When we determined how Earth really works

Jonathan Amos elaborates on the significance of the plate tectonics theory and how it explains earth's movements

When we determined how Earth really works

What would you put on your list of the great scientific breakthroughs of the 20th Century? General relativity? Quantum mechanics? Something to do with genetics, perhaps? One discovery that ought to be on everyone’s rundown is plate tectonics — the description of how the rigid outer shell of our planet (its lithosphere) moves and is recycled. This year, the theory celebrates its 50th anniversary.

The truly great ideas in science not only seem brilliantly simple and intuitive when they come into focus, they also then have this extraordinary power to answer so many other questions in nature. Plate tectonics is a perfect example of this. It tells us why the Himalayas are so tall; why Mexico experiences damaging earthquakes; why the monkeys in South America look different from the ones in Africa; and why Antarctica went into a deep freeze. But when you’re on the inside of the bubble, trying to make all the pieces of evidence fit into a coherent narrative — the solution seems very far from obvious.

“We had no idea what were the cause of earthquakes and volcanoes and things like that,” recalls Dan McKenzie. “It’s extraordinarily difficult now to put yourself back into the state of mind that we had when I was an undergraduate.” Dan is regarded as one of the architects of modern plate tectonics theory. In 1967, he published a paper in the journal Nature called ‘The North Pacific: An Example of Tectonics on a Sphere’ with Robert Parker, another Cambridge University graduate. It drew on a rash of post-war discoveries to paint a compelling picture of how the sea floor in that part of the globe was able to move, much like a curved paving stone, initiating earthquakes where it interacted with the other great slabs of solid rock covering the Earth.

Although seen as an ‘aha!’ moment, it was actually a long run-up to that point with a group of committed scientists sprinting and dipping for the line. The story goes back to 1915 to Alfred Wegener, the German polar explorer and meteorologist, who we most associate with the idea of continental drift. Alfred could see that the continents were not static, that they must have shifted over time, and that the coastlines of South America and Africa looked a suspiciously snug fit, as if they were once joined together. But he couldn’t devise a convincing mechanism to drive the motion.

Things really had to wait for World War II and the technologies it spawned, such as echosounders and magnetometers. Developed to hunt down submarines and to find mines, these capabilities were put to work in peacetime to investigate the properties of the sea floor. And it was these investigations that revealed how plates are made at mid-ocean ridges and destroyed at their margins where they underthrust the continents.

“Plate tectonics really comes from the oceans. It was when we discovered the oceanic ridges, subduction zones and transform faults, and so forth,” said John Dewey from Oxford University. In the 1960s, there was this massively increased knowledge through oceanographic expeditions. “Until that time we’d been looking down microscopes at thin sections of rock, looking at faults and outcrops on land,” said John.

One of the key observations was that of sea-floor spreading — the process of creating new crust at the ridges by upwelling magma. As the rock cools and moves away from a ridge, it locks into its minerals the direction of Earth’s magnetic field. And as the field reverses, as it does every few hundred thousand years, so does the polarity in the rocks, presenting a zebra-like, striped pattern to traversing research ships and theirmagnetometers. In 1967, all roads led to the spring meeting of the American Geophysical Union. Some 70 abstracts were submitted on sea-floor spreading alone. A heady time, it must have been. The coherent narrative of plate tectonics was about to fall rapidly into place.

As to the mechanism that eluded Alfred, scientists can now see how the weight of underthrusting plates plays such a major role in driving the whole system. Much as the slinky dog needs no encouragement once it has started its journey downstairs, so the descending rock appears to have an unstoppable momentum. Tony Watts, an Oxford geologist, explains: “We know that the fastest moving plates, the ones spreading the fastest, have very long slabs, long pieces of lithosphere, that are going under at ocean trenches. So, it looks as though something called ‘trench pull’ is a very important force and it’s generally agreed to be larger than ‘ridge push’.”

Nothing is ever done and dusted in science. There is still a lively debate for example about precisely when and how plate tectonics got going on Earth. More than four billion years ago as the result of asteroid impacts, argued one recent Nature Geoscience paper.

Today, we have extraordinary tools such as GPS and satellite radar interferometry that allow us to watch the march of the plates.

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