The Continents That Wouldn't Sit Still: How Wegener's Idea Was Eventually Accepted

Here's a small thing I do on beach walks. I pick up a pebble, turn it over, and think, "You've been somewhere before you were here."

Most of the time that's as far as it goes. But a pebble is a good place to start this story, because the question behind it is a big one: if rock can travel, how far can it go? And what if whole continents have been doing the same, only much more slowly?

That was the question Alfred Wegener asked, and it's a good one to follow, partly because he was right about a lot of it, and partly because the way he turned out to be wrong is just as interesting.

A jigsaw you can see on any globe

Start with something familiar. Look at a world map and your eye drifts to the east coast of South America and the west coast of Africa. They look like two pieces of a jigsaw. (I'll admit that the first time someone pointed this out to me, I thought, "Well, obviously," and then felt a bit cheated by how obvious it was.)

People had noticed this for a long time before Wegener. A matching shape on its own proves very little. Plenty of things look like other things. It's the sort of observation that makes a good conversation and a poor argument.

Wegener was a German scientist who worked in meteorology and what we'd now call geophysics, and he also went on research expeditions to Greenland. He took the matching shapes seriously, but he wanted more than a resemblance. Around 1912 he began presenting his idea, and in 1915 he published a book, The Origin of Continents and Oceans, setting out the case. Over the next several years he revised it as he gathered more evidence.

His idea was that the continents had once been joined in a single great landmass, which he called Pangaea (roughly "all land"), and that they had since moved apart.

What else lined up

The coastline match was only the opening move. Wegener's strength was that he drew in evidence from several fields at once, which is a habit I admire (and rarely manage myself).

Fossils. A small freshwater reptile called Mesosaurus turns up in rocks of the same age in South America and in southern Africa. It lived in fresh water and wasn't built for crossing an ocean. A fossil plant called Glossopteris, a seed-bearing fern, shows up across the southern continents: South America, Africa, India, Australia and Antarctica. If those landmasses had always been far apart, how did the same creatures and plants get there?

Rocks and mountains. Some belts of rock seem to stop dead at one coast and carry on, with the same character and age, on another continent across the Atlantic. The Appalachian mountains of North America and certain mountain belts in Britain and Scandinavia are the usual example. Close the Atlantic in your mind and they line up like the two halves of a torn page.

That torn-page comparison is one Wegener himself used: if you rip a newspaper and fit the edges back together, you check whether the lines of print continue across the tear. One matching line might be chance. Many matching lines are hard to wave away.

Ancient climates. This is where his meteorology background shows. Rocks scratched and scoured by ancient ice sheets are found in places that are now warm, such as parts of southern Africa, India and Australia. Meanwhile, coal, which forms from lush plant growth, is found in places that are now very cold. Either the climate did strange things, or the land had been somewhere else.

(A tidy detail: the glacial evidence makes much more sense once you put the southern continents back together around the South Pole, rather than scattering them across the globe.)

So why didn't everyone cheer?

This is the part I find most worth thinking about, because it's easy to tell the story as "clever man, stubborn old scientists," and that's not quite fair.

Wegener's weak point was the how. He had a lot of evidence that the continents had moved, but he couldn't give a convincing reason why they would.

He suggested forces connected to the Earth's rotation and to tides, with the continents somehow ploughing through the ocean floor. When other scientists did the sums, those forces were far too weak. And the picture itself seemed physically unlikely: how would a continent shove its way through solid ocean crust without crumpling or breaking up? Some of his estimates for how fast the continents moved were also much too high.

Here is a thought experiment that shows why this mattered. Imagine a friend tells you your garden shed has crept two metres across the lawn overnight. You might look at the grass, see the flattened patch and think, "Well, something has clearly happened." But if they can't say what could have pushed a shed, and the only suggestion is "maybe the wind," and you know the wind can't shift a shed, you'd be wise to stay unconvinced. Not because you doubt your own eyes, but because a good explanation has to be physically possible.

Wegener's critics were, in that sense, asking a fair question. A theory that describes what without being able to say how is unfinished. Science tends to wait for the second half.

There were also other explanations on the table. For the matching fossils, many geologists proposed land bridges, strips of land that once connected the continents and later sank. It's not a silly idea. It simply turned out not to fit the physics of how rock floats on the material beneath it.

A few people did take continental drift seriously, especially some who worked on the geology of the southern continents, where the matching evidence was strongest. Arthur Holmes, a British geologist, suggested that slow circulation in the Earth's hot, soft interior could be the engine. That was a real step towards an answer, though it was still an idea rather than something anyone could test.

So the picture in the 1920s and 30s wasn't "everyone laughed." It was closer to "interesting, but the mechanism doesn't work, so we'll hold off." I find that reasonable, even if it makes me wish the evidence had come faster.

The sea floor changes the conversation

For a long time, nobody could see the ocean floor, which is a big gap if your question is about how continents move. The answer, it turned out, was hiding there.

Around the middle of the twentieth century, better echo-sounding (bouncing sound off the sea bed and timing the return) and other survey methods allowed scientists to map the ocean floor in much greater detail. Some of what they found was surprising:

• A huge chain of underwater mountains, the mid-ocean ridges, running down the middle of the Atlantic and winding through other oceans.
• Deep trenches along some ocean edges.
• Earthquakes arranged in neat belts rather than scattered at random.

Then came the idea that pulled these together. In the early 1960s the American geologist Harry Hess proposed what became known as sea-floor spreading: at the ridges, new rock rises from below, forms new ocean floor, and the older floor moves away on both sides, like a very slow conveyor belt. The trenches are where old ocean floor sinks back down.

That changed the picture completely. Wegener thought continents ploughed through the ocean floor. In the newer view, continents don't plough through anything. They ride along with the ocean floor, as part of larger slabs of the Earth's outer shell.

The magnetic stripes

Then came evidence that you could actually measure. Lava contains tiny iron-rich mineral grains, and as the rock cools they line up with the Earth's magnetic field, a bit like compass needles being locked in place. So new rock at a ridge carries a record of the field's direction at the moment it set.

The Earth's magnetic field has flipped direction many times in the past. (Yes, really: north and south have swapped over geological time. It's well established, if a little dizzying.) So if sea-floor spreading is real, you'd expect stripes of alternating magnetic direction in the ocean floor, matching on either side of a ridge like a mirror image.

That is exactly what surveys found. Patterns of stripes lay parallel to the ridges, symmetrical on either side. To me this is the satisfying kind of evidence, where an idea makes a prediction that could easily have failed, and the sea floor said yes.

There was one more clue. The ocean floor turns out to be quite young. Nothing in the ocean floor is much older than about 180 million years, while some continental rocks are billions of years old. That makes sense if ocean floor is constantly made at ridges and recycled at trenches, while continental rock is lighter and tends to stay at the surface.

A bit of arithmetic

Here is a back-of-the-envelope calculation, just to see whether the idea is plausible. (I like doing these at the kitchen table. Nobody has ever asked me to stop.)

Say a plate moves at about 3 centimetres a year. That's roughly the pace at which a fingernail grows, give or take. Over 100 million years:

3 cm × 100,000,000 = 300,000,000 cm = 3,000 kilometres.

That's about the distance that the South Atlantic is wide at its narrowest. A speed too slow to notice in a human lifetime, given enough time, can open an ocean. This is also where Wegener's critics and Wegener were both partly right: the movement was real, but much slower and gentler than some of his estimates, and it needed time on a scale that's hard to feel.

From "drift" to "plate tectonics"

By the late 1960s these threads (the ridges, the stripes, the earthquake belts, the trenches) had come together into the framework we now call plate tectonics. The Earth's outer shell is broken into a number of large plates and several smaller ones that move slowly relative to each other. Most of the dramatic geology happens at their edges, where they pull apart, slide past or push together.

The "why" Wegener couldn't supply is now understood in broad terms: the Earth's interior is hot, and heat moving