IT WAS 1967, and a profound shift in our understanding of the planet was taking place. In San Francisco, hippies gathered to celebrate counterculture and jolt our social consciousness. Meanwhile, in the far south of California, a young geologist was working on an idea that would cause as profound a revolution in Earth science as the discovery of DNA in biology. Dan McKenzie spent the Summer of Love figuring out the theory of plate tectonics.
For decades, suspicion had been growing that Earth’s surface wasn’t static, that the continents could and did move. In the early 1900s, Alfred Wegener proposed that continents drifted like giant icebergs floating on the ocean. It certainly looked as if they did, since the outlines of Africa and South America, and their rock types and fossils, matched up in ways that suggested the two continents had once been joined.
But Wegener couldn’t say why continents moved – so few people took his idea seriously. The fact remained, however, that certain things taking place on the surface of the planet just didn’t make sense. Volcanic eruptions were a mystery, earthquakes even more so. And no one quite knew why mountain ranges formed.
Advertisement
In many ways, it wasn’t surprising that people didn’t understand what was happening, since there was so little to go on. In the early 20th century, there was no seismology, no accurate location data for earthquakes and no detailed knowledge of the sea floor. That all changed after the second world war.
In the late 1940s, researchers set out on ocean-mapping expeditions that led to the discovery of a global system of oceanic ridges. They had curious features. Running the length of the mid-Atlantic ridge, for instance, was a rift valley flanked by vast mountain ranges. The idea that some kind of movement might be taking place began to take hold.
As the cold war set in, seismometers were deployed to detect underground nuclear tests. They also spotted earthquakes – in specific areas, including along the mid-ocean ridges. Magnetometers designed to track submarines found patterns of magnetism in the rocks along the mid-Atlantic ridge that suggested crust was forming there and spreading outwards.
These findings demanded explanation. If more of this outer layer of the planet was being created, was this being balanced by destruction elsewhere? The earthquakes showed that dynamic processes were affecting parts of Earth’s surface, which geologists began to refer to as plates, but exactly how remained unclear.
At the Scripps Institution of Oceanography in La Jolla, California, McKenzie began pondering how plates might move. At the time, everyone was thinking about what happened at the margins of plates; no one had worked out how entire plates might move relative to each other.
McKenzie had a stroke of insight: the plates were rigid. “The crucial idea was that the interior of plates did not deform internally,” he says. It doesn’t seem much, but by treating plates as rigid, it was now possible to think of them as geometric figures, like paving stones on a sphere. He and his colleague Bob Parker realised that the relative motion between two plates could be described by drawing a circle on the planet’s surface whose axis goes through its centre – an idea that dates back to an 18th-century theorem by the Swiss mathematician Leonhard Euler. This allowed the pair to calculate the relative motion of plates in the North Pacific. They found it tallied exactly with earthquake activity in the region, showing that their calculations were correct and that this movement must not only be happening but also be the likely cause of the earthquakes.
It was the last piece of the puzzle. Suddenly, they could see not just that plates shifted, but also that each was moving relative to the other. The entire surface of Earth was covered in rigid plates jammed up against each other, yet still in motion. McKenzie and Parker submitted their work to Nature, knowing they had hit on something big. As it happened, they weren’t the only ones.
They say every idea has its time, and that was certainly true for plate tectonics. Unknown to McKenzie, Jason Morgan at Princeton University had arrived at exactly the same conclusion and had given a talk on the subject early on in 1967. Morgan could even identify three types of plate boundary: the ridges where new crust is formed, trenches where crust disappears and faults where crust neither disappears nor appears. The idea of plate tectonics was born.
Historians accord ownership of the theory to both McKenzie and Morgan, who is now a visiting scholar at Harvard University. But McKenzie says Morgan has priority. “There’s no question about that at all,” he says. “Morgan talked about it before I had even thought about it. It’s just I didn’t know about that.”
“The impact was immediate and profound. Geology was completely transformed”
What is indisputable is that geology was transformed in 1967. “The discovery of plate tectonics completely and profoundly changed the subject,” says Nicky White, a geologist at the University of Cambridge. Earthquakes happen when pressure that’s building up between moving plates is suddenly released. Volcanoes appear when magma from below Earth’s upper mantle rises up at plate boundaries, and mountains form when one plate inexorably crunches into another. The idea of plate tectonics is so obvious in hindsight that we forget it is only 50 years old.
The discovery rippled through geology with incredible speed. “The impact was immediate and profound to the extent that I was being taught about it in primary school in Dublin by 1973, which is amazingly fast penetration,” says White.
McKenzie’s world immediately changed too. “It was astonishing,” he says. “Overnight, I became famous. I went from being a completely unknown, first-year postdoc to somebody who was invited to every international conference.” But, “I didn’t have a job. I was still a postdoc.”
The jobs offers would roll in: a full professorship in Manchester and a grand one in Zurich. But McKenzie chose a more junior position at the University of Cambridge, where he’d studied for his first degree – in physics. “I didn’t really fancy being the youngest whiz kid at somewhere like Zurich,” says McKenzie. “I got married and stayed at Cambridge. And for a bit, we were very poor.”
He kept working on plate tectonics, though, using it to explain the evolution of the Indian Ocean. Other researchers did, too, notably Xavier Le Pichon at the Lamont-Doherty Earth Observatory, who showed that Earth’s surface comprised six major, interacting plates, and that the crust forming at ocean ridges was balanced by losses at subduction zones where one plate slipped beneath another.
But though plate tectonics explained a lot, some mysteries remained. Not all earthquakes and volcanic eruptions happen at plate boundaries for example. We now know that plates are not entirely rigid – and their flexure can lead to intraplate earthquakes. Also, massive plumes of hot magma rising up from the lower mantle can cause volcanic activity, as in Hawaii.
McKenzie is still at Cambridge, working not just on Earth’s crust and upper mantle, but also on the tectonics of Mars and Venus. The focus of terrestrial plate tectonics research has shifted to the dynamics of the plates. Do those sinking at subduction zones stop at the core-mantle boundary, or do they go deeper? Knowing that could be key to understanding the forces driving plate tectonics. “Nobody really understands what’s going on down there. That’s going to be a really big thing,” says McKenzie. And as with the discovery of plate tectonics originally, “it’s now accessible with the technology we have.”
This article appeared in print under the headline “The day the Earth moved”
