Storm in a Teacup: The Physics of Everyday Life(80)

But the meandering of the poles is only the start. The Earth’s fickle magnetic field has far more to offer than navigational assistance. And the clues it leaves behind have provided the final confirmation of one of the most controversial, simple, and profound ideas that geologists have ever had. The continents, the immense rocky masses that dominate Earth’s surface, are moving.


IN THE 1950S, human civilization was whooshing into a new technological and scientific era. The foundations of our modern society were being laid: Microwave ovens, Legos, Velcro, and the bikini had all arrived and were working their way into popular use. Humanity was coming to terms with the arrival of the atomic age, social rules were being completely rewritten, and credit cards had just been invented. And yet, in the midst of all this galloping modernity, we couldn’t make sense of the planet we were living on. Geologists had been fantastic at cataloging the Earth’s rocks, but they couldn’t explain the Earth itself. Where did all these mountains come from? Why is this volcano here? Why are some rocks so old and some so new? Why are the rocks different everywhere you look?

One of the many observations crying out for a satisfactory explanation was that the east coast of South America and the west coast of Africa looked as though they had once fit together like jigsaw-puzzle pieces. The rocks matched, the shapes matched, and the fossils matched. How could all that possibly be coincidence? But most scientists just saw it as an unimportant curiosity; it was almost unthinkable that anything that big could go anywhere. In the early 1900s, a German researcher named Alfred Wegener finally gathered together all the evidence and proposed the idea of “continental drift.” Wegener suggested that South America and Africa had once been connected, and that one of these huge land masses had broken away from the other and drifted across the face of the planet. Very few scientists took this idea seriously, because the idea of something as gigantic as a continent just drifting 3,000 miles to the west seemed ludicrous. If that was true, what was doing the pushing? Wegener himself suggested that the continents plowed through the oceanic rock, but couldn’t provide any evidence. There was no “how” and no “why,” and the theory was quickly shelved. No one else had any better ideas, and the question was left alone.

By the 1950s, there still weren’t any better ideas around, but there were some new measurements. The lava spewed out by volcanoes had iron-rich compounds in it, and it was discovered that each speck of one of those compounds could act like a compass needle, twisting around to line up with the local magnetic field. The really useful part was that when the lava cooled down and formed solid rock, the tiny iron minerals couldn’t move anymore, so they were locked in position. These tiny frozen compasses meant that a record of the Earth’s magnetic field was built into the rock at the moment it formed. When geologists used this record to look at the changes in the magnetic field through the ages, something even more curious came to light. The direction of the Earth’s magnetic field seemed to reverse every few hundred thousand years. It completely flipped, so that south became north and north became south. It didn’t seem to matter too much, but it was very odd.

Then the geologists got to the sea floor. One of the many unexplained phenomena of the Earth’s structure was that several oceans had ridges of underwater mountains running in vast lines across the flat plains of the ocean floor. No one knew what they were doing there. The most famous is the mid-Atlantic ridge, a line of volcanoes that starts above water (the country we call Iceland is just the protruding end of this ridge) and then disappears underwater, where it zigzags all the way down the center of the Atlantic Ocean almost to Antarctica. Then, in 1960, magnetic measurements showed that the magnetism of the rocks surrounding that ridge was very strange indeed. It was striped, and the stripes ran parallel to the ridge. As you went away from the central ridge, the sea-floor rocks had magnetism that pointed north, then south, then north again, and these stripes ran the length of the mountain range. And it got weirder. If you looked at the other side of the ridge, the magnetic stripes there were an exact mirror image.

In 1962 two British scientists, Drummond Hoyle Matthews and Fred Vine, made the link.?? With hindsight, you can almost hear the solid “click” as all the strange pieces of geology dropped into place. What if, they said, the sea-floor volcanoes are building new sea floor as the continents move apart? The magnetism at the ridge lines up with today’s magnetic field. But as the continents move apart, that rock from the ridges is carried out to both sides of the volcanoes and new rock is made. When the Earth’s magnetic field reverses, the magnetism of the new lava will also reverse, starting a new stripe that points in the opposite direction. The reason the stripes are a mirror image of each other is that each stripe represents a period of one magnetic alignment, before it flips back the other way. Other discoveries around the same time showed the places where old sea floor was being destroyed, which was important because the planet itself stays the same size. On the other side of South America, the Andes mountain range exists because that’s where old sea floor from the Pacific is being pushed underneath the continent, back down into the Earth’s mantle. Once you know that the continents can be shunted around, colliding and separating, creating and destroying sea floor as they go, the patterns of geology make sense. This was the seminal moment in geology, the discovery of plate tectonics. Plate tectonics is now the backbone of everything we understand about why the Earth is the way it is.