by Bill Bryson
At all events, mobile crust was an idea whose time had finally come. A symposium of many of the most important figures in the field was convened in London under the auspices of the Royal Society in 1964, and suddenly, it seemed, everyone was a convert. The Earth, the meeting agreed, was a mosaic of interconnected segments whose various stately jostlings accounted for much of the planet’s surface behaviour.
The name “continental drift” was fairly swiftly discarded when it was realized that the whole crust was in motion and not just the continents, but it took a while to settle on a name for the individual segments. At first people called them “crustal blocks” or sometimes “paving stones.” Not until late 1968, with the publication of an article by three American seismologists in the Journal of Geophysical Research, did the segments receive the name by which they have since been known: plates. The same article called the new science plate tectonics.
Old ideas die hard and not everyone rushed to embrace the exciting new theory well into the 1970s, one of the most popular and influential geological textbooks, The Earth by the venerable Harold Jeffreys, strenuously insisted that plate tectonics was a physical impossibility, just as it had in the first edition way back in 1924. It was equally dismissive of convection and sea-floor spreading. And in Basin and Range, published in 1980, John McPhee noted that even then one American geologist in eight still didn’t believe in plate tectonics.
Today we know that the Earth’s surface is made up of eight to twelve big plates (depending on how you define big) and twenty or so smaller ones, and that they all move in different directions and at different speeds. Some plates are large and comparatively inactive, others small but energetic. They bear only an incidental relationship to the land masses that sit upon them. The North American plate, for instance, is much larger than the continent with which it is associated. It roughly traces the outline of the continent’s western coast (which is why that area is so seismically active, because of the bump and crush of the plate boundary), but ignores the eastern seaboard altogether and instead extends halfway across the Atlantic to the mid-ocean ridge. Iceland is split down the middle, which makes it tectonically half American and half European. New Zealand, meanwhile, is part of the immense Indian Ocean plate even though it is nowhere near the Indian Ocean. And so it goes for most plates.
The connections between modern land masses and those of the past were found to be infinitely more complex than anyone had imagined. Kazakhstan, it turns out, was once attached to Norway and New England. One corner of Staten Island, but only a corner, is European. So is part of Newfoundland. Pick up a pebble from a Massachusetts beach and its nearest kin will now be in Africa. The Scottish Highlands and much of Scandinavia are substantially American. Some of the Shackleton Range of Antarctica, it is thought, may once have belonged to the Appalachians of the eastern US. Rocks, in short, get around.
The constant turmoil keeps the plates from fusing into a single immobile plate. Assuming things continue much as at present, the Atlantic Ocean will expand until eventually it is much bigger than the Pacific. Much of California will float off and become a kind of Madagascar of the Pacific. Africa will push northward into Europe, squeezing the Mediterranean out of existence and thrusting up a chain of mountains of Himalayan majesty running from Paris to Calcutta. Australia will colonize the islands to its north and connect by some isthmian umbilicus to Asia. These are future outcomes, but not future events. The events are happening now. As we sit here, continents are adrift, like leaves on a pond. Thanks to Global Positioning Systems we can see that Europe and North America are parting at about the speed a fingernail grows—roughly two metres 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 of lifetimes that keeps us from appreciating the changes. Look at a globe and what you are seeing really is a snapshot of the continents as they have been for just one-tenth of 1 per cent of the Earth’s history.
Earth is alone among the rocky planets in having tectonics and why this should be is a bit of a mystery. It is not simply a matter of size or density—Venus is nearly a twin of Earth in these respects and yet has no tectonic activity—but it may be that we have just the right materials in just the right measures to keep the Earth bubbling away. It is thought—though it is really nothing more than a thought—that tectonics is an important part of the planet’s organic well-being. As the physicist and writer James Trefil has put it, “It would be hard to believe that the continuous movement of tectonic plates has no effect on the development of life on earth.” He suggests that the challenges induced by tectonics—changes in climate, for instance—were an important spur to the development of intelligence. Others believe the driftings of the continents may have produced at least some of the Earth’s various extinction events. In November 2002 Tony Dickson of Cambridge University produced a report, published in the journal Science, strongly suggesting that there may well be a relationship between the history of rocks and the history of life. What Dickson established was that the chemical composition of the world’s oceans has altered abruptly and dramatically at times throughout the past half-billion years and that these changes often correlate with important events in biological history—the huge outburst of tiny organisms that created the chalk cliffs of England’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 so dramatically from time to time, but the opening and shutting of ocean ridges would be an obvious possible culprit.
The boundaries of Earth’s main tectonic plates. The slow, steady movements of these plates, and the pressures that build up where they meet, leave the boundary zones dangerously susceptible to earthquakes. (credit 12.8)
At all events, plate tectonics explained not only the surface dynamics of the Earth—how an ancient Hipparion got from France to Florida, for example—but also many of its internal actions. Earthquakes, the formation of island chains, the carbon cycle, the locations of mountains, the coming of ice ages, the origins of life itself—there was hardly a matter that wasn’t directly influenced by this remarkable new theory. Geologists, as McPhee has noted, found themselves in the giddying position where “the whole earth suddenly made sense.”
But only up to a point. The distribution of continents in former times is much less neatly resolved than most people outside geophysics think. Although textbooks give confident-looking representations of ancient land masses with names like Laurasia, Gondwana, Rodinia and Pangaea, these are sometimes based on conclusions that don’t altogether hold up. As George Gaylord Simpson observes in Fossils and the History of Life, species of plants and animals from the ancient world have a habit of appearing inconveniently where they shouldn’t and 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 distributions of a genus of ancient 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 known connection to Gondwana. This troubling discrepancy was—and continues to be—mostly ignored. Similarly, a Triassic reptile called lystrosaurus has been found from Antarctica all the way to Asia, supporting the idea of a former connection between those continents, but it has never turned up in South America or Australia, which are believed to have been part of the same continent at the same time.
There are also many surface features that tectonics can’t explain. Take Denver. It is, as everyone knows, a mile high, but that rise is comparatively recent. When dinosaurs roamed the Earth, Denver was part of an ocean bottom, many thousands of metres lower. Yet the rocks on which Denver sits are not fractured or deformed in the way they would be if Denver had been pushed up by colliding plates, and anyway Denver was too far from the plate edges to be susceptible to their actions. It would be as if you pushed against the edge of
a rug hoping to raise a ruck at the opposite end. Mysteriously and over millions of years, it appears that Denver has been rising, like baking bread. So, too, has much of southern Africa; a portion of it 1,600 kilometres across has risen about one and a half kilometres in a hundred million years without any known associated tectonic activity. Australia, meanwhile, has been tilting and sinking. Over the past hundred million years, as it has drifted north towards Asia, its leading edge has sunk by nearly 200 metres. It appears that Indonesia is very slowly drowning, and dragging Australia down with 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 in 1930, 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 lies there yet, but about a metre closer to North America than on the day he died.
Alfred Wegener readies a weather balloon for launch in Greenland in 1930. Soon after, he died there. (credit 12.9)
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 continental drift theories was even published.
The other principal player in the emergence of tectonics theory, Harry Hess, was also at Princeton at the time, and would spend the rest of his career there. One of his students was a bright young fellow named Walter Alvarez, who would eventually change the world of science in a quite different way.
As for geology itself, its cataclysms had only just begun, and it was young Alvarez who helped to start the process.
Radiantly destructive lava flows from Mount Etna to Valle del Bove in Italy following an eruption in January 1992. (credit p4.1)
BANG!
People knew for a long time that there was something odd about the earth beneath Manson, Iowa. In 1912, a man drilling a well for the town water supply reported bringing up a lot of strangely deformed rock—“crystalline clast breccia with a melt matrix” and “overturned ejecta flap,” as it was later described in an official report. The water was odd, too. It was almost as soft as rainwater. Naturally occurring soft water had never been found in Iowa before.
Though Manson’s strange rocks and silken waters were matters of curiosity, forty-one years would pass before a team from the University of Iowa got around to making a trip to the community, then as now a town of about two thousand people in the northwest part of the state. In 1953, after sinking a series of experimental bores, university geologists agreed that the site was indeed anomalous and attributed the deformed rocks to some ancient, unspecified volcanic action. This was in keeping with the wisdom of the day, but it was also about as wrong as a geological conclusion can get.
The trauma to Manson’s geology had come not from within the Earth, but from at least one hundred million miles beyond. Some time in the very ancient past, when Manson stood on the edge of a shallow sea, a rock about a mile and a half across, weighing 10 billion tons and travelling at perhaps two hundred times the speed of sound, ripped through the atmosphere and punched into the Earth with a violence and suddenness that we can scarcely imagine. Where Manson now stands became in an instant a hole three miles deep and more than 20 miles across. The limestone that elsewhere gives Iowa its hard, mineralized water was obliterated and replaced by the shocked basement rocks that so puzzled the water driller in 1912.
The Manson impact was the biggest thing that has ever occurred on the mainland United States. Of any type. Ever. The crater it left behind was so colossal that if you stood on one edge you would only just be able to see the other side on a good day. It would make the Grand Canyon look quaint and trifling. Unfortunately for lovers of spectacle, 2.5 million years of passing ice sheets filled the Manson crater right to the top with rich glacial till, then graded it smooth, so that today the landscape at Manson, and for miles around, is as flat as a table top. Which is of course why no one has ever heard of the Manson crater.
At the library in Manson they are delighted to show you a collection of newspaper articles and a box of core samples from a 1991–2 drilling programme—indeed, they positively bustle to produce them—but you have to ask to see them. Nothing permanent is on display and nowhere in the town is there any historical marker.
To most people in Manson the biggest thing ever to happen was a tornado that rolled up Main Street in 1979, tearing apart the business district. One of the advantages of all that surrounding flatness is that you can see danger from a long way off. Virtually the whole town turned out at one end of Main Street and watched for half an hour as the tornado came towards them, hoping it would veer off, then prudently scampered when it did not. Four of them, alas, didn’t move quite fast enough and were killed. Every June now Manson has a week-long event called Crater Days, which was dreamed up as a way of helping people forget that unhappy anniversary. It doesn’t really have anything to do with the crater. Nobody’s figured out a way to capitalize on an impact site that isn’t visible.
“Very occasionally we get people coming in and asking where they should go to see the crater and we have to tell them that there is nothing to see,” says Anna Schlapkohl, the town’s friendly librarian. “Then they go away kind of disappointed.” However, most people, including most Iowans, have never heard of the Manson crater. Even for geologists it barely rates a footnote. But for one brief period in the 1980s, Manson was the most geologically exciting place on Earth.
The story begins in the early 1950s when a bright young geologist named Eugene Shoemaker paid a visit to Meteor Crater in Arizona. Today Meteor Crater is the most famous impact site on Earth and a popular tourist attraction. In those days, however, it didn’t receive many visitors and was still often referred to as Barringer Crater, after a wealthy mining engineer named Daniel M. Barringer who had staked a claim on it in 1903. Barringer believed that the crater had been formed by a 10 million tonne meteor, heavily freighted with iron and nickel, and it was his confident expectation that he would make a fortune digging it out. Unaware that the meteor and everything in it would have been vaporized on impact, he wasted a fortune, and the next twenty-six years, cutting tunnels that yielded nothing.
Meteor Crater in Arizona, at nearly a mile across and several hundred feet deep, is now perhaps the world’s best known impact crater, but for much of the twentieth century it was thought to be the result of an explosion from within the Earth. It was created by a meteor strike 50,000 years ago. (credit 13.1)
By the standards of today, crater research in the early 1900s was a trifle unsophisticated, to say the least. The leading early investigator, G. K. Gilbert of Columbia University, modelled the effects of impacts by flinging marbles into pans of oatmeal. (For reasons I cannot supply, Gilbert conducted these experiments not in a laboratory at Columbia but in a hotel room.) Somehow, from this Gilbert concluded that the Moon’s craters were indeed formed by impacts—in itself quite a radical notion for the time—but that the Earth’s were not. Most scientists refused to go even that far. To them, the Moon’s craters were evidence of ancient volcanoes and nothing more. The few craters that remained evident on the Earth (most had been eroded away) were generally attributed to other causes or treated as fluky rarities.
A meteor caught by a lucky photographer streaks across the night sky of Finland. The momentary flare of such shooting stars generally comes from small fragments of rock, often no larger than a mustard seed, that are consumed before they reach the Earth’s surface. (credit 13.2)
By the time Shoemaker came along, a common view was that Meteor Crater had been formed by an underground steam explosion. Shoemaker knew nothing about underground steam explosions—he couldn’t: they don’t exist—but he did know all about blast zones. One of his first jobs out of college had been to study explosion rings at the Yucca Flats nuclear test site in Nevada. He concluded, as Barringer had before him, that there was nothing at Meteor Crater to suggest volcanic
activity, but that there were huge distributions of other stuff—anomalous fine silicas and magnetites principally—that suggested an impact from space. Intrigued, he began to study the subject in his spare time.
One of a set of teaching cards illustrating the different types of comets along with their elliptical orbits. The illustrations at bottom left and right show two different meteor showers, one over Europe in 1836 and one over Niagara Falls in 1833. (credit 13.3)
Working first with his colleague Eleanor Helin and later with his wife, Carolyn, and associate David Levy, Shoemaker began a systematic survey of the inner solar system. They spent one week each month at the Palomar Observatory in California looking for objects, asteroids primarily, whose trajectories carried them across the Earth’s orbit.
“At the time we started, only slightly more than a dozen of these things had ever been discovered in the entire course of astronomical observation,” Shoemaker recalled some years later in a television interview. “Astronomers in the twentieth century essentially abandoned the solar system,” he added. “Their attention was turned to the stars, the galaxies.”
What Shoemaker and his colleagues found was that there was more risk out there—a great deal more—than anyone had ever imagined.
Asteroids, as most people know, are rocky objects orbiting in loose formation in a belt between Mars and Jupiter. In illustrations they are always shown as existing in a jumble, but in fact the solar system is quite a roomy place and the average asteroid actually will be about one and a half million kilometres from its nearest neighbour. Nobody knows even approximately how many asteroids there are tumbling through space, but the number is thought to be probably not less than a billion. They are presumed to be a planet that never quite made it, owing to the unsettling gravitational pull of Jupiter, which kept—and keeps—them from coalescing.