The Science of Discworld Revised Edition

nowadays find rocks in regions with warm climates that were clearly laid down in regions with cold climates? For example, remains of ancient glaciers 420 million years old can still be seen in the Sahara Desert, and fossil ferns are found in Antarctica. Pretty much everyone else thought that the climate must have changed: Wegener became convinced that the climate had stayed much the same, give or take the odd ice age, and the continents had shifted. Perhaps they’d been driven apart by convection in the mantle – he wasn’t sure.
    This was considered a crazy idea: it wasn’t suggested by a geologist,
and
it ignored all sorts of inconvenient evidence,
and
the alleged fit between South America and Africa wasn’t all that good anyway,
and
– to top it all – there was no conceivable mechanism for carting continents around. Certainly not convection, which was too weak. Great A’Tuin may lug a planet around on its back, but that’s fantasy: in the real world, there seemed to be no conceivable way for it to happen.
    We use the word ‘conceivable’ because a number of very bright and very reputable scientists were busily making one of the subject’s worst, and commonest, errors. They were confusing ‘I can’t see a way for this to happen’ with ‘There
is
no way for this to happen.’ One of them, it pains one of us to admit, was a mathematician, and a brilliant one, but when his calculations told him that the Earth’s mantle couldn’t support forces strong enough to move continents, it didn’t occur to him that the theories on which those calculations were based might be wrong. His name was Sir Harold Jeffreys, and he really should have been more imaginative, because it wasn’t just the
shapes
of the land on either side of the Atlantic that fitted. The geology fitted too, and so did the fossil record. There is, for example, a fossil beast called
Mesosaurus
. It lived 270 million years ago, and is found only in South America and Africa. It couldn’t have swum the Atlantic, but it could have evolved on Pangea and spread to both continents before they drifted apart.
    In the 1960s, however, Wegener’s ideas became orthodox and the theory of ‘continental drift’ became established. At a meeting of leading geologists, a Ponder Stibbons-like young man named Edward Bullard and two colleagues enlisted the aid of a new piece of kit called a computer. They instructed the machine to find the
best
fit between Africa and South America,
and
North America,
and
Europe, allowing for a bit of breakage but not too much. Instead of using today’s coastline, which was never a very sensible idea but made it possible to claim that the fit wasn’t actually that good, they used the contour corresponding to a depth of 3200 feet (1000 m) underwater, whose shape is less likely to have been changed by erosion. The fit
was
good, and the geology across the join matched amazingly well. And even though the people at the conference came out just as divided in their opinions as they’d been when they went in, somehow continental drift had become the consensus.
    Today we have much more evidence, and a fair idea of the mechanism. Down the middle of the Atlantic Ocean, and elsewhere in other oceans, there runs a ridge – roughly north–south and about midway between South America and Africa. Volcanic material is welling up along that ridge, and spreading sideways. It’s been spreading for 200 million years, and it’s still doing it today: we can even send deep-sea submarines down there to watch. It’s not spreading at speeds humans can see – America moves about three-quarters of an inch (2 cm) further away from Africa every year, about the same rate that your fingernails grow – but today’s instruments can easily measure such a change.
    The most striking evidence for continental drift is magnetic: the rocks on either side bear a curious pattern of magnetic stripes, reversing polarity from north to south and back again, and that pattern is
symmetric
on either side of the ridge – making it clear that the stripes were frozen in place as the rocks cooled in the Earth’s magnetic field. Whenever the Earth’s dynamo flipped polarity, as it does from time to time, the rock immediately adjacent to the ridge-line, on either side, got the same new polarity. As the rocks then spread apart, they took the same patterns of stripes with them.
    The surface of the Earth is not a solid sphere. Instead, the continents and the ocean-beds