When she became chief of the seismological department of the Danish geodetic institution in 1928, she was in charge of interpreting the data from her seismographs and writing up bulletins. Throughout her twenty-five years in that position, she ran the office alone, rarely even having secretarial help. One of the bugaboos of the job was making sure that the Scoresbysund seismological station she had set up in northeastern Greenland was kept staffed. It was so remote that its keeper had contact with the home office just once a year, when a boat showed up. Keepers kept quitting. As for scientific research, that was not part of Lehmann’s job description and was not encouraged. But it was tolerated.
This was no barrier to Lehmann. She was famous for her limitless ability for hard work and for her irritable intelligence. A relative recalls her telling him: “You should know how many incompetent men I had to compete with—in vain.” And she was tenacious, perhaps imperious. A colleague recalled that she was extraordinarily sensitive to noise—another type of wave—and once, at a conference in Zurich with him, persuaded him to swap his quiet downscale hotel room for her expensive one because, despite the cost, her hotel management couldn’t guarantee her that it would be quiet. At age 102, mainly blind but professionally active, she was still going to her summer cottage in Holte, on the outskirts of Copenhagen: “Of course I am in the summerhouse,” she said, offended, when a telephone caller was surprised to find her there.
She insisted that seismograms from different stations be read by the same person, giving a single person a way to track a pulse of waves from station to station. And all the while, she was perfecting what a later seismologist called “a black art”: the ability to listen to the story the shock waves traveling through the Earth were telling her.
Then, on June 7, 1929, an earthquake with the magnitude of 7.8 struck near the small town of Murchison on New Zealand’s south island. Lehmann’s network of observatories registered some P waves in part of the Earth’s interior where they were not expected. She made a bold conjectural leap: What if there were something else inside the liquid core Jeffreys had discovered, something through which waves might travel faster than in the rest of the core?
This was before the time of computers. The calculations to test a theory like hers were done by hand. Lehmann didn’t even have an assistant. Her cousin’s son, Nils Groes, witnessed her technique. One summer Sunday he sat with her in her garden in Copenhagen, watching her sort through cards organized in cardboard oatmeal boxes on a table she had set up on the lawn. Contained on the cards was information about earthquake times, the shapes of the waves they produced, and their velocity. Her conclusion once she’d crunched the numbers after the New Zealand earthquake was that the Earth had a second core nestled within the fluid one. It was a stunning find, missed by all the eminent physicists of the day. Ever cautious, she did not declare that the new part of the Earth was solid, just that it was different. She was such a superb mathematician that she calculated the inner core’s radius at nearly what today’s accepted measurement is: 1,215 kilometers. She called it the “inner” core and promptly wrote to Jeffreys, the king of seismology, to tell him what she’d found—and what he’d missed. He fobbed her off. For four years. Finally, tired of waiting for him to take a look at her data, she published her famous “P’” paper in 1936. Many of the world’s geophysical luminaries accepted the idea immediately, but it took Jeffreys a few years. By 1947, it was included in seismological textbooks.
Lehmann’s finding, and subsequent ones by other researchers that the inner core is solid and that the whole of the core is mainly iron, underpins the development of today’s theory of the geomagnetic field. Seismology remains a critical piece of the scientific efforts to look inside the inner Earth, tracking ever finer details about its architecture, topography, and chemistry. It took up a whole session at the Nantes conference, where seismologists minutely parsed, for example, findings on two big odd zones toward the bottom of the mantle underneath the Atlantic and the Pacific. These zones seem to have sharp edges and may be chemically distinct from the rest of the mantle. Seismic readings suggest they may be made of among the most primordial stuff in the core.
Lehmann, who retired in 1953, became even more prolific after she could stop chasing keepers in Greenland, often traveling to the United States and Canada to collaborate with colleagues. In 1962, Jeffreys wrote to Bohr, asking whether she had ever been recognized for scientific excellence in Denmark. Bohr wrote to Nørlund—his brother-in-law and Lehmann’s former boss—recommending that she receive the gold medal of the Danish Academy of Sciences and Letters. She got it in 1965. More than twenty years later, when she was ninety-nine, Lehmann wrote her final scientific paper. That was just as British and American physicists were learning how to read another set of clues about the inner Earth: the satellite images that could examine what was going on at the boundary between the top of Jeffreys’s liquid core and the bottom of the mantle. But rather than the surprise of a previously unknown architecture, the satellite images were showing the contortions over time of the seat of the Earth’s magnetic power: the molten liquid with its long-limbed gyre and warring factions. Those movements, in turn, determine how strong the Earth’s magnetic shield is and whether the poles are gearing up for a move.
The whole idea that they may be poised to switch again is a far reach from what Brunhes announced in his 1906 paper. His conclusion then was that the poles had at one time been on opposite sides of the planet from where he knew them to be in that year. He refused to go further, saying it was too early to make any attempts to figure out when the reversal had happened. But more than one reversal? Reversals that seem somehow to be a critical component of the dynamo at the heart of the Earth? Reversals that could affect life as we know it? Yikes!
CHAPTER 19
pharaohs, fairies, and a tar-paper shack
Inside the conference center’s auditorium in Nantes, the scientists were struggling. Not with the new findings on the inner workings of the Earth. Or with the elegant math that described them. But with the tiny font size of the print on the screen at the front of the cavernous room. Some of the conference participants were covertly bringing out binoculars. Others were taking photographs with their iPhones and then zooming in on the information using their touchscreens. This is the mind of the scientist: If there are barriers to getting data, you figure out your own way of leaping over them. It was that instinct that drove the scientific community after Brunhes’s paper on reversals in 1906. Skepticism reigned. Had the planet’s magnetic field really reversed direction? And if so, how can we be sure?
Theoretical questions hinged on whether such a dramatic perturbation of the poles was even possible. If it was, what was the mechanism? What was the purpose of a reversal? Could it have happened more than once? Could it even be a recurring feature of the planet’s magnetic landscape?
The practical questions were no less pressing. What if Brunhes’s terracotta didn’t mean what he thought it meant? Modern inquisitors might question whether Brunhes’s finding meant that the poles were stable but the European continent had rotated 180 degrees on the Earth’s surface. But at the beginning of the twentieth century, most geologists thought the continents were fixed, so they were looking for a different explanation for Brunhes’s discovery. What if scientists had a faulty understanding of the way the magnetic memory of rocks worked? What if Brunhes’s chunk of rock had merely been hit by lightning and that had shifted its dip? What if rocks could change their magnetic memory on their own, without any influence from the Earth’s poles?
It was this latter issue that dominated magnetic studies for decades after Brunhes’s paper. If a rock could spontaneously change its record of magnetic coordinates, then the whole idea that the field had reversed would be in question, as would many other aspects of rock magnetism. Before that question could be settled, other findings supporting Brunhes began trickling in from other parts of the world. Scientists had begun their meticulous
job of collecting new data points. The most compelling findings were in a modest three-page paper in Proceedings of the Imperial Academy by the Japanese geologist Motonori Matuyama in 1929. A professor at Kyoto University, Matuyama also studied at the University of Chicago.
Japan is a global volcano hot spot. It sits at the juncture of four tectonic plates along what’s known as the Pacific Ring of Fire. Recently analyzed undersea marine sediments show that volcanoes have been active in the area for 10 million years and that the past 2 million years have been a period of extreme volcanic activity. In other words, the Japanese are keenly interested in what happens under the Earth’s crust, and that island nation has produced some of the world’s most eminent experts in all things inner-Earth. Including lava.
In theory, as Melloni and then Brunhes had reasoned, lava would take on a record of the intensity and direction of the magnetic field from the time and place where it cooled, in effect becoming a sort of sophisticated fossil compass showing inclination, declination, and field strength. So in 1926, Matuyama went looking for ancient basalt in a cave celebrated for that type of rock in Japan. He carefully measured its magnetic coordinates while it was in the cave and then took a sample for later examination. Its field pointed in the exact opposite direction from where the Earth’s field pointed in 1926. Matuyama then embarked on a systematic examination of basalts that had spewed forth from volcanoes over many millions of years in Japan, Korea, and Northeast China, then called Manchuria. His findings were that some of the rocks’ fields were aligned with today’s north and some of them were aligned with the south. Few were aligned anywhere in between.
The south-aligned rocks were from different geological periods: Some were Miocene, meaning they were as much as 23 million years old. Some were from the Quaternary, making them as much as 2.6 million years old. His conclusions were staggering. Not only was there more proof that the poles had reversed, but now there was evidence of more than one reversal. Each appeared to have lasted for a long period. Even more astonishing, Matuyama could put rough dates on when some of those reversals had taken place. All of a sudden, it looked as though geologists might be able to make a clock going back over the Earth’s distant past, describing where the poles had been during each era. It was a new way of seeing the planet, akin to the first maps Edmond Halley had produced showing the wavy contours of declination across the Atlantic Ocean.
One hitch in this analysis was the possibility that rocks could change their own magnetic memory. Through the 1930s and 1940s, this was an intractable problem. Rocks were tricky. Even iron, the standard material for compass needles, could lose its magnetic sensitivity. That’s why sailors in centuries past had carried a lodestone as a “keeper” to keep the iron magnetized. They would stroke it across the compass’s needle every now and again to remagnetize the iron. Geophysicists handled the confusion over spontaneous rock reversals by ignoring pole reversals until they could get more data, a phenomenon the American geophysicist Allan Cox and his colleagues later put down to “the embarrassing lack, even at so late a date, of a theory adequate to account for the present geomagnetic field, let alone reversed magnetic fields which may or may not have existed earlier in the earth’s history.”
One clue to the solution came from the work of Louis Néel, once offered a job at the observatory in Clermont-Ferrand, where Brunhes had worked. Néel eventually went to Grenoble, where he set up that university’s world-famous geophysics program. That’s where Philippe Cardin worked, who gave the public lecture at the conference in Nantes. But in 1931 as Néel was considering a position in Clermont-Ferrand, Brunhes’s legacy in magnetism was on his mind. So was the mystery of precisely how and why a rock retained its magnetic memory. Taking a page out of quantum mechanics, Néel began to question whether every molecule in a substance was magnetized in precisely the same way. What if there were differences? In a series of discoveries that won him the Nobel Prize in 1970, Néel found that there were. In the years following the Second World War, he advanced the concept of ferromagnetism, and, in 1949, discovered ferrimagnetism, which is a related but slightly different phenomenon. In doing so, it’s said that Néel took the magic out of magnetism, because he could finally explain why a material could hold its magnetic charge.
The reason goes back to the unpaired spinning electron.
The electron’s motion makes a tiny circulating current. That, in turn, creates a magnetic field with two poles. In most materials that make up our universe, the magnetic fields of unpaired spinning electrons cancel each other out, so the material doesn’t hold a magnetic charge. It’s a nano zero-sum game. It’s why so few materials retain magnetization over time. Sometimes, though, when electrons are unpaired, they don’t cancel out but reinforce each other by lining up. It’s the opposite of what you’d expect, and that makes these substances odd. When the electrons line up rather than neutralizing each other, the material ends up being magnetized, either for a while or, sometimes, permanently—as long as it doesn’t get heated up past its Curie point. The permanent type is called remanent magnetism, after the Latin word for “remaining.” This can get a lot more complicated. Even the Encyclopedia of Geomagnetism and Paleomagnetism says there are too many types of remanent magnetism for it to review. The type we’re interested in here is the magnetism a rock acquires under natural conditions as it cools. It’s commonly called natural remanent magnetism. In the days since Brunhes, scientists have learned how to strip away from rocks little bits of magnetism that came from other outside sources in order to reveal natural remanent magnetism. Carlo Laj, a French geophysicist, went back to Pont Farin and redid Brunhes’s experiments after stripping away extraneous magnetic influences. His paper, published in 2002, showed that Brunhes’s findings were absolutely correct.
Néel found that there are crucial differences in how the electrons decide to arrange themselves in order to enhance their magnetic fields. The differences determine the tenacity of a material’s magnetic field. In some substances the orbitals where electrons live overlap across atoms. And in some of those cases, when orbitals overlap, the electrons in adjacent atoms are then forced to line up in the same direction. That magnifies the magnetic pull of a material. When that happens, the material is called “ferromagnetic.” The common ferromagnets are iron, nickel, and cobalt and some compounds they are in. The name comes from the Latin word for iron: ferrum. The iron in a compass needle is a ferromagnet.
There’s a catch, though. The enhanced magnetic field within the groups of atoms or molecules is confined to domains, or neighborhoods, within a material. And while the field is strong within that neighborhood, it can be offset by an opposite field in the next neighborhood. So the material as a whole is not necessarily magnetized. That’s why your car keys aren’t magnetized, as a rule. But ferromagnetic materials can be magnetized if you put them in the presence of a strong magnet. The magnet’s power can make the unpaired electrons spin in the same direction, no matter which domain they’re in. That’s how the keepers kept the compass needles working. Stroking the needle with the lodestone made the domains line up. Ferromagnets can keep this strong magnetic charge for a time, but not permanently.
And then there’s more permanent magnetism, like the lodestone. Sometimes the way the atoms line up means that the opposite spins of the electrons don’t fully cancel each other out. Instead, they line up in, you could say, teams of uneven sizes, in alternating rows. One team is spinning in one direction. The other team spins in the opposite. The direction of spin of the bigger team wins out for the material as a whole and the material locks in on its magnetic direction. This is called ferrimagnetism. This arrangement of spins is far more stable than that of the ferromagnetic materials. It’s less apt to be changed or lost. The best example on Earth is the lodestone, the same magnetite that Homer wrote about and that Gilbert experimented on and that first sparked human investigation of magnetism. Magnetite is a type of iron oxide made up of a molecule of three iron atoms—each w
ith four unpaired spinning electrons—connected by four oxygen atoms. It can hold its magnetic charge for millions of years, unless it is heated up past its Curie point. Some rare earth elements are also ferrimagnetic.
Once Néel worked out the difference between the ferros and ferris—in my giddier moments I call them the pharaohs and the fairies—he looked at fine-grained volcanic rocks and found that they often contained enough of the right size of ferrimagnetic grains of iron oxides to bind their magnetic memory for millions of years, unless heated. The same phenomenon holds true in some types of sedimentary rocks, like iron-rich terracottas.
During the same period after the Second World War, John Graham, a keen young geology graduate student at the Carnegie Institute of Washington, DC, launched a series of expeditions to test the magnetism of rocks across the United States. Pictures from the era show a truck made into a roving rock-sampling lab, complete with a spare tire strapped to the hood. Disconcertingly, in light of the European and Asian findings, Graham found rocks in the same layer that seemed to be pointing magnetically in different directions. Could they have reversed themselves spontaneously?
He turned to Néel. Néel, a theoretician, predicted that it was possible and set out several rare scenarios in which it could happen. Supporting his theory, Japanese scientists showed in the laboratory that some lavas from Mount Haruna were susceptible to reversing their own fields, as long as they were cooled at a specific rate and contained a specific chemical composition. Yet Jan Hospers, a graduate student at Cambridge, who examined layers of lava flows from the highly volcanic Iceland, found clear evidence of not just one or two but three reversals of the whole field over time. He concluded in 1951 that “the earth’s magnetization has suffered repeated reversals, and that rock magnetism can be used for geological correlation. . . .”
The Spinning Magnet Page 16
