When Kornprobst was a graduate student at the Sorbonne in Paris in the late 1950s, his professors used to save their highest ridicule for two theories: that the continents drift and that the magnetic poles reverse. Today, Kornprobst said, craning his neck to point up a hill behind a Citroën car dealership in the town of Boisséjour near Clermont-Ferrand, both are geological gospel.
The hill was the site of another piece of the Brunhes puzzle. Shortly after assuming the directorship of the observatory in Clermont-Ferrand, Brunhes had read three key scientific papers that had launched his quest to understand more about magnetism. Like other scientists of his generation, he knew that the magnetic field was shifty. It was thought to be composed of three parts: declination, inclination, and strength. (Today, they are compressed into direction and strength, but declination is still noted on any high-quality map.) Any point on the planet could be described by those three magnetic coordinates. It was more complex than the typical two-dimensional geographic mapping coordinates of latitude and longitude. The problem was, magnetic coordinates changed slightly over time.
Declination is straightforward. Compasses point home to the magnetic pole along magnetic lines of force that converge at the poles. But beginning in the eleventh century, the Chinese scientist Shen Kuo realized that the spot the compass points to is different from the Earth’s geographical pole. The geographic North Pole, for example, can be found directly underneath the North Star. Shen wrote about this in his 1088 chronicle Meng Chhi Pi Than (Dream Pool Essays). By the early fifteenth century, European mariners knew this too. The angle of difference between the geographic pole and the magnetic line of force is known as declination. By convention, if you are east of geographic north, your declination is positive; west is negative. Declination changes depending on where you are on Earth. It also changes if you stay put but measure it over time. Not only do the magnetic poles themselves shift around, but the magnetic field lines that the compass responds to do too. It’s counterintuitive, especially if you’ve done those magnetic experiments with iron filings shaken onto a piece of white paper laid over a bar magnet. The filings arrange themselves with spooky precision along the magnet’s lines of force, intersecting at the poles. But the Earth’s field lines are not tidy like the ones on the white paper. They are stretchy and prone to wild distortions. Over decades, those changes could be dramatic. For example, modern reconstructions show that declination in London in 1653 was positive. By 1669, it was negative. After gyrating greatly in the ensuing years, by 2018 it looks headed to become positive again.
Inclination, or magnetic dip, was a later discovery. Measurements began sporadically in England in 1576, and the idea gained prominence after the publication in 1581 of The Newe Attractive, a pamphlet written by the Elizabethan mariner Robert Norman. After having spent a couple of sharp-eyed decades at sea, Norman became a master instrument maker in London. His revolutionary work on magnetic dip stemmed from years of experimentation with the sea compasses he was making. He discovered that if you have a compass needle moving freely in a sphere and you point it at the horizon, it will be pulled either up or down by the Earth’s magnetic force, depending on where you are compared to the magnetic equator. In London, Norman measured the dip at an angle of 71 degrees, 50 minutes. At the equator it doesn’t dip at all, remaining horizontal. At what we call today’s North Pole, it will point straight down, and at the South Pole straight up.
The fact that things changed so much implied that the Earth’s magnetic force was a vast entity moving to its own inscrutable rhythms. Scientists of the nineteenth century were determined to crack the code. One goal was to measure regional variations in magnetic fields over time, to reconstruct the Earth’s magnetic life back through the centuries. That’s where Brunhes entered the picture.
The first of the three critical papers that caught Brunhes’s attention was by Macedonio Melloni, an Italian physicist who founded the Vesuvius Observatory just outside Naples in 1848, the same year revolutions were bubbling over in cities around Europe. That meteorological observatory was set up in part to monitor the activity of Mount Vesuvius, the volcano that had erupted in 79 CE, destroying the Roman settlements of Pompeii and Herculaneum, killing at least 1,000. Today Vesuvius is one of the few active volcanoes in Europe. Melloni selected the observatory’s site, designed its building, and chose the instruments. But months after it opened, Melloni was fired and only just avoided being banished from Naples. He was caught up in a mass ouster of academics following an insurrection against King Ferdinand II of Naples. A military letter recently uncovered in the Naples state archives disclosed that political leaders had deemed Melloni “bad” because he was friendly with some of Europe’s ultra-liberal thinkers and radicals. Among them were famous scientists, including the great British physicist Michael Faraday.
Melloni was no stranger to bitter scientific controversy. He had been drummed out of Paris’s scientific circles earlier in his career for findings that linked heat and light. By 1834, that same work captivated London and Melloni became one of the most celebrated physicists in Europe.
After being fired from the Vesuvius Observatory for being a radical liberal, he took himself to Portici, just outside Naples, and reignited an earlier interest in measuring the magnetism of volcanic rocks from Italy and Iceland. Melloni had developed a simple process reminiscent of the early Chinese compasses: a pair of needles, nine centimeters in length, magnetized and hung one above the other by silk thread. When he passed a piece of lava near the upper one, he could measure whether it attracted the needle away from the orientation of the lower needle, and by how much.
He found that all the lavas made the needle move. And he went on to make a bold contention: The lavas had captured the precise magnetic coordinates of the spot on the Earth where they were when they cooled. The idea was that a piece of lava laid down in Italy would have different magnetic coordinates from one in Bolivia. We could say that the electrons had developed a magnetic memory, aligning in a sort of magnetic fingerprint that helped identify the declination and inclination and intensity of where they were located.
Melloni went further. In his lab, he heated lava rocks until they were red hot, at which point they lost their original magnetic memory. When they cooled, they acquired a new one. The flaw in his research was that he didn’t systematically determine whether a batch of lava showed the same clear magnetic orientation across its flow. His results were fascinating but not conclusive. He died in a cholera epidemic in 1854.
By 1899, Giuseppe Folgheraiter had taken Melloni’s findings to another level. This was the second key paper Brunhes read. Based in Rome, Folgheraiter examined archeologically dated terracotta clay pots from ancient Greek, Roman, and Etruscan civilizations and found that they retained a strong magnetic orientation, even over many centuries. He surmised that they held the coordinates of the magnetic field from the time they were baked.
The third important paper was by Pierre Curie, the French physicist who went on to win the Nobel Prize with his wife, Marie Curie, and Henri Becquerel for their work on radioactive substances. Pierre Curie discovered in 1895 that any solid heated to a high enough temperature loses its magnetic properties. The temperature depends on the material and is in the hundreds of degrees Celsius. In a nutshell, the unpaired electrons become excited and confused by all that heat and refuse to line up in the same direction. In certain and relatively rare materials like terracotta and lava, when the atoms cool down again, their unpaired electrons line up once more in a field, taking on the coordinates of whatever field they are in at the time. The temperature that suspends their magnetism is known as the Curie point, and today is an undisputed rule of physics.
Brunhes, sitting in his Rabanesse tower a few years later, put all these pieces together. He was in the perfect place to do it. There he was, nestled in the remains of a string of ancient volcanoes in central France. Obviously, there had been hot lava. Some places also had natural terracotta laid down in a layered sedimentary bed. Terracotta contains iron-based molecules, and Folgheraiter had shown that it retained a magnetic signature. What Brunhes needed was an undisturbed seam of terracotta that had been heated up when lava poured over it, Kornprobst explained to me. It was hard to find. But there was some in Boisséjour, near Clermont-Ferrand. So he traveled there, likely by mule, where the Gravenoire volcano had erupted 60,000 years earlier, and collected a few samples.
One of the car salesmen came out to see what Kornprobst was doing. Kornprobst explained that he was retracing the steps of a famous French physicist who had hacked a small piece out of this hill a century before. The salesman shrugged and went back inside the dealership. In the end, the tiny cube of terracotta clay Brunhes cut out of the hill in Boisséjour didn’t tell him much. But, more determined than ever to find more magnetic evidence, he set about trying to find a much better volcanic site.