lakes.
Bullard of Cambridge University in 1949 that this fluid part of the Earth’s core revolves in away that makes it, in effect, an electrical motor, creating the Earth’s magnetic field. Theassumption is that the convecting fluids in the Earth act somehow like the currents in wires.
Convection as a process was first deduced by the eccentric Count von Rumford at the end ofthe eighteenth century. Sixty years later an English vicar named Osmond Fisher prescientlysuggested 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.
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. Theseismologists resolutely based their conclusions on the behavior of Hawaiian volcanoes,which don’t blow out sideways. Almost the only person who believed that something reallybad might happen was Jack Hyde, a geology professor at a community college in Tacoma. Hepointed out that St. Helens didn’t have an open vent, as Hawaiian volcanoes have, so anypressure building up inside was bound to be released dramatically and probablycatastrophically. However, Hyde was not part of the official team and his observationsattracted little notice.
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.
So all that can be said is that at some slightly indeterminate point as we head toward thecenter of Earth we leave the asthenosphere and plunge into pure mantle. Considering that itaccounts for 82 percent of the Earth’s volume and 65 percent of its mass, the mantle doesn’tattract a great deal of attention, largely because the things that interest Earth scientists andgeneral readers alike happen either deeper down (as with magnetism) or nearer the surface (aswith earthquakes). We know that to a depth of about a hundred miles the mantle consistspredominantly of a type of rock known as peridotite, but what fills the space beyond isuncertain. According to a Nature report, it seems not to be peridotite. More than this we donot know.
We were beginning to get a vague idea of the Earth’s layered interior—though it really wasonly vague. Not until 1936 did a Danish scientist named Inge Lehmann, studyingseismographs of earthquakes in New Zealand, discover that there were two cores—an innerone that we now believe to be solid and an outer one (the one that Oldham had detected) thatis thought to be liquid and the seat of magnetism.
We know that Earth’s magnetic field changes in power from time to time: during the age ofthe dinosaurs, it was up to three times as strong as now. We also know that it reverses itselfevery 500,000 years or so on average, though that average hides a huge degree ofunpredictability. The last reversal was about 750,000 years ago. Sometimes it stays put formillions of years—37 million years appears to be the longest stretch—and at other times it hasreversed after as little as 20,000 years. Altogether in the last 100 million years it has reverseditself about two hundred times, and we don’t have any real idea why. It has been called “thegreatest unanswered question in the geological sciences.”
By the 1960s scientists had grown sufficiently frustrated by how little they understood ofthe Earth’s interior that they decided to try to do something about it. Specifically, they got theidea to drill through the ocean floor (the continental crust was too thick) to the Mohodiscontinuity and to extract a piece of the Earth’s mantle for examination at leisure. Thethinking was that if they could understand the nature of the rocks inside the Earth, they mightbegin to understand how they interacted, and thus possibly be able to predict earthquakes andother unwelcome events.
Until slightly under a century ago, what the best-informed scientific minds knew aboutEarth’s interior was not much more than what a coal miner knew—namely, that you could digdown through soil for a distance and then you’d hit rock and that was about it. Then in 1906,an Irish geologist named R. D. Oldham, while examining some seismograph readings from anearthquake in Guatemala, noticed that certain shock waves had penetrated to a point deepwithin the Earth and then bounced off at an angle, as if they had encountered some kind ofbarrier. From this he deduced that the Earth has a core. Three years later a Croatianseismologist named Andrija Mohorovi?i′c was studying graphs from an earthquake in Zagrebwhen he noticed a similar odd deflection, but at a shallower level. He had discovered theboundary between the crust and the layer immediately below, the mantle; this zone has beenknown ever since as the Mohorovi?i′c discontinuity, or Moho for short.
The BBC documentary had suggested that the existence of so much ash in Nebraska was asurprise. In fact, Nebraska’s huge ash deposits had been known about for a long time. Foralmost a century they had been mined to make household cleaning powders like Comet andAjax. But curiouslyno one had ever thought to wonder where all the ash came from.
The convulsive force was so great that the water rushed out of the city’s harbor and returnedin a wave fifty feet high, adding to the destruction. When at last the motion ceased, survivorsenjoyed just three minutes of calm before a second shock came, only slightly less severe thanthe first. A third and final shock followed two hours later. At the end of it all, sixty thousandpeople were dead and virtually every building for miles reduced to rubble. The San Franciscoearthquake of 1906, for comparison, measured an estimated 7.8 on the Richter scale andlasted less than thirty seconds.
Newspapers gave daily reports on the best places to get a view. Television crews repeatedlyflew in helicopters to the summit, and people were even seen climbing over the mountain. Onone day, more than seventy copters and light aircraft circled the summit. But as the dayspassed and the rumblings failed to develop into anything dramatic, people grew restless, andthe view became general that the volcano wasn’t going to blow after all.
1We know that thesurface is dominated by silicates, which are relatively light and not heavy enough to accountfor the planet’s overall density. Therefore there must be heavier stuff inside. We know that togenerate our magnetic field somewhere in the interior there must be a concentrated belt ofmetallic elements in a liquid state. That much is universally agreed upon. Almost everythingbeyond that—how the layers interact, what causes them to behave in the way they do, whatthey will do at any time in the future—is a matter of at least some uncertainty, and generallyquite a lot of uncertainty.
At least theoretically, there is no upper limit for an earthquake—nor, come to that, a lowerlimit. The scale is a simple measure of force, but says nothing about damage. A magnitude 7quake happening deep in the mantle—say, four hundred miles down—might cause no surfacedamage at all, while a significantly smaller one happening just four miles under the surfacecould wreak widespread devastation. Much, too, depends on the nature of the subsoil, thequake’s duration, the frequency and severity of aftershocks, and the physical setting of theaffected area. All this means that the most fearsome quakes are not necessarily the mostforceful, though force obviously counts for a lot.
Although they tend to cluster in certain places—notably around the rim of the Pacific—theycan occur almost anywhere. In the United States, only Florida, eastern Texas, and the upperMidwest seem—so far—to be almost entirely immune. New England has had two quakes ofmagnitude 6.0 or greater in the last two hundred years. In April 2002, the region experienceda 5.1 magnitude shaking in a quake near Lake Champlain on the New York–Vermont border,causing extensive local damage and (I can attest) knocking pictures from walls and childrenfrom beds as far away as New Hampshire.
Voorhies sent samples to colleagues all over the western United States asking if there wasanything about it that they recognized. Several months later a geologist named BillBonnichsen from the Idaho Geological Survey got in touch and told him that the ash matcheda volcanic deposit from a place called Bruneau-Jarbidge in southwest Idaho. The event thatkilled the plains animals of Nebraska was a volcanic explosion on a scale previouslyunimagined—but big enough to leave an ash layer ten feet deep almost a thousand miles awayin eastern Nebraska. It turned out that under the western United States there was a hugecauldron of magma, a colossal volcanic hot spot, which erupted cataclysmically every600,000 years or so. The last such eruption was just over 600,000 years ago. The hot spot isstill there. These days we call it Yellowstone National Park.
Today, the site of Voorhies’s discovery is called Ashfall Fossil Beds State Park, and it has astylish new visitors’ center and museum, with thoughtful displays on the geology of Nebraskaand the history of the fossil beds. The center incorporates a lab with a glass wall throughwhich visitors can watch paleontologists cleaning bones. Working alone in the lab on themorning I passed through was a cheerfully grizzled-looking fellow in a blue work shirt whomI recognized as Mike Voorhies from a BBC television documentary in which he featured.
Tokyo has already suffered one of the most devastating earthquakes in modern times. OnSeptember 1, 1923, just before noon, the city was hit by what is known as the Great Kantoquake—an event more than ten times more powerful than Kobe’s earthquake. Two hundredthousand people were killed. Since that time, Tokyo has been eerily quiet, so the strainbeneath 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. Nobodycares to guess how many people might die, but the potential economic cost has been put ashigh as $7 trillion.
They don’t get a huge number of visitors to Ashfall Fossil Beds State Park—it’s slightly inthe middle of nowhere—and Voorhies seemed pleased to show me around. He took me to thespot atop a twenty-foot ravine where he had made his find.
Our own attempts to penetrate toward the middle have been modest indeed. One or two SouthAfrican gold mines reach to a depth of two miles, but most mines on Earth go no more thanabout a quarter of a mile beneath the surface. If the planet were an apple, we wouldn’t yethave broken through the skin. Indeed, we haven’t even come close.
The largest earthquake since the scale’s invention was (depending on which source youcredit) either one centered on Prince William Sound in Alaska in March 1964, whichmeasured 9.2 on the Richter scale, or one in the Pacific Ocean off the coast of Chile in 1960,which was initially logged at 8.6 magnitude but later revised upward by some authorities(including the United States Geological Survey) to a truly grand-scale 9.5. As you will gatherfrom this, measuring earthquakes is not always an exact science, particularly wheninterpreting readings from remote locations. At all events, both quakes were whopping. The1960 quake not only caused widespread damage across coastal South America, but also set offa giant tsunami that rolled six thousand miles across the Pacific and slapped away much ofdowntown Hilo, Hawaii, destroying five hundred buildings and killing sixty people. Similarwave surges claimed yet more victims as far away as Japan and the Philippines.
. . even collapsed scaffolding erected around the Capitol Building in Washington, D.C.” OnJanuary 23 and February 4 further quakes of similar magnitude followed. New Madrid hasbeen silent ever since—but not surprisingly, since such episodes have never been known tohappen in the same place twice. As far as we know, they are as random as lightning. The nextone could be under Chicago or Paris or Kinshasa. No one can even begin to guess. And whatcauses these massive intraplate rupturings? Something deep within the Earth. More than thatwe don’t know.
Exactly what happens isn’t known, but it is felt pretty certain that it is connected with the corespinning and with its being liquid. Bodies that don’t have a liquid core—the Moon and Mars,for instance—don’t have magnetism.
The distance from the surface of Earth to the center is 3,959 miles, which isn’t so very far.
Therefore the materials in the upper and lower mantle must at least occasionally mix.
We may be going through a reversal now. The Earth’s magnetic field has diminished byperhaps as much as 6 percent in the last century alone. Any diminution in magnetism is likelyto be bad news, because magnetism, apart from holding notes to refrigerators and keeping ourcompasses pointing the right way, plays a vital role in keeping us alive. Space is full ofdangerous cosmic rays that in the absence of magnetic protection would tear through ourbodies, 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 spacecalled the Van Allen belts. They also interact with particles in the upper atmosphere to createthe bewitching veils of light known as the auroras.
For pure, focused, devastation, however, probably the most intense earthquake in recordedhistory was one that struck—and essentially shook to pieces—Lisbon, Portugal, on All SaintsDay (November 1), 1755. Just before ten in the morning, the city was hit by a suddensideways lurch now estimated at magnitude 9.0 and shaken ferociously for seven full minutes.
The luckiest person on that day was a graduate student named Harry Glicken. He had beenmanning an observation post 5.7 miles from the mountain, but he had a college placementinterview on May 18 in California, and so had left the site the day before the eruption. Hisplace 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.
Interestingly, even though the hole was modest, nearly everything about it was surprising.
How and when the Earth got its crust are questions that divide geologists into two broadcamps—those who think it happened abruptly early in the Earth’s history and those who thinkit happened gradually and rather later. Strength of feeling runs deep on such matters. RichardArmstrong of Yale proposed an early-burst theory in the 1960s, then spent the rest of hiscareer fighting those who did not agree with him. He died of cancer in 1991, but shortlybefore his death he “lashed out at his critics in a polemic in an Australian earth science journalthat charged them with perpetuating myths,” according to a report inEarth magazine in 1998.
Four years later, Soviet scientists decided to try their luck on dry land. They chose a spot onRussia’s Kola Peninsula, near the Finnish border, and set to work with the hope of drilling toa depth of fifteen kilometers. The work proved harder than expected, but the Soviets werecommendably persistent. When at last they gave up, nineteen years later, they had drilled to adepth of 12,262 meters, or about 7.6 miles. Bearing in mind that the crust of the Earthrepresents only about 0.3 percent of the planet’s volume and that the Kola hole had not cuteven one-third of the way through the crust, we can hardly claim to have conquered theinterior.
Ninety minutes after the blast, ash began to rain down on Yakima, Washington, acommunity of fifty thousand people about eighty miles away. As you would expect, the ashturned day to night and got into everything, clogging motors, generators, and electricalswitching equipment, choking pedestrians, blocking filtration systems, and generally bringingthings to a halt. The airport shut down and highways in and out of the city were closed.
But to return to Washington. Mount St. Helens lost thirteen hundred feet of peak, and 230square miles of forest were devastated. Enough trees to build 150,000 homes (or 300,000 insome reports) were blown away. The damage was placed at $2.7 billion. A giant column ofsmoke and ash rose to a height of sixty thousand feet in less than ten minutes. An airlinersome thirty miles away reported being pelted with rocks.
In a setting like this”—he made a sweeping gesture across the vast and unvarying prairie—“you wouldn’t know where to begin. There could be really magnificent stuff out there, butthere’s no surface clues to show you where to start looking.”
Beneath the mantle are the two cores—a solid inner core and a liquid outer one. Needless tosay, our understanding of the nature of these cores is indirect, but scientists can make somereasonable assumptions. They know that the pressures at the center of the Earth aresufficiently high—something over three million times those found at the surface—to turn anyrock there solid. They also know from Earth’s history (among other clues) that the inner coreis very good at retaining its heat. Although it is little more than a guess, it is thought that inover four billion years the temperature at the core has fallen by no more than 200°F. No oneknows exactly how hot the Earth’s core is, but estimates range from something over 7,000°Fto 13,000°F—about as hot as the surface of the Sun.
A big part of the reason for our ignorance, interestingly enough, is that traditionally therehas been little effort to coordinate what’s happening on top of the Earth with what’s going oninside. According to Shawna Vogel: “Geologists and geophysicists rarely go to the samemeetings or collaborate on the same problems.”
Every attempt ended in failure. The deepest they penetrated was only about 600 feet. TheMohole became known as the No Hole. In 1966, exasperated with ever-rising costs and noresults, Congress killed the project.
The movements occur not just laterally as the Earth’s plates move across the surface, but upand down as well, as rocks rise and fall under the churning process known as convection.
Even more unnerving, because they are less well understood and capable of occurringanywhere at any time, are the rarer type of shakings known as intraplate quakes. Thesehappen away from plate boundaries, which makes them wholly unpredictable. And becausethey come from a much greater depth, they tend to propagate over much wider areas. Themost notorious such quakes ever to hit the United States were a series of three in NewMadrid, Missouri, in the winter of 1811–12. The adventure started just after midnight onDecember 16 when people were awakened first by the noise of panicking farm animals (therestiveness 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 theEarth. Emerging from their houses, locals found the land rolling in waves up to three feet highand opening up in fissures several feet deep. A strong smell of sulfur filled the air. Theshaking lasted for four minutes with the usual devastating effects to property. Among thewitnesses was the artist John James Audubon, who happened to be in the area. The quakeradiated outward with such force that it knocked down chimneys in Cincinnati four hundredmiles away and, according to at least one account, “wrecked boats in East Coast harbors and .
St. Helens started its ominous rumblings on March 20. Within a week it was eruptingmagma, albeit in modest amounts, up to a hundred times a day, and being constantly shakenwith earthquakes. People were evacuated to what was assumed to be a safe distance of eightmiles. As the mountain’s rumblings grew St. Helens became a tourist attraction for the world.
At first they thought the animals were buried alive, and Voorhies stated as much in aNational Geographic article in 1981. “The article called the site a ‘Pompeii of prehistoricanimals,’ ” he told me, “which was unfortunate because just afterward we realized that theanimals hadn’t died suddenly at all. They were all suffering from something calledhypertrophic pulmonary osteodystrophy, which is what you would get if you were breathing alot of abrasive ash—and they must have been breathing a lot of it because the ash was feetthick for hundreds of miles.” He picked up a chunk of grayish, claylike dirt and crumbled itinto my hand. It was powdery but slightly gritty. “Nasty stuff to have to breathe,” he went on,“because it’s very fine but also quite sharp. So anyway they came here to this watering hole,presumably seeking relief, and died in some misery. The ash would have ruined everything. Itwould have buried all the grass and coated every leaf and turned the water into an undrinkablegray sludge. It couldn’t have been very agreeable at all.”
Seismologists insist that there is no evidence to support such a thesis.
I asked him in what way it was a dumb place to hunt for bones. “Well, if you’re looking forbones, you really need exposed rock. That’s why most paleontology is done in hot, dry places.
“It was a dumb place to look for bones,” he said happily. “But I wasn’t looking for bones. Iwas thinking of making a geological map of eastern Nebraska at the time, and really just kindof poking around. If I hadn’t gone up this ravine or the rains hadn’t just washed out that skull,I’d have walked on by and this would never have been found.” He indicated a roofedenclosure nearby, which had become the main excavation site. Some two hundred animalshad been found lying together in a jumble.
From 650 to 2,700 km (400 to 1,700 mi) is the lower mantle. From 2,700 to 2,890 km (1,700 to 1,900 mi) is the"D" layer. From 2,890 to 5,150 km (1,900 to 3,200 mi) is the outer core, and from 5,150 to 6,378 km (3,200 to3,967 mi) is the inner core.
Helens’s behavior primarily had seen only Hawaiian volcanoes in action, and they, it turnedout, were not the same thing at all.
“He died a bitter man,” reported a colleague.
Perhaps nothing better demonstrates our inadequate grasp of the dynamics of the Earth’sinterior than how badly we are caught out when it acts up, and it would be hard to come upwith a more salutary reminder of the limitations of our understanding than the eruption ofMount St. Helens in Washington in 1980.
Even the one part of it we can see, the crust, is a matter of some fairly strident debate.
At just about the time that Lehmann was refining our basic understanding of the Earth’sinterior by studying the seismic waves of earthquakes, two geologists at Caltech in Californiawere devising a way to make comparisons between one earthquake and the next. They wereCharles Richter and Beno Gutenberg, though for reasons that have nothing to do with fairnessthe scale became known almost at once as Richter’s alone. (It has nothing to do with Richtereither. A modest fellow, he never referred to the scale by his own name, but always called it“the Magnitude Scale.”)The Richter scale has always been widely misunderstood by nonscientists, though perhapsa little less so now than in its early days when visitors to Richter’s office often asked to seehis celebrated scale, thinking it was some kind of machine. The scale is of course more anidea than an object, an arbitrary measure of the Earth’s tremblings based on surfacemeasurements. It rises exponentially, so that a 7.3 quake is fifty times more powerful than a6.3 earthquake and 2,500 times more powerful than a 5.3 earthquake.
Less than two years after the Unzen catastrophe another group of volcano watchers, led byStanley Williams of the University of Arizona, descended into the rim of an active volcanocalled Galeras in Colombia. Despite the deaths of recent years, only two of the sixteenmembers of Williams’s party wore safety helmets or other protective gear. The volcanoerupted, killing six of the scientists, along with three tourists who had followed them, andseriously injuring several others, including Williams himself.
We all know what happened next. At 8:32 A.M. on a Sunday morning, May 18, the northside of the volcano collapsed, sending an enormous avalanche of dirt and rock rushing downthe mountain slope at 150 miles an hour. It was the biggest landslide in human history andcarried enough material to bury the whole of Manhattan to a depth of four hundred feet. Aminute later, its flank severely weakened, St. Helens exploded with the force of five hundredHiroshima-sized atomic bombs, shooting out a murderous hot cloud at up to 650 miles anhour—much too fast, clearly, for anyone nearby to outrace. Many people who were thought tobe in safe areas, often far out of sight of the volcano, were overtaken. Fifty-seven people werekilled. Twenty-three of the bodies were never found. The toll would have been much higherexcept that it was a Sunday. Had it been a weekday many lumber workers would have beenworking within the death zone. As it was, people were killed eighteen miles away.
“I’m a little embarrassed to tell you,” Voorhies said, smiling briefly, “that the first I thoughtabout it was when an editor at the National Geographic asked me the source of all the ash andI had to confess that I didn’t know. Nobody knew.”
It has been calculated that if you sunk a well to the center and dropped a brick into it, it wouldtake only forty-five minutes for it to hit the bottom (though at that point it would beweightless since all the Earth’s gravity would be above and around it rather than beneath it).
We know amazingly little about what happens beneath our feet. It is fairly remarkable tothink that Ford has been building cars and baseball has been playing World Series for longerthan we have known that the Earth has a core. And of course the idea that the continents moveabout on the surface like lily pads has been common wisdom for much less than a generation.
Nearly all geology texts tell you that continental crust is three to six miles thick under theoceans, about twenty-five miles thick under the continents, and forty to sixty miles thickunder big mountain chains, but there are many puzzling variabilities within thesegeneralizations. The crust beneath the Sierra Nevada Mountains, for instance, is only aboutnineteen to twenty-five miles thick, and no one knows why. By all the laws of geophysics theSierra Nevadas should be sinking, as if into quicksand. (Some people think they may be.)1For those who crave a more detailed picture of the Earths interior, here are the dimensions of the variouslayers, using average figures: From 0 to 40 km (25 mi) is the crust. From 40 to 400 km (25 to 250 mi) is theupper mantle. From 400 to 650 km (250 to 400 mi) is a transition zone between the upper and lower mantle.
IN THE SUMMER of 1971, a young geologist named Mike Voorhies was scouting around onsome grassy farmland in eastern Nebraska, not far from the little town of Orchard, where hehad grown up. Passing through a steep-sided gully, he spotted a curious glint in the brushabove and clambered up to have a look. What he had seen was the perfectly preserved skull ofa young rhinoceros, which had been washed out by recent heavy rains.
