Back and forth it went. The Earth’s magnetic rock record was reliable. The Earth’s magnetic rock record was tainted. And this was the only tool the geophysicists knew of to determine whether the poles had reversed. They were flummoxed. Dueling theories about how rocks’ orientations were laid down continued for years, even as evidence mounted from around the world that the Earth’s magnetic field had reversed many times. By 1963, a poll of twenty-eight leading paleomagnetic researchers attending a meeting in Munich found only half could support the idea that the poles had switched places, but each of them believed that some rocks could switch their own magnetic field independently.
A year later, in a development that marked a shift in the research away from Europe and toward the Americas, there was more evidence than ever that reversals were part of the planet’s inner makeup. In 1964, Allan Cox, Richard Doell, and Brent Dalrymple of the US Geological Survey in Menlo Park, California, published their landmark paper “Reversals of the Earth’s Magnetic Field” in Science. They had gone looking for the ultimate proof that the rocks could tell the story of what had gone on in the Earth’s core. That meant they needed rocks showing magnetic memory from the same time periods at different places across the Earth’s crust. And that meant knowing with a high degree of precision how old the rocks were. They used a new technique involving the radioactive decay of potassium-40 to argon-40. Today, it’s known as K-Ar dating, after the chemical symbols of the elements. (K is for potassium and Ar is for argon.) By determining how much argon-40 is in a sample compared to the radioactive potassium-40 it would have started with, you can tell how long it’s been since the rock crystallized.
With the help of scientists around the world, they collected sixty-four samples of volcanic rocks from North America, including Hawaii; Europe; and Africa, and analyzed their ages using the K-Ar dating method. At the same time, they looked at the rocks’ magnetic signals. The Geological Survey gave them a small tar-paper shack, where they could work out what it all meant. Their results produced nothing less than the first global magnetic calendar going back 4 million years, describing epochs in the Earth’s history when the poles had been where they are today and others when they had been on opposite sides of the planet. Those early findings already showed some of the peculiarities that we now know characterize reversals: They last a long time in geological terms, and long enough to be captured in the rock record; they are of irregular length; sometimes, the poles try to reverse but fail.
Most intriguingly, Cox’s group pinpointed the last time the poles reversed to 780,000 years ago. That’s before our species, modern humans, was on the Earth. And they decided to name the current epoch in honor of Brunhes. The epoch that preceded this one is called the Matuyama. Others are named after Gauss and Gilbert. Nearly sixty years after his paper on the terracottas of Pont Farin, Brunhes’s contribution to the discipline of geomagnetism was formally acknowledged.
While Cox, Doell, and Dalrymple dealt at length in their paper with the idea that rocks could spontaneously reverse their polarity, they concluded that such events were rare. In fact, they were so rare that they did not negate the robust evidence from around the world supporting the switching of the poles.
The poles really do switch places sometimes.
At last, another element of the Earth’s turbulent past was starting to come into focus. Now, what the magnetic researchers wanted was to piece together reversals going even further back in time, hopefully back to the birth of the planet’s own magnetic field when the Earth was about 1 billion years old, or perhaps even younger. How often had the poles flipped, and were they conspiring to do so again?
CHAPTER 20
zebra skins under the sea
Any disagreements over rock magnetism through the first decades of the twentieth century paled in comparison to the most anguished geological question of that era: Do the continents move?
The idea had roots in the early nineteenth century, when Alexander von Humboldt, who traveled extensively in South America, noted how that continent’s eastern flank would nestle neatly under Africa’s west shoulder if the two were put together. By 1912, the German geophysicist and meteorologist Alfred Wegener gave two public talks that took the idea much further. He proposed that all the continents had once fitted together like a giant jigsaw puzzle, making a supercontinent that had later pulled apart. The Earth’s crust was not fixed; it was malleable. Wegener named the supercontinent Pangaea, after the Greek words pan and Gaia, or “all Mother Earth.” As evidence, he pointed not only to the shapes of the continents, but also to similarities across the modern continents of geological features and species. He even drew a map, which was later disparaged for its lack of precision, showing where the continents might once have been when they formed Pangaea.
Laid up by an injury in the First World War, Wegener refined his ideas and published a book on them in 1915. It was a scientific scandal. Wegener was criticized for the scientific no-no of being a proselytizer for his unorthodox and unpopular idea, which became known as “continental drift.” Protocol demanded that he seek truth, not converts, his critics said. Ostracized, he could not find work at a university in his home country, and finally took a position in Austria. Fifteen years after he published his book, and long before the scandal abated, he died, trapped in a storm while trying to ferry supplies to a meteorological station in Greenland by dogsled. He was fifty.
Why was it such a flash point? Cambridge’s Sir Edward Bullard, one of the British lions of geophysics, who first repudiated Wegener’s ideas and then championed them, wrote about the backlash in a retrospective essay in the 1970s. “There is always a strong inclination for a body of professionals to oppose an unorthodox view. Such a group has a considerable investment in orthodoxy: they have learned to interpret a large body of data in terms of the old view, and they have prepared lectures and perhaps written books with the old background. To think the whole subject through again when one is no longer young is not easy and involves admitting a partially misspent youth.” Until the 1950s, believing in Wegener’s theory of continental drift was “unusual and a little reprehensible,” Bullard wrote.
But then a set of clues supporting the idea began to emerge, rather haltingly. In the early 1950s, Edward “Ted” Irving began working on his PhD in geophysics at Cambridge, focusing on rock magnetism. One of his fellow students was Jan Hospers, who had looked at the Icelandic lavas and had seen reversals. Irving began to study a magnificent stretch of exposed sandstone in the northwest of Scotland. The Torridonian sandstones were hues of red or purple or brown and they ran horizontally for seventy miles along coastal mountains in beds at times 18,000 feet thick. They were untouched, as much as 700 million years old, fine-grained, sprinkled with magnetite and hematite, and strongly magnetized. Irving took four hundred samples.
But when he analyzed them, he found that their magnetic fields pointed to the northwest and southeast, far from the present-day geographic pole. He toyed with the concept that the rocks had reversed their magnetic direction on their own, but began exploring another idea, along with Kenneth Creer, a postgraduate student at Cambridge. They found that the older the rock was, the farther away from the current pole the field pointed. Could it be that the poles wandered across the face of the Earth? They plotted their “paleopoles” on a map charting the possible sites of the magnetic north pole back 700 million years. In 1954, they presented their “polar wander path” at the meeting of the British Association for the Advancement of Science. The idea caught on. An article in Time magazine that year rather excitedly tracked the “north pole’s travels” at 14,000 miles in 700 million years, working out to 1.3 inches a year, complete with a diagram.
This was not the poles switching places during a reversal, which scientists had been considering since Brunhes’s paper in 1906. This was the poles meandering far from the axis on which the planet spins in an apparently determined progression. It was different from the observations of lon
g-term idiosyncrasies in the magnetic field, or secular variation, which saw the poles shifting around, but always in the vicinity of the geographical poles. If the idea that the poles could wander to remote parts of the planet had been true, it would have added another level of mystery to the workings of the inner Earth. Scientists didn’t have a scrap of theory to explain it.
They didn’t need one, as it happened. Irving and Creer never imagined that the poles were doing that type of broad-scale roaming. Even as they were charting their polar wander path, they thought it was far more likely that the poles had stayed put (more or less) and that the rocks themselves had moved, embedded in Scotland, which was moving too. They re-dubbed the phenomenon “apparent polar wander.” Irving realized that what he was tracking was not the changing position of ancient poles but the changing position of ancient latitudes and therefore continents. He had read his Wegener. Using this “apparent polar wander” information, he could retrace the march of the continents over time, each compared to the others. It was like seeing lost worlds spring back to life.
To test his theory, he got blocks of basalt from seven ancient lava flows in India and found that, based on the information contained within their magnetic readings, the Indian continent had moved north by 53 degrees of latitude and rotated counterclockwise by 28 degrees from about the time the dinosaurs died out 65 million years ago. The shocking conclusion from his results, when he put them all together, was not only that the continents had moved but that they had moved immense distances over time.
His ideas were so controversial that when he wrote up his findings for his doctoral thesis, the Cambridge examiners refused to award him the degree. In 1954, Irving took himself off to the Australian National University in Canberra, had a beer for solace with his new boss, dusted himself off, and pressed on, finding ever more evidence for his theory. He ended up in Canada, drawn by his Canadian wife and the allure of the country’s pre-Cambrian shield, where some of the planet’s oldest rocks are exposed.
At the same time, in the years following the Second World War, another set of clues was finding its way into the mix. Geophysicists had begun a more thorough examination of the seafloor. It was a whole new discipline called marine geology, and it involved ships dragging echo sounders, dredgers, and seismographs across the ocean, as well as machines to drill cores in the seabed. It began as a military enterprise by governments wanting tactical information about the shape of the ocean floor. At that time, even some eminent geologists contended that the ocean contained a network of sunken continents that could surface almost at will. Many thought that continents had once been ocean bottoms. Most thought of the seafloor as featureless and barren. The new findings painted a different picture. They said that the bottom of the ocean was not made of the same stuff as the continents. Ocean floor was basalt. It was thinner and far younger than the continents, with nothing more than about 200 million years old. As for the ocean’s hills, they were underwater volcanoes.
Beginning in 1956, some of that deep seabed topography could be visualized for the first time. A team at the Lamont Geological Laboratory at Columbia University in New York (today, it’s the Lamont-Doherty Earth Observatory), under the direction of the eminent geophysicist Maurice “Doc” Ewing (he was a great friend of Inge Lehmann, and her collaborator), had begun collecting tens of thousands of deep-ocean soundings made over many years in journeys across the Atlantic Ocean. Marie Tharp, one of the mathematicians who worked on them, recalled that a neat-handed colleague recorded the data using a crow feather quill pen, writing in India ink on blue linen pages, a document that became the bible of the field. Tharp helped plot the data on physiographic maps that could show the ocean floor’s profile as if it were being spotted from a low-flying airplane. The early, patchy images clearly showed a deep cleft valley slicing between a curving line of mountains on either side. When Tharp showed it to her boss, Bruce Heezen, he groaned, said it looked “too much like continental drift,” and dismissed it as “girl talk.” Nevertheless, Tharp persisted and by 1956 had a convincing map of the Mid-Atlantic Ridge. When Heezen and Ewing presented the map to a meeting in Toronto of the American Geophysical Union that year, scientists reacted with amazement, skepticism, and scorn, Tharp wrote. A mid-ocean ridge representing a seam in the ocean floor where new floor was being created, forcing continents to move across the face of the Earth? Silly girl.
As the 1960s dawned, it occurred to geophysicists on either side of the Atlantic Ocean that they might be able to take magnetic readings of rocks on the seafloor by towing a magnetometer behind a ship. Results began to emerge from the deep ocean, including a batch from the East Pacific Ocean Basin. Intriguingly, they showed bands of magnetic readings, fields pointing in alternating directions, right across the deep seafloor, lining up parallel to the East Pacific Ridge and symmetrical on either side of it. The results were published, but without explanation. No one could understand what they meant.
Enter the Canadian Lawrence Morley, a specialist in rock magnetism with the Geological Survey of Canada. He became obsessed with the findings on the ocean bands, admitting in a later essay that he neglected all his other duties while he tried to figure them out. He was familiar with magnetic readings over broad terrestrial landscapes because he had done them from the air to find petroleum and minerals. On land, they were a mishmash of polarities. They were not in tidy patterns like those on the ocean floor. But he was certain the ocean stripes must be related to remanent magnetism, as those on land were. Then he discovered a 1961 paper describing the evolution of the ocean basins by seafloor spreading.
In a flash, he put three concepts, until then unconnected, together into a single theory: Wegener’s continental drift, seafloor spreading, and reversal of the poles. In his view, the stripes on the ocean floor came from hot magma continually rising from the inner Earth at seams in the Earth’s crust, creating brand-new ocean floor. As the bands cooled in the water past their Curie point, the ferrimagnetic materials in them took on the direction of the magnetic field from that time. They would spread from the ridge, or seam, in the crust, symmetrically on either side toward the continents, laying down precise records of the magnetic field. Over millions of years, as new seafloor was born, it became an archive of pole reversals. If you colored the bands of negative polarity in black, the picture looked like a zebra skin, spreading out from a central meridian: black, white, black, white, and so on.
Morley swiftly wrote up a paper explaining his hypothesis and tried, as he said, “desperately” to get it published. Nature rejected it in February 1963, saying it lacked the space to carry the article. It languished for months on the desk of an anonymous reviewer for the Journal of Geophysical Research before again being rejected in late August. In a note appended to the rejection, the reviewer said the idea was interesting, and added, in a remark so snide that it has been engraved in scientific memory, that it was “more appropriately discussed at a cocktail party than published in a serious scientific journal.” On September 7, 1963, Nature published an article by the Cambridge geophysicists Frederick Vine and Drummond Matthews, who had independently come to the same conclusions as Morley, based on magnetic readings from ocean ridges in the Indian, Atlantic, and Pacific Oceans and the paper on seafloor spreading. The Cambridge team was gracious. Today, the idea is known as the Vine-Matthews-Morley hypothesis. For his part, Morley abandoned rock magnetism and became a pioneer in remote sensing with satellites.
By late 1966, there was a continental divide on the idea of continental drift. Most American geophysicists rejected it, while most Europeans accepted it. Bullard recounted arriving in New York that year at a key symposium on the history of the Earth’s crust. On the first day, Ewing, whose own team had produced the first maps of the mid-ocean ridge system, said to him: “You don’t believe all this rubbish, do you Teddy?”
In the meantime, Cox, Doell, and Dalrymple, who had sweated over their magnetic chronology of the Earth, took a look at the zebr
a stripes now being produced by magnetometers dragged over spreading ocean floor ridges. By then, geophysicists were able to calculate with mathematical models how long it had taken for a stripe to form, meaning that the patterns were not only chronicles of magnetic direction, but also of time. When Cox and his group placed the ridge readings against the timeline they had put together in their 1964 paper, the two correlated. Cox said: “I felt cold chills. This was the most exciting moment of my scientific career.”
Wegener’s idea, combined with the Vine-Matthews-Morley theory, Tharp’s maps, Irving’s work on the shifting latitudes, and Cox’s magnetic clock, led to what became known in 1968 as the theory of plate tectonics. That’s the idea that the Earth’s crust moves slowly across the mantle on about twenty plates of different sizes, some huge, others small. Some parts of the seabed floor are spreading at ridges or rifts; others are being consumed, or subducted, underneath one another at trenches. It’s the ultimate in recycling: Old floor gets destroyed along one plate boundary, and at another, new floor gets made. Moving plates can also cause continents to collide. That’s what happened about 50 million years ago when India and Asia crashed into each other. Rather than one plate sinking under the other, the plates rose up to make the Himalayas. At still other boundaries, the plates move in opposite directions against each other, grating, creating fault zones that are prone to earthquakes. One of the few on land today is the San Andreas fault zone that slashes through California, more or less parallel to the coast.
The Spinning Magnet Page 17
