by John Dvorak
Besides giving her son an unusual middle name—which makes it easy for science historians to identify him—his mother, Henrietta, or “Hettie,” had another major influence. She was an accomplished mountaineer, having scaled, before her son’s birth, several high peaks, first in the Alps, then in the Rockies, at a time when few women engaged in such activity. Her most notable feat was to be the first person to reach the summit of Peak No. 7 in the Valley of Ten Peaks rimming Moraine Lake in the Canadian Rockies. She made her ascent in 1906. The next year, in her honor, the 10,630-foot-high peak was named Mount Tuzo.*
Inspired by his mother, the son also became a mountaineer, being the first to make a solo ascent of Mount Hague in Montana. It was a desire for such strenuous outings that made him decide to be a geologist.
In 1930, Wilson graduated from the University of Toronto—the same university Andrew Lawson had graduated from nearly 50 years earlier—and accepted a job with the Canadian Geological Survey—again, as Lawson had after his graduation. Wilson’s first job was to prepare a geologic map of southern Nova Scotia, an assignment that was filled with adventure. On one occasion, when he was in a canoe, he spotted a moose swimming in the water and pulled up to it. He jumped on the moose’s back and rode it a short distance. Another time, when he and his camp mates were short of food, he again saw a moose while he was in a canoe, though this time he hit the animal on the head with an axe, killing it and providing his camp with fresh meat.
He worked for the Canadian Geological Survey for ten years. At one point, he was assigned the task of preparing a geologic map of all of southeastern Canada. That experience, he would say, conditioned his mind: Everywhere he worked and all the data he compiled indicated the Earth’s surface was stable. Seldom did he encounter a fault or a fold. He saw that old rocks, such as those of the Mesozoic Age, lay atop even older rocks, those of the Paleozoic, that were on top of even older rocks of the Precambrian—and all lay in correct sequence with no sign of having been fractured or displaced. From that, Wilson concluded—and for years argued passionately for—the view that, by and large, the Earth’s surface was immobile, that at the largest scale the present outlines and configurations of the continents and ocean basins had not changed in any significant way over the eons of Earth history. It was an idea, Wilson reminded others during debates, that had originated hundreds of years earlier by the most venerated of scientists, Isaac Newton, who suggested the entire Earth, including its outer rim, was rigid. And Wilson saw no geologic evidence to the contrary.
Wilson, of course—as did almost every geologist of the time—passionately discounted any theory to the contrary, which included the one proposed in 1912 by German meteorologist Alfred Wegener that the continents had shifted their positions a significant amount during geologic history, an idea that was later corrupted into the phrase “continental drift.”
Wegener based his theory on a host of observations, including evidence of glaciation in tropical regions, the discovery of identical fossils at widely distant locations—such as Glossopteris, a giant fern-like plant with tongue-shaped leaves that could be found in Africa, South America, Australia, India, and Antarctica—and the existence of similar rock sequences on opposite sides of the Atlantic Ocean. Wilson and others thought these were misinterpretations of the geologic record. Also, Wegener could offer no physical mechanism to explain why the continents should move. As Wilson wrote in 1949, “Continental drift is without cause or a physical theory.” Later, he would recant that statement and confess that at that time he had been “too stupid to accept, until I was fifty” the idea that the Earth’s surface was, indeed, highly mobile. But that was years in the future. Throughout most of his career, he maintained that the Earth was rigid and strong, and that the surface did not slide any appreciable amount.
By the 1950s, now a professor at the University of Toronto and having become somewhat of a scientific gadfly, Wilson began to temper his stance for a rigid Earth and conceded that the cooling of a hot interior would have caused the planet to contract, giving rise to mountain ranges and island chains. By 1960 he modified his view again, this time arguing that the recent detailed mapping of the mid-ocean ridge—which was now known to extend beyond the north Atlantic Ocean as a long continuous line of mountains that snakes along the ocean bottom like the seams of a baseball—showed that this was the place where the Earth’s surface was being pulled apart, driven by a general expansion of the entire plate. He suggested that the reason for the expansion—which was counter to his earlier claim that the planet was contracting—was either because the Earth’s interior had a high concentration of radioactive elements and thus the interior was getting hotter, or because the force of gravity was decreasing. Neither explanation is true, so it was fortunate Wilson’s pursuit of expansionism was short-lived.
Just a year later, in 1961, he dramatically changed his idea about the planet and its global dynamics yet again. By then, a host of new discoveries—many of them made during the International Geophysical Year*—had been announced and debated and confirmed. For example, it was now known that the deepest part of an ocean is not in the middle, but along the edges at narrow trenches; that the magnetic poles had apparently wandered greatly; and that ocean basins were much younger than continents. These seemingly disparate observations—and many others—forced Wilson to reconsider what he thought he knew about the Earth. The man whose groundbreaking insights caused Wilson’s reevaluation was Harry Hess of Princeton University.
It is fair to say that Hess had a strange fixation on rocks, as his wife, Annette, could attest. On the only vacation they ever intentionally took away from geology, which was a honeymoon on Nantucket Island, she would remember the island as having “only one rock, and that was brought in as a monument.” She also remembered that her new husband “used to look longingly at it.”
Hess was a quiet, unpretentious man always sporting a small mustache and always toying with a lit cigarette. His office was one of legendary clutter, filled with bathymetric charts that he had assembled during scientific cruises. He had an uncanny ability to assimilate widely disparate observations into a theoretical whole, as he displayed in 1962, when he proposed a new idea about the origin of the seafloor, an idea so radical that instead of calling it “science,” he referred to it as “geopoetry.”
The idea was this: Material from the Earth’s interior rose along the mid-ocean ridge, causing the ocean basins to spread apart. A case in point was the northern Atlantic Ocean, where Hess and others had conducted oceanographic cruises and shown that the entire Atlantic Ocean was spreading apart. The spreading explained why the coastal outline of Brazil in South America fits neatly against the west coast of Africa, a fact long noticed by cartographers but, until now, never satisfactorily explained. Overall, the seafloor moves away from the mid-ocean ridge like the top of a conveyor belt. And if seafloor is being created along a ridge, Hess reasoned, it must be consumed somewhere—and he pointed to deep ocean trenches, which he described as “jaw crushers,” as the places where seafloor was slipping back into the Earth.
If true, it was a revolutionary idea. But how could it be proved that the seafloor was sliding slowly and steadily like the top of a conveyor belt? Wilson knew of a way.
He thought back to the afternoon he had stood on Mauna Loa. Conventional scientific wisdom held that the long chain of islands and shoals had formed simultaneously and the volcanic fires had ended sequentially from northwest to southeast, accounting for the current activity at the southeastern end at Mauna Loa and Kilauea. But if the seafloor moved, and if there was a single source of magma, then the chain of islands would not only show an age progression but new islands were still being created. Wilson explained it this way: Imagine lying on your back at the bottom of a shallow stream, blowing bubbles to the surface through a straw. The bursting bubbles are the Hawaiian Islands, and they lie in a line because they were swept along the surface of the moving stream.
Th
e older volcanoes, he predicted, would lie farther from the source. And that was exactly what was revealed a decade later when radiometric age dating was applied to Hawaiian rocks, but that was still in the future. For his purpose, Wilson had to rely on a less direct argument.
He knew the Hawaiian Islands are not the only long chain of Pacific islands that show an apparent age progression. The Austral, Society, and Tuamotos also have active volcanoes at the southeast ends and the islands to the northwest appear to be progressively older.
He wrote this idea in a paper and submitted it to a leading scientific journal in the United States. It was rejected on the grounds that the idea of age progression in the origin of the Hawaiian Islands was at odds with everything else currently known about the geology of the major islands—that is, Kauai, Oahu, Molokai, Maui, and Hawaii. Undeterred, he sent it to a leading journal in Canada and it was published immediately.
Publication, however, does not insure interest, and his paper, “A Possible Origin of the Hawaiian Islands,” was ignored, at least for now. Today it is regarded as one of the pioneering papers—one of the crucial steps—that led to a revolution in understanding internal Earth dynamics. It was the first time anyone had pointed to evidence, no matter how crude or circumstantial, that the Earth’s surface was highly mobile, that a vast area—in this case the entire floor of the Pacific basin, one-third of the Earth’s surface—was moving as a single, coherent, gigantic block over distances of many hundreds of miles.
But Wilson realized that a key element was still missing. If one looked at any of the new detailed bathymetric maps of the Pacific basin—such as the one prepared in 1963 by Gleb Udintsev of the Soviet Union’s Institute of Oceanology, which shows how crucial international cooperation was in the 1960s in arriving at a theory of global dynamics despite the Cold War—one could see in the southeastern Pacific a mid-ocean ridge known as the East Pacific Rise, where, according to Hess, seafloor was being created, and along the west and north edges of the Pacific basin, running along the edge of the Mariana Islands, Japan, and the Aleutian Islands, deep trenches where the seafloor was being consumed. But what was happening elsewhere along the edge of the Pacific? Specifically, what was happening for hundreds of miles along the California coast where there was no mid-ocean ridge and no trench?
It was here, Wilson realized, that the final crucial piece to the global puzzle would be found. And to do so, he would have to explain the San Andreas Fault.
In the fall of 1964, Wilson arrived at Cambridge University in England to begin a one-year collaboration with British scientists who were trying to make sense of the huge volumes of data being accumulated about the seafloor. Little is known about the way he spent his first few months in England except that immediately after the new year, he went on vacation.
He hired a yacht and went sailing with his wife and two daughters on the south coast of Turkey. Whether he completely blanked his mind and relaxed on the trip or concentrated intently on the implications of seafloor spreading and the origin of Pacific islands is not known. But after a month, when he returned to Cambridge, he was brimming with a new idea.
John Dewey, a lecturer in structural geology at Cambridge, remembers Wilson bursting into his office at the Sedgwick Museum. “Dewey,” Wilson exclaimed, “I have just discovered a new class of fault.”
“Rubbish,” Dewey responded. “We know about the geometry and kinematics of every kind of fault known to mankind.”
Wilson then grinned and produced a simple model he had cut from colored paper. It consisted of two L-shaped pieces hooked together like a pair of hockey sticks. He slid the pieces back and forth so that the inside edge of one L-shaped piece slid against the inside edge of the other. He explained that where the two pieces of paper were sliding was a new type of fault, a transform fault, because it transformed the spreading movement of one mid-ocean ridge to the spreading movement of an adjacent, offset ridge.
“I was transfixed,” Dewey remembered of the moment, “both by the realization that I was seeing something profoundly new and important, and by the fact that I was talking to a very clever and original man.”
To put this in context, for the last few years scientists had looked at maps of the seafloor and seen the mid-ocean ridge that runs along the center of the Atlantic Ocean. The ridge actually consists of straight segments offset by faults, sometimes for hundreds of miles. The favored interpretation was that the faults were evidence that the entire ocean was being torn open, from coast to coast. But Wilson disagreed. The faults were evidence of tearing, but only between adjacent ridge segments. This new view suggested that the opening of an ocean basin was concentrated at the ridge and along the connecting faults—the transform faults. The rest of the seafloor simply drifted along as a rigid block until it was consumed at a deep trench.
Wilson gave a name to these rigid blocks; he called them “plates.” Most of the floor of the Pacific Ocean, from the East Pacific Rise to the deep trenches, is a plate—the Pacific plate. From the eastern edge of the Pacific plate to the mid-ocean ridge in the North Atlantic Ocean is the North American plate. In all, Wilson suggested, there were a dozen or so major plates and many minor ones, each one sliding slowly across the Earth’s surface. And the places where they met—the plate boundaries—were the most geologically active regions of the planet.
Suddenly, much became clear—at least to Wilson. If one returns to Mallet’s map of 1857 that showed the major seismic zones of the Earth, these zones, which lie mostly along the edge of the Pacific and across southern Asia and into the Mediterranean, identify some of Wilson’s plate boundaries. If one examines modern maps of worldwide seismicity, plate boundaries are defined in great detail. And those boundaries are of three types: the two suggested by Hess, spreading at mid-ocean ridges and the “jaw crushers” at deep trenches—the word “subduction” would not be used until 1969—and the transform faults suggested by Wilson.
But in 1965, while Wilson was still at Cambridge, the idea of moving plates was not yet accepted by most members of the geological community. So he needed a test.
After studying several maps, including the Udintsev map, which he had pasted to the wall of his office at Cambridge, he finally realized that the San Andreas must be a major transform fault system. He knew of Hill and Dibblee’s claim of hundreds of miles of horizontal displacement along the fault—though once, when Wilson attended one of Hill’s presentations, he had immediately dismissed the idea—and it was their claim that directed his attention to the San Andreas. More important, the south end of the fault seemed to connect with the East Pacific Rise, which oceanographers had traced as far north as the west coast of Mexico. But what was at the north end of the fault?
Hess happened to be visiting Cambridge in 1965, and he and Wilson, along with young British researcher Frederick Vine, were in Wilson’s office one morning discussing the San Andreas Fault. Vine’s specialty was the study of magnetic field anomalies on the seafloor, and he and others had shown a few years earlier that strong magnetic anomalies were always associated with the mid-ocean ridge.
“Look, there should be a ridge here,” said Wilson, pointing to a map where the north end of the San Andreas Fault projected into the ocean.
“Well, if you’re going to put a ridge there, then there ought to be some magnetic expression,” responded Hess. Vine then hustled to the library to retrieve the appropriate map that would show the magnetic field.
Lo and behold, as Vine would recall, when they unrolled the map there it was: a linear magnetic anomaly where Wilson had indicated the crest of a mid-ocean ridge should be. Vine later admitted he stood there amazed. The map had been in the library for four years, yet no one had ever noticed so prominent a feature. Later, he and Wilson would give it a name—the Juan de Fuca Ridge, named after the nearby strait that separates Vancouver Island and Washington State.
That act of discovery elevated the San Andreas Fault from a featur
e of local interest to one of global importance. Ever since—as almost every geologic textbook now proclaims, and almost every speaker who has ever made a presentation about faults and global dynamics asserts—the San Andreas Fault has been known as a plate boundary, the line where the immense mass of the Pacific plate grinds against the equally formidable mass of the North American plate. And as a result, earthquakes must be a common occurrence in California, which they were.
But Wilson realized something else. If tectonic places could slide for hundreds or perhaps thousands of miles, then by necessity plate boundaries had to evolve. And if plate boundaries evolved, then a feature such as the San Andreas must have had a beginning. Soon after Wilson made his suggestion that the San Andreas Fault was a plate boundary, he and others engaged in finding an answer to this additional question—and they figured out how the fault had originated.
An abundance of paleontological, paleomagnetic, lithologic, and paleoglacial evidence—offered first by Wegener then by others, which led to the discovery of the mid-oceanic ridge, magnetic lineations, and additional fossil discoveries—points to the existence, a very long time ago, of a single large continental mass—a supercontinent—that scientists now refer to as Pangaea.
Inevitably, because the forces that drive the movement of plates today were in existence back then, Pangaea broke up. It first fractured along an east-west line and the two giant parts drifted apart, forming the Tethys Sea, a large ancestor of the Mediterranean Sea. Another fracture split Pangaea along a north-south line and the large fragments that drifted to the west became the two American continents while those that drifted east became Eurasia and Africa. If one wonders what the splitting and separation of the continents looked like during an early stage, one has only to examine the Red Sea, where, running along the seafloor, is a young mid-ocean ridge that is driving Africa and the Arabian Peninsula away from each other—the ridge defined, in part, by a line of persistent earthquake activity.