The monsoon rains had been falling for two days when the earthquake hit, and the ground of the Shillong Plateau in northeastern India was saturated. By 5:15 on the afternoon of June 12, 1897, when the earth began shaking, the land was so wet that much of it melted away beneath people’s feet, a process geologists refer to as “extensive liquefaction.” Land slid. Bridges sank. Sand boils and mud volcanoes erupted. Every single building in an area about the size of Louisiana was reduced to rubble. A cleft that ran for miles cracked open in the subterranean crustal plate. More than 1,500 people died. Known as the Assam earthquake, it has been estimated at a magnitude of 8.7 and is considered one of the largest in modern history.
A dozen primitive seismographs in Europe captured the movement of the Earth as the crust ruptured, tracking the waves of shocks and aftershocks as they flowed from one side of the planet to the other, through the center. Seismology was experimental at that time and geologists were only beginning to be able to read the story the graph lines could tell. But the Irish geologist Richard Dixon Oldham happened to be in India working with the Geological Survey of India just then. All corners of society dissuaded him from investigating the huge earthquake. They had different priorities, being fixated not on death and destruction but on the Diamond Jubilee celebrations of Queen Victoria, just eleven days away. Nevertheless, Oldham went to the site, looked at the seismographic records, and produced a carefully written report. But he kept thinking about those waves, and by 1906, the year of Brunhes’s paper on the terracotta and the year after Einstein’s on the special theory of relativity, Oldham put out a journal article for which he continues to be remembered: the first description of the internal structure of the Earth based on measured observations.
Oldham’s revelation was to be able to look at the seismographs of the Assam earthquake and separate out two different types of waves—P waves and S waves. Then, he could see that they had traveled at different speeds. P, or primary, waves are the fast ones, moving at thirty times the velocity of sound. S, or secondary, waves are slower. Not only that, but Oldham found that some of the waves had passed straight from one side of the Earth to the other, while others had taken a detour. The only way he could make sense of what he saw in the waves was to deduce that the Earth had a core made of a different material from its surroundings, and that the different material was changing the path of the waves.
At that time, six competing theories about the structure of the inner Earth were in play among geologists, mathematicians, and physicists. Oldham’s finding torpedoed five of them. The inner Earth theorists fell into the same two eighteenth-century factions that had fought over the source of the magma that had erupted from the volcanoes of the Chaîne des Puys in France: the Vulcanists and the Neptunists. At that time the Vulcanists believed that the crust of the Earth had been formed by heat, while the Neptunists thought it was the result of Noah’s flood covering the Earth. While their passionate disagreement ostensibly centered on fire versus water, it really came down to a dispute over how old the Earth was. And that was really about when the Earth would end.
And while the Ussher calculation that the Earth had been born in 4004 BCE was losing credibility as evidence stacked up about the planet’s far older origin, the primary geological textbook for both camps was still the Bible. Geology was theology. That’s why people thought the planet’s time of death was inextricably tied to its time of birth. The Old Testament Book of Genesis was their particular guide to interpret their findings in the planet’s rock record. By the late nineteenth century, the two groups of theorists had turned their attention to the structure of the inner Earth and had become known as the solidists and the fluidists. Among the players in this pitched century-long discussion were Ampère, Davy, and the Irish-Scottish physicist William Thomson, who became Lord Kelvin; the scale of absolute temperature measurement is named after him. A disciple of Faraday, Thomson died in 1907.
Some of the fluidists were convinced that the Earth was filled with a central primitive heat that had melted everything inside, leaving only a thin crust overtop. In that model, volcanoes and earthquakes were a direct conduit to the seething cauldron below. Others in the same bloc said the crust was thick, but still enclosed a bubbling liquid that was a by-product of the formation of the Earth, an “ejectum from the solar furnace.” Still another analysis was that the planet held all three states of matter. Deep inside it was gas, surrounded by liquid and then crusted over by a solid.
And then there was the camp of the hard-boiled egg: The Earth was solid from core to crust. This theory’s most famous proponent was Thomson, who declared that the whole planet must be tougher than steel and immovable within. Otherwise, he argued, “its figure must yield to the distorting forces of the moon and sun.” In other words, any liquid within the Earth would be shaped by the violence of tides, just as the ocean was, throwing the planet out of balance throughout the course of every day. Thomson even delivered a lecture in Baltimore in 1884 on the topic, in which he twirled a raw egg and a hard-boiled one to demonstrate his theory. The raw egg wobbled a great deal; the hard-boiled spun like the Earth. It was good theater, if questionable science. A variation on Thomson’s theory was that the Earth was very nearly solid, with a thin liquid layer just under the crust.
The sixth idea was that the Earth had a thick crust, liquid interior, and solid core. This was closest to what Oldham’s interpretation of the Assam earthquake seismological data supported. The other theories soon withered. The fallout was akin to that from J. J. Thomson’s discovery in 1897 of the electron, the first subatomic particle, for which he got the Nobel Prize in 1906, leading to widespread adoption of the theory of atomism, even by previous skeptics, and Bohr’s model of the atom.
The magnitude of Oldham’s finding is difficult to overstate. Geophysics had evolved from Aristotle’s idea that the Earth was an immutable dullard in a glorious heaven, to Gilbert’s contention that the Earth had a magnetic soul, to the surprising finding in the English garden of John Welles in 1634 that the magnetic field was constantly on the move, to Brunhes’s finding that the entire direction of the field had switched at least once, to Oldham’s charting of the shape of part of the Earth’s interior. Oldham’s finding began to get at the heart of why the magnetic field was so kaleidoscopic. This new information held out the possibility that the magnetic signals that people had been measuring for hundreds of years were a proxy for the architecture and even the strategy locked within that hidden place. Seismometers could finally pierce the crust, allowing scientists to peer within the heart of the planet for the first time. The key was to understand that the speed and direction of the waves contained information about the chemical composition and state of the matter they were traveling through.
Around the time that Oldham was working up his 1906 paper, Inge Lehmann felt her first earthquake. She was in her teens, she recalled, at home in Copenhagen, when the lamp swayed and the floor began to move. She didn’t reveal whether that earthquake, whose epicenter was never discovered, sparked her lifelong love affair with seismic waves. But three decades after Oldham’s great revelation, she published one of the most important discoveries ever made about the composition of the Earth’s deep interior and therefore about how our planet came to be. The Cambridge physicist Sir Harold Jeffreys had already concluded, in 1929, that because S waves could not pass through the core, it must be completely fluid. It was the first evidence to support Halley’s idea from the late seventeenth century that the core was liquid. It was a huge breakthrough and richly symbolic: The underworld of myth and Old Testament was now laid bare. Jeffreys, writing to Lehmann, then a seismologist in Copenhagen, about the reaction of his American colleague, the Jesuit priest James Macelwane, said: “I should have thought a good Jesuit would have jumped at the discovery of hell, but he reacts all wrong.”
But then Lehmann took a closer look at the seismic shocks that traveled through the Earth and saw a slightly different picture. There was a discrepancy in the waves that could only be explained if Jeffreys’s liquid core had another core nestled within it, somehow different from what surrounded it. Famously, the name of Lehmann’s 1936 paper that explained the idea was simply “P’,” after the P waves her seismometer was reading. (P’ represents the type of P wave that passes through the mantle into the core and then into the mantle again.)
The tale of Lehmann’s discovery is another confluence of unlikely events that characterize so much of the exploration of the Earth’s electromagnetic field and interior. The only woman in the emerging international field of seismology, she was born in 1888, the child of an eminent Danish family that included artists, politicians, scientists, and a surgeon. Her father, Alfred, was a professor of psychology at the University of Copenhagen who launched the practice of experimental psychology in Denmark. He was so immersed in his work that his family only saw him when they ate together and occasionally when he took them on walks on Sundays. Lehmann’s parents sent her to one of the first coeducational schools in Denmark, run by Hanna Adler, whose sister was Niels Bohr’s mother. Bohr, who was three years older than Lehmann, occasionally taught there. Adler, one of the first women to get a university degree in physics, famously traveled around the United States gaining entrée into the best society by trading on her ability to explain Maxwell’s equations, then new. It was similar to the tack Ørsted had taken to get into influential company in Europe at the beginning of the 1800s by carting around a brand-new voltaic pile.
In what turned out to be a boon for the field of geophysics, Adler not only believed in educating girls and boys together, but she also believed in treating them as equals. Each of her students, male and female, studied academic subjects as well as woodworking, soccer, and needlepoint. Lehmann, who died at age 104 in 1993, wrote later in life that Adler recognized no difference in the intellectual ability of boys and girls. Neither did the teachers she hired. Lehmann loved mathematics, and as a treat, her math teacher gave her tougher problems to solve, much to her parents’ dismay. They felt she was too weak to take on the extra work. Lehmann later wrote that she had simply been bored.
Lehmann hit up hard against a different philosophy from Adler’s when she entered Newnham College at the University of Cambridge in 1910 after a stint at the University of Copenhagen. At Cambridge, Lehmann experienced “severe restrictions” on her movements as a woman, she later wrote, “restrictions completely foreign to a girl who had moved freely amongst boys and young men at home.” And while Newnham College was established for female students, the university itself didn’t allow women to earn degrees until 1948, Cambridge being the last university in the United Kingdom to do so.
Lehmann had a breakdown in 1911, which has been put down to too much work. She returned to Copenhagen, honing her mathematical skills in an actuarial office, where she calculated risks of death for insurance policies. At the age of thirty-two, she finally got the equivalent of an advanced degree in physical sciences and mathematics from the University of Copenhagen. She remained in actuarial work for a couple of years until she happened on the geophysicist Niels Erik Nørlund, director of the Danish geodetic institution Den Danske Gradmaaling. (Nørlund was married to the sister of Niels Bohr. Denmark’s intelligentsia was small and well connected, then as now, and Bohr, the superstar physicist, was the node around which some of it rotated.) Nørlund recognized Lehmann’s mathematical genius and in 1925 asked her to become his assistant and to set up a network of seismological observatories in Denmark and Greenland. She had never seen a seismograph before and taught herself how to interpret its squiggles before finally being sent on a three-month training trip to study with European experts. Seismology became her passion.
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!