12 THE EARTH MOVES
《万物简史英文版》章节: 12 THE EARTH MOVES,宠文网网友提供全文无弹窗免费在线阅读。!
Mr. Hapgood briskly dismissed any such notions, noting that the geologists K. E. Casterand J. C. Mendes had done extensive fieldwork on both sides of the Atlantic and hadestablished beyond question that no such similarities existed. Goodness knows what outcropsMessrs. Caster and Mendes had looked at, beacuse in fact many of the rock formations onboth sides of the Atlanticare the same—not just very similar but the same.
This was not an idea that flew with Mr. Hapgood, or many other geologists of his day. Thetheory Hapgood alluded to was one first propounded in 1908 by an amateur Americangeologist named Frank Bursley Taylor. Taylor came from a wealthy family and had both themeans and freedom from academic constraints to pursue unconventional lines of inquiry. Hewas one of those struck by the similarity in shape between the facing coastlines of Africa andSouth America, and from this observation he developed the idea that the continents had onceslid around. He suggested—presciently as it turned out—that the crunching together ofcontinents could have thrust up the world’s mountain chains. He failed, however, to producemuch in the way of evidence, and the theory was considered too crackpot to merit seriousattention.
In Germany, however, Taylor’s idea was picked up, and effectively appropriated, by atheorist named Alfred Wegener, a meteorologist at the University of Marburg. Wegenerinvestigated the many plant and fossil anomalies that did not fit comfortably into the standardmodel of Earth history and realized that very little of it made sense if conventionallyinterpreted. Animal fossils repeatedly turned up on opposite sides of oceans that were clearlytoo wide to swim. How, he wondered, did marsupials travel from South America to Australia?
How did identical snails turn up in Scandinavia and New England? And how, come to that,did one account for coal seams and other semi-tropical remnants in frigid spots likeSpitsbergen, four hundred miles north of Norway, if they had not somehow migrated therefrom warmer climes?
Wegener developed the theory that the world’s continents had once come together in asingle landmass he called Pangaea, where flora and fauna had been able to mingle, before thecontinents had split apart and floated off to their present positions. All this he put together in abook called Die Entstehung der Kontinente und Ozeane, or The Origin of Continents and Oceans, which was published in German in 1912 and—despite the outbreak of the FirstWorld War in the meantime—in English three years later.
Because of the war, Wegener’s theory didn’t attract much notice at first, but by 1920, whenhe produced a revised and expanded edition, it quickly became a subject of discussion.
Everyone agreed that continents moved—but up and down, not sideways. The process ofvertical movement, known as isostasy, was a foundation of geological beliefs for generations,though no one had any good theories as to how or why it happened. One idea, which remainedin textbooks well into my own school days, was the baked apple theory propounded by theAustrian Eduard Suess just before the turn of the century. This suggested that as the moltenEarth had cooled, it had become wrinkled in the manner of a baked apple, creating oceanbasins and mountain ranges. Never mind that James Hutton had shown long before that anysuch static arrangement would eventually result in a featureless spheroid as erosion leveledthe bumps and filled in the divots. There was also the problem, demonstrated by Rutherfordand Soddy early in the century, that Earthly elements hold huge reserves of heat—much toomuch to allow for the sort of cooling and shrinking Suess suggested. And anyway, if Suess’stheory was correct then mountains should be evenly distributed across the face of the Earth,which patently they were not, and of more or less the same ages; yet by the early 1900s it wasalready evident that some ranges, like the Urals and Appalachians, were hundreds of millionsof years older than others, like the Alps and Rockies. Clearly the time was ripe for a newtheory. Unfortunately, Alfred Wegener was not the man that geologists wished to provide it.
For a start, his radical notions questioned the foundations of their discipline, seldom aneffective way to generate warmth in an audience. Such a challenge would have been painfulenough coming from a geologist, but Wegener had no background in geology. He was ameteorologist, for goodness sake. A weatherman—a German weatherman. These were notremediable deficiencies.
And so geologists took every pain they could think of to dismiss his evidence and belittlehis suggestions. To get around the problems of fossil distributions, they posited ancient “landbridges” wherever they were needed. When an ancient horse named Hipparion was found tohave lived in France and Florida at the same time, a land bridge was drawn across theAtlantic. When it was realized that ancient tapirs had existed simultaneously in SouthAmerica and Southeast Asia a land bridge was drawn there, too. Soon maps of prehistoricseas were almost solid with hypothesized land bridges—from North America to Europe, fromBrazil to Africa, from Southeast Asia to Australia, from Australia to Antarctica. Theseconnective tendrils had not only conveniently appeared whenever it was necessary to move aliving organism from one landmass to another, but then obligingly vanished without leaving atrace of their former existence. None of this, of course, was supported by so much as a grainof actual evidence—nothing so wrong could be—yet it was geological orthodoxy for the nexthalf century.
Even land bridges couldn’t explain some things. One species of trilobite that was wellknown in Europe was also found to have lived on Newfoundland—but only on one side. Noone could persuasively explain how it had managed to cross two thousand miles of hostileocean but then failed to find its way around the corner of a 200-mile-wide island. Even moreawkwardly anomalous was another species of trilobite found in Europe and the PacificNorthwest but nowhere in between, which would have required not so much a land bridge as aflyover. Yet as late as 1964 when the Encyclopaedia Britannica discussed the rival theories, itwas Wegener’s that was held to be full of “numerous grave theoretical difficulties.”
To be sure, Wegener made mistakes. He asserted that Greenland is drifting west by about amile a year, which is clearly nonsense. (It’s more like half an inch.) Above all, he could offerno convincing explanation for how the landmasses moved about. To believe in his theory youhad to accept that massive continents somehow pushed through solid crust, like a plowthrough soil, without leaving any furrow in their wake. Nothing then known could plausiblyexplain what motored these massive movements.
It was Arthur Holmes, the English geologist who did so much to determine the age of theEarth, who suggested a possible way. Holmes was the first scientist to understand thatradioactive warming could produce convection currents within the Earth. In theory thesecould be powerful enough to slide continents around on the surface. In his popular andinfluential textbook Principles of Physical Geology , first published in 1944, Holmes laid outa continental drift theory that was in its fundamentals the theory that prevails today. It wasstill a radical proposition for the time and widely criticized, particularly in the United States,where resistance to drift lasted longer than elsewhere. One reviewer there fretted, without anyevident sense of irony, that Holmes presented his arguments so clearly and compellingly thatstudents might actually come to believe them.
Elsewhere, however, the new theory drew steady if cautious support. In 1950, a vote at theannual meeting of the British Association for the Advancement of Science showed that abouthalf of those present now embraced the idea of continental drift. (Hapgood soon after citedthis figure as proof of how tragically misled British geologists had become.) Curiously,Holmes himself sometimes wavered in his conviction. In 1953 he confessed: “I have neversucceeded in freeing myself from a nagging prejudice against continental drift; in mygeological bones, so to speak, I feel the hypothesis is a fantastic one.”
Continental drift was not entirely without support in the United States. Reginald Daly ofHarvard spoke for it, but he, you may recall, was the man who suggested that the Moon hadbeen formed by a cosmic impact, and his ideas tended to be considered interesting, evenworthy, but a touch too exuberant for serious consideration. And so most American academicsstuck to the belief that the continents had occupied their present positions forever and thattheir surface features could be attributed to something other than lateral motions.
Interestingly, oil company geologists had known for years that if you wanted to find oil youhad to allow for precisely the sort of surface movements that were implied by plate tectonics.
But oil geologists didn’t write academic papers; they just found oil.
There was one other major problem with Earth theories that no one had resolved, or evencome close to resolving. That was the question of where all the sediments went. Every yearEarth’s rivers carried massive volumes of eroded material—500 million tons of calcium, forinstance—to the seas. If you multiplied the rate of deposition by the number of years it hadbeen going on, it produced a disturbing figure: there should be about twelve miles ofsediments on the ocean bottoms—or, put another way, the ocean bottoms should by now bewell above the ocean tops. Scientists dealt with this paradox in the handiest possible way.
They ignored it. But eventually there came a point when they could ignore it no longer.
In the Second World War, a Princeton University mineralogist named Harry Hess was putin charge of an attack transport ship, the USS Cape Johnson. Aboard this vessel was a fancynew depth sounder called a fathometer, which was designed to facilitate inshore maneuvers during beach landings, but Hess realized that it could equally well be used for scientificpurposes and never switched it off, even when far out at sea, even in the heat of battle. Whathe found was entirely unexpected. If the ocean floors were ancient, as everyone assumed, theyshould be thickly blanketed with sediments, like the mud on the bottom of a river or lake. ButHess’s readings showed that the ocean floor offered anything but the gooey smoothness ofancient silts. It was scored everywhere with canyons, trenches, and crevasses and dotted withvolcanic seamounts that he called guyots after an earlier Princeton geologist named ArnoldGuyot. All this was a puzzle, but Hess had a war to take part in, and put such thoughts to theback of his mind.
After the war, Hess returned to Princeton and the preoccupations of teaching, but themysteries of the seafloor continued to occupy a space in his thoughts. Meanwhile, throughoutthe 1950s oceanographers were undertaking more and more sophisticated surveys of theocean floors. In so doing, they found an even bigger surprise: the mightiest and mostextensive mountain range on Earth was—mostly—underwater. It traced a continuous pathalong the world’s seabeds, rather like the stitching on a baseball. If you began at Iceland, youcould follow it down the center of the Atlantic Ocean, around the bottom of Africa, and acrossthe Indian and Southern Oceans, below Australia; there it angled across the Pacific as ifmaking for Baja California before shooting up the west coast of the United States to Alaska.
Occasionally its higher peaks poked above the water as an island or archipelago—the Azoresand Canaries in the Atlantic, Hawaii in the Pacific, for instance—but mostly it was buriedunder thousands of fathoms of salty sea, unknown and unsuspected. When all its brancheswere added together, the network extended to 46,600 miles.
A very little of this had been known for some time. People laying ocean-floor cables in thenineteenth century had realized that there was some kind of mountainous intrusion in the mid-Atlantic from the way the cables ran, but the continuous nature and overall scale of the chainwas a stunning surprise. Moreover, it contained physical anomalies that couldn’t be explained.
Down the middle of the mid-Atlantic ridge was a canyon—a rift—up to a dozen miles widefor its entire 12,000-mile length. This seemed to suggest that the Earth was splitting apart atthe seams, like a nut bursting out of its shell. It was an absurd and unnerving notion, but theevidence couldn’t be denied.
Then in 1960 core samples showed that the ocean floor was quite young at the mid-Atlanticridge but grew progressively older as you moved away from it to the east or west. Harry Hessconsidered the matter and realized that this could mean only one thing: new ocean crust wasbeing formed on either side of the central rift, then being pushed away from it as new crustcame along behind. The Atlantic floor was effectively two large conveyor belts, one carryingcrust toward North America, the other carrying crust toward Europe. The process becameknown as seafloor spreading.
When the crust reached the end of its journey at the boundary with continents, it plungedback into the Earth in a process known as subduction. That explained where all the sedimentwent. It was being returned to the bowels of the Earth. It also explained why ocean floorseverywhere were so comparatively youthful. None had ever been found to be older than about175 million years, which was a puzzle because continental rocks were often billions of yearsold. Now Hess could see why. Ocean rocks lasted only as long as it took them to travel toshore. It was a beautiful theory that explained a great deal. Hess elaborated his ideas in animportant paper, which was almost universally ignored. Sometimes the world just isn’t readyfor a good idea.
Meanwhile, two researchers, working independently, were making some startling findingsby drawing on a curious fact of Earth history that had been discovered several decades earlier.
In 1906, a French physicist named Bernard Brunhes had found that the planet’s magnetic fieldreverses itself from time to time, and that the record of these reversals is permanently fixed incertain rocks at the time of their birth. Specifically, tiny grains of iron ore within the rockspoint to wherever the magnetic poles happen to be at the time of their formation, then staypointing in that direction as the rocks cool and harden. In effect they “remember” where themagnetic poles were at the time of their creation. For years this was little more than acuriosity, but in the 1950s Patrick Blackett of the University of London and S. K. Runcorn ofthe University of Newcastle studied the ancient magnetic patterns frozen in British rocks andwere startled, to say the very least, to find them indicating that at some time in the distant pastBritain had spun on its axis and traveled some distance to the north, as if it had somehowcome loose from its moorings. Moreover, they also discovered that if you placed a map ofEurope’s magnetic patterns alongside an American one from the same period, they fit togetheras neatly as two halves of a torn letter. It was uncanny.
Their findings were ignored too.
It finally fell to two men from Cambridge University, a geophysicist named DrummondMatthews and a graduate student of his named Fred Vine, to draw all the strands together. In1963, using magnetic studies of the Atlantic Ocean floor, they demonstrated conclusively thatthe seafloors were spreading in precisely the manner Hess had suggested and that thecontinents were in motion too. An unlucky Canadian geologist named Lawrence Morley cameup with the same conclusion at the same time, but couldn’t find anyone to publish his paper.
In what has become a famous snub, the editor of the Journal of Geophysical Research toldhim: “Such speculations make interesting talk at cocktail parties, but it is not the sort of thingthat ought to be published under serious scientific aegis.” One geologist later described it as“probably the most significant paper in the earth sciences ever to be denied publication.”
At all events, mobile crust was an idea whose time had finally come. A symposium ofmany of the most important figures in the field was convened in London under the auspices ofthe Royal Society in 1964, and suddenly, it seemed, everyone was a convert. The Earth, themeeting agreed, was a mosaic of interconnected segments whose various stately jostlingsaccounted for much of the planet’s surface behavior.
The name “continental drift” was fairly swiftly discarded when it was realized that thewhole crust was in motion and not just the continents, but it took a while to settle on a namefor the individual segments. At first people called them “crustal blocks” or sometimes “pavingstones.” Not until late 1968, with the publication of an article by three Americanseismologists in the Journal of Geophysical Research , did the segments receive the name bywhich they have since been known: plates. The same article called the new science platetectonics.
Old ideas die hard, and not everyone rushed to embrace the exciting new theory. Well intothe 1970s, one of the most popular and influential geological textbooks, The Earth by thevenerable Harold Jeffreys, strenuously insisted that plate tectonics was a physicalimpossibility, just as it had in the first edition way back in 1924. It was equally dismissive ofconvection and seafloor spreading. And in Basin and Range, published in 1980, John McPheenoted that even then one American geologist in eight still didn’t believe in plate tectonics.
Today we know that Earth’s surface is made up of eight to twelve big plates (depending onhow you define big) and twenty or so smaller ones, and they all move in different directionsand at different speeds. Some plates are large and comparatively inactive, others small butenergetic. They bear only an incidental relationship to the landmasses that sit upon them. TheNorth American plate, for instance, is much larger than the continent with which it isassociated. It roughly traces the outline of the continent’s western coast (which is why thatarea is so seismically active, because of the bump and crush of the plate boundary), butignores the eastern seaboard altogether and instead extends halfway across the Atlantic to themid-ocean ridge. Iceland is split down the middle, which makes it tectonically half Americanand half European. New Zealand, meanwhile, is part of the immense Indian Ocean plate eventhough it is nowhere near the Indian Ocean. And so it goes for most plates.
The connections between modern landmasses and those of the past were found to beinfinitely more complex than anyone had imagined. Kazakhstan, it turns out, was onceattached to Norway and New England. One corner of Staten Island, but only a corner, isEuropean. So is part of Newfoundland. Pick up a pebble from a Massachusetts beach, and itsnearest kin will now be in Africa. The Scottish Highlands and much of Scandinavia aresubstantially American. Some of the Shackleton Range of Antarctica, it is thought, may oncehave belonged to the Appalachians of the eastern U.S. Rocks, in short, get around.
The constant turmoil keeps the plates from fusing into a single immobile plate. Assumingthings continue much as at present, the Atlantic Ocean will expand until eventually it is muchbigger than the Pacific. Much of California will float off and become a kind of Madagascar ofthe Pacific. Africa will push northward into Europe, squeezing the Mediterranean out ofexistence and thrusting up a chain of mountains of Himalayan majesty running from Paris toCalcutta. Australia will colonize the islands to its north and connect by some isthmianumbilicus to Asia. These are future outcomes, but not future events. The events are happeningnow. As we sit here, continents are adrift, like leaves on a pond. Thanks to Global PositioningSystems we can see that Europe and North America are parting at about the speed a fingernailgrows—roughly two yards in a human lifetime. If you were prepared to wait long enough,you could ride from Los Angeles all the way up to San Francisco. It is only the brevity oflifetimes that keeps us from appreciating the changes. Look at a globe and what you areseeing really is a snapshot of the continents as they have been for just one-tenth of 1 percentof the Earth’s history.
Earth is alone among the rocky planets in having tectonics, and why this should be is a bitof a mystery. It is not simply a matter of size or density—Venus is nearly a twin of Earth inthese respects and yet has no tectonic activity. It is thought—though it is really nothing morethan a thought—that tectonics is an important part of the planet’s organic well-being. As thephysicist and writer James Trefil has put it, “It would be hard to believe that the continuousmovement of tectonic plates has no effect on the development of life on earth.” He suggeststhat the challenges induced by tectonics—changes in climate, for instance—were animportant spur to the development of intelligence. Others believe the driftings of thecontinents may have produced at least some of the Earth’s various extinction events. InNovember of 2002, Tony Dickson of Cambridge University in England produced a report,published in the journal Science, strongly suggesting that there may well be a relationshipbetween the history of rocks and the history of life. What Dickson established was that thechemical composition of the world’s oceans has altered abruptly and vigorously throughoutthe past half billion years and that these changes often correlate with important events inbiological history—the huge outburst of tiny organisms that created the chalk cliffs ofEngland’s south coast, the sudden fashion for shells among marine organisms during the Cambrian period, and so on. No one can say what causes the oceans’ chemistry to change sodramatically from time to time, but the opening and shutting of ocean ridges would be anobvious possible culprit.
At all events, plate tectonics not only explained the surface dynamics of the Earth—how anancient Hipparion got from France to Florida, for example—but also many of its internalactions. Earthquakes, the formation of island chains, the carbon cycle, the locations ofmountains, the coming of ice ages, the origins of life itself—there was hardly a matter thatwasn’t directly influenced by this remarkable new theory. Geologists, as McPhee has noted,found themselves in the giddying position that “the whole earth suddenly made sense.”
But only up to a point. The distribution of continents in former times is much less neatlyresolved than most people outside geophysics think. Although textbooks give confident-looking representations of ancient landmasses with names like Laurasia, Gondwana, Rodinia,and Pangaea, these are sometimes based on conclusions that don’t altogether hold up. AsGeorge Gaylord Simpson observes in Fossils and the History of Life, species of plants andanimals from the ancient world have a habit of appearing inconveniently where they shouldn’tand failing to be where they ought.
The outline of Gondwana, a once-mighty continent connecting Australia, Africa,Antarctica, and South America, was based in large part on the distribution of a genus ofancient tongue fern called Glossopteris, which was found in all the right places. However,much later Glossopteris was also discovered in parts of the world that had no knownconnection to Gondwana. This troubling discrepancy was—and continues to be—mostlyignored. Similarly a Triassic reptile called Lystrosaurus has been found from Antarctica allthe way to Asia, supporting the idea of a former connection between those continents, but ithas never turned up in South America or Australia, which are believed to have been part ofthe same continent at the same time.
There are also many surface features that tectonics can’t explain. Take Denver. It is, aseveryone knows, a mile high, but that rise is comparatively recent. When dinosaurs roamedthe Earth, Denver was part of an ocean bottom, many thousands of feet lower. Yet the rockson which Denver sits are not fractured or deformed in the way they would be if Denver hadbeen pushed up by colliding plates, and anyway Denver was too far from the plate edges to besusceptible to their actions. It would be as if you pushed against the edge of a rug hoping toraise a ruck at the opposite end. Mysteriously and over millions of years, it appears thatDenver has been rising, like baking bread. So, too, has much of southern Africa; a portion ofit a thousand miles across has risen nearly a mile in 100 million years without any knownassociated tectonic activity. Australia, meanwhile, has been tilting and sinking. Over the past100 million years as it has drifted north toward Asia, its leading edge has sunk by some sixhundred feet. It appears that Indonesia is very slowly drowning, and dragging Australia downwith it. Nothing in the theories of tectonics can explain any of this.
Alfred Wegener never lived to see his ideas vindicated. On an expedition to Greenland in1930, he set out alone, on his fiftieth birthday, to check out a supply drop. He never returned.
He was found a few days later, frozen to death on the ice. He was buried on the spot and liesthere yet, but about a yard closer to North America than on the day he died.
Einstein also failed to live long enough to see that he had backed the wrong horse. In fact,he died at Princeton, New Jersey, in 1955 before Charles Hapgood’s rubbishing of continentaldrift theories was even published.
The other principal player in the emergence of tectonics theory, Harry Hess, was also atPrinceton at the time, and would spend the rest of his career there. One of his students was abright young fellow named Walter Alvarez, who would eventually change the world ofscience in a quite different way.
As for geology itself, its cataclysms had only just begun, and it was young Alvarez whohelped to start the process.
PART IV DANGEROUS PLANETThe history of any one part of theEarth, like the life of a soldier, consistsof long periods of boredom