To drive the countryside with Kornprobst was to learn to read the tale of the Earth’s tormented dramas and fiery convulsions, its traumatic passage through time. After decades of practice, it came as easily to him as breathing. “Basement! Three hundred million years old!” he declared as he maneuvered down a narrow road, pointing to his right at a fragment of the continental crust. A little farther, gesturing to something that looked precisely the same as everything else around it: “A small volcano from fifteen million years ago. Only its chimney is left.” Farther still: “Basalt. Ten thousand years.” Two beats later: “Lava. Forty thousand years.” Then: “Basalt!”
We were on our way to the outskirts of the tiny village of Pont Farin, perhaps a two-hour drive from Clermont-Ferrand, to see if we could find Brunhes’s seam of terracotta. The snow had melted overnight. The damp chill had lifted. The sky was clear cerulean blue and the fields of central France were waking from winter, a few green patches standing out here and there among the tidy rectangular farms. Just weeks later, these fields would be sown with corn, potatoes, and wheat, nurtured by the rich black soil that is the legacy of this area’s ancient string of volcanoes. Volcanoes have fed the livelihood of every generation that has settled here, with crops, pastures, thick creams, crumbly cheeses, tannic wines, and eventually industries that relied on stream water made pure by the exquisite filter of the rocks.
More than a century earlier, Brunhes had put out feelers to the road engineers of this rural heart of France, asking them to be on the lookout for a formation of sedimentary terracotta that had been covered over with hot lava from an ancient volcano. Like so many other geophysicists of his day, Brunhes was trying to find rocks that had lost and then gained a magnetic fingerprint after being superheated with lava. According to Melloni’s findings, Folgheraiter’s conclusions, and Curie’s rule, terracotta in that formation ought to have the same magnetic orientation as the material that later spilled on top of it, the lava or basalt, a fine-grained black rock. The larger goal was for physicists to try to track long-term magnetic variations through rock samples taken from different parts of the world, reconstructing the evolution of the Earth’s magnetic field over time and trying to work out what could possibly cause it to change.
And then one day, one of Brunhes’s friends, a Monsieur Vinay—history has not recorded his first name—an engineer with the road and bridge works administration, told him about a new road he had helped excavate near Pont Farin, or Pontfarein, as it was called then. The construction of the road exposed exactly the configuration Brunhes was looking for: a long seam of terracotta covered with basalt. Brunhes packed his chisels, got on his horse, and set off.
Kornprobst was zooming down the Giscard d’Estaing autoroute, named after the former French president, heading south. He had his maps out. The road cut was going to be tricky to find. Even Pont Farin was not obvious, tucked away on a loop of rural road that Google Maps barely notices. I could see that he was a little nervous. He speeded up, darted over, passed a string of cars, and then zipped back into the right-hand lane. Another car honked at him angrily and then whipped past. Kornprobst took his right hand from the gearshift, held it up in front of the rearview mirror, palm facing in, and gave a dismissive wave, mouth set in a line of determined nonchalance.
He scanned the landscape. To a geophysicist, coming to this part of France, known as the Cantal, was like reminiscing with a cherished friend over a sumptuous dinner. The autoroute cut through layers of sedimentary rock laid down 30 million years ago during the Oligocene, the epoch when truly archaic animals gave way to some we would recognize today: elephants, pigs, horses, and apes. All that sediment lay on top of a hard granitic basement, forged in heat long ago and then cracked in places and thrust to the surface by the gyrations of the crust itself. Underneath all of it were the remains of magma beds that had fed volcanoes much older than the Chaîne des Puys. Formed many millions of years ago, they had spilled such immense quantities of basalt that it formed a lake of the stuff. Basalt is beloved by geologists because it crystallizes in the mantle and is therefore the closest thing to original lava they ever see. Now the volcanoes, once the greatest Europe had ever seen, were worn down to shadows of their former glory by winds, rain, and time. And the lake of basalt had been transformed into a vast fertile plain.
By 10:45 a.m., we were on picturesque back roads, arriving in Les Ternes, a village of six hundred built in tiers up the side of the road. Tidy stacked stone walls lined its few streets. Kornprobst suggested casually that we stop for a coffee at the restaurant that stood below the town’s sixteenth-century castle. Just as casually, he approached two middle-aged men sitting at its splendid bar. One was in blue jeans, the other, camouflage fatigue pants. Did they know Pont Farin?
Mais oui!
Kornprobst listened intently as they described how to get there: up to the top of the village to the road running along a fenced pasture, and bear to the left. He thanked them, and then couldn’t resist. Did they know that a famous physicist discovered just a little way from here that the poles reverse? That north becomes south and south becomes north? That a visitor from overseas—he pointed to me then—was here to write about this very, very famous physicist? The men looked at me, smiled politely, shrugged at Kornprobst, and went back to their drinks.
There were no markings on the road once we found it at the top of the village, just a tiny metal flag on a post naming the road as D57. Again, Kornprobst consulted his map. Yes, yes, we were on the right track, he murmured as we wound our way through the fields. Past more stone fences and low roofs covered with moss. And then, Pont Farin. Proudly, Kornprobst stopped the car and snapped a photograph of me standing in front of the village sign, clutching my notebook.
“Village” may be an overstatement. It had two houses and a stream. But farther on, past the houses, was another yet smaller curve. Kornprobst pulled tentatively onto the bank of the road. Was this it? He hesitated. No. He pulled forward again and then spotted something that looked similar. Here! He was jubilant. He threw open the trunk of the car and retrieved his geologist’s toolkit: a compass in a battered red casing and a hammer.
We set off at a clip down the narrow road. It hugged the side of a hill, a sharp cliff off to the left down to the river far below. This was the road Brunhes’s friend had helped make more than a hundred years before. I couldn’t help but marvel at the fact that it was still there. No village sprawl. No road widenings. No big-box stores or industrial complexes paving over the old site. This part of France has remained largely unchanged for hundreds of years. Birds were chirping merrily. The scent of manure was strong. The sun was finally warm enough to encourage us to shed layers of clothing. We were utterly alone.
Ah! Terracotta! Kornprobst cried, pointing to a sprinkle of brittle red flakes in the pool of a stream trickling down the bank of the hill on our right. We were getting close. The first time he came to find Brunhes’s terracotta, he failed, he told me. He had been expecting a quarry. Instead, it was a nearly untouched outcropping covered by the detritus of a century. I was expecting a sign or some small mark of the scientific revolution that had been wrought in this place. There was nothing. A more forgotten corner of France would be hard to imagine. We had been walking for a while by now. He spotted a house just around a curve and stopped short. Too far. We turned back. Now he was even more intent, stooping down to peer at the bank for the telltale red of the terracotta seam and occasionally banging at it with the hammer.
Suddenly, he leapt across the stream and clambered up the side of the hill. It was strewn with plastic oil containers and discarded wine bottles. Young trees were struggling to survive in the thick layer of soil. He leaned down with the hammer and whacked at a piece of moss. A chunk of terracotta rolled out into his hands. His face cracked into the gleeful smile of a child as he handed it to me.
This jagged piece of rock nestled in the palm of my hand had been laid down right here 10 to 15 million years before, undisturbed until this moment. Five million years ago, a volcano erupted, spilling lava over the expanse of terracotta, heating it up to as much as 700 degrees Celsius, well above its Curie point. Recall that terracotta contains a lot of iron. Each iron atom has four unpaired electrons, and they got so hot when the lava covered them that they lost the magnetic orientation they had held for all those millions of years prior. Then, when they cooled, shortly after the volcano erupted, they realigned themselves with the magnetic field’s direction and intensity of that particular spot on the Earth at that time. Enough unpaired electrons in the terracotta lined up with the Earth’s magnetic field that the rock’s magnetic flow locked into the direction it had at that time. In effect, the terracotta became a crust-bound magnet.
But when Brunhes took his samples from this very seam of terracotta back to his laboratory in the Rabanesse tower in Clermont-Ferrand—he had a lot of trouble getting perfectly shaped samples because the terracotta was so fragile—he found that the magnetic dip, fixed by time and heat into the rock to show the direction of the field it had cooled in, pointed in exactly the opposite direction from what he believed to be north. It was the same story with a piece of the basalt he had broken some chisels on as he extracted it from just above the terracotta.
To him, the conclusion was inescapable: When the terracotta heated up and then cooled underneath its layer of lava, the magnetic north pole had been on the opposite side of the Earth from where it was in France in 1905. He published a paper on his findings in 1906. It was a momentous couple of years for the science of electromagnetism. J. J. Thomson won the Nobel Prize for discovering the electron, the first subatomic particle, in 1906. Albert Einstein published his special theory of relativity in 1905, laying the groundwork for the vast electromagnetic infrastructure that is the central nervous system of modern civilization. It would be another fourteen years after Thomson’s prize until the proton was discovered in 1920 and a dozen more until the neutron was identified in 1932. And it was even longer, until after the Second World War, before the role of the unpaired spinning electron in the phenomenon of magnetism would be fully understood and the science of the reversals of the Earth’s magnetic field became incontrovertible.
But in 1906, the implications of Brunhes’s claim that the poles had once been reversed were so staggering as to make it unbelievable to most scientists. They spent decades deriding it and questioning whether the Earth’s rocks were a reliable record of magnetic memory. They still hadn’t worked out how or why or on what schedule the Earth’s magnetic field’s declination, dip, and strength changed. The idea that the field could reverse direction would mean that they had critically miscalculated the nature of the force itself. And that they had no clue about what made it function in the first place. Apart from being a revolutionary idea, it was an assault on scientific pride. And it was easy to scoff at because the conclusive proof that would eventually convince the scientific world still lay concealed in the Earth’s crust, under the sea and deep within its core.
Brunhes never published on the magnetic field again, although he continued to research it. On Sunday, May 8, 1910, he had just returned to Clermont-Ferrand from a trip to the Cantal, where he had been taking measurements in some mines. The weather was foul, and it was snowing heavily. Despite the snow and his own fatigue, he left the Rabanesse tower near midnight to find out the results of a civic election in a nearby town. Shortly after, police officers making their rounds in the ancient streets near the tower found a man unconscious on the ground. It was Brunhes. They took him home, but he’d had a massive stroke. He died at noon on Tuesday, May 10. He was forty-two.
He did not live to see his grand new observatory, Les Landais, which opened two years later. He did not live to see the vindication of his outrageous theory of reversal, or to discover that the Earth’s magnetic direction had flipped not just once but hundreds of times in its history. He would never know that this current magnetic phase of the Earth’s history is named after him. The man who traveled by mule and horse did not live to see all the new data from satellites orbiting Earth showing that its field is becoming daily more disturbed, and that part of it in the southern hemisphere has already changed direction. He could not have known that a century after his death scientists would be debating whether the poles are gathering strength to reverse once again. Or that the vast electromagnetic infrastructure humans have built will be in danger when the poles switch places. He could never have foreseen that scientists are struggling to understand what such a reversal could mean for life on Earth, the great spinning magnet that is our world, or the likelihood of it being utter catastrophe for human civilization.