Moving Continents

A Little History Of Science: Moving Continents

Earthquakes are deadly and terrifying. Deadly because of the wholesale destruction they cause, terrifying because the earth should not move beneath our feet. And yet it does, all the time, if mostly unseen and unfelt. Like so much of science, understanding the earth’s structure is about measuring the unseen, unfelt part – and convincing others that you’re right. The continents and ocean floors do move beneath us.

What we experience of the earth’s history in our lives is a tiny snapshot, the smallest of moments in a very long process. Geologists have scientific techniques, but they must also use their imaginations, thinking ‘outside the box’. All good scientists do, even if they are working in the laboratory, checking their ideas against the evidence at hand.

Our nineteenth-century geologists used the traditional tools: fossil finds, analysing and classifying rocks, looking at the effects of earthquakes and volcanoes. All this they wove into a reasonable history of the earth. Much of what they learned still holds true today.

But there were a number of problems that nagged at them, and needed a new kind of bold idea. The old ‘catastrophists’ had relied on the idea of different sorts of forces, or perhaps even miraculous interventions – great floods such as Noah’s Flood described in the Bible. Instead, the new focus would be on time – immense periods of time called ‘deep time’. What was the earth like, 200 million years ago, or twice or three times that number of years ago?

How could deep time help answer three key questions? First, why did the major continents look as if they could be cut out from the oceans and stuck together, like pieces in an enormous jigsaw puzzle? The east coast of South America would fit pretty snugly into the west coast of Africa. Was this an accident?

Second, why were the rock formations of South Africa so like ones found in Brazil, on the other side of the Atlantic Ocean? Why, in such a small island as Great Britain, were there dramatic variations between the Highlands of Scotland, with its crags and lochs, and the gently rolling Weald of Sussex in the south? Indeed, had Britain always been separated from the European mainland? Or Alaska from Asia?

Third, there were some odd patterns in the locations of plants and animals. Why were some species of snail found both in Europe and in eastern North America, but not on the other side of the American continent, on the west? Why were the marsupials in Australia so different from those found elsewhere? In the 1850s Darwin and Wallace pioneered some answers, and the theory of evolution helped explain a lot. Darwin ran some very smelly experiments, keeping seeds sitting in tubs of seawater in his study for months on end. He wanted to give the seeds an experience like a long sea journey. Then he planted them to see if they could germinate and grow. Sometimes they did, so that was one answer. Darwin also found ways to discover if birds could transport seeds, insects and other living things over very long distances. And they could, but this didn’t explain all the puzzles.

There was one radical idea that could explain a great deal. This theory was that the continents had not always been where they are now, or that they had once been joined by strips of land, ‘land bridges’. Many geologists from the late nineteenth century thought that there had once been land bridges in several places. There was good evidence that Britain had once been connected to Europe. It would explain very effectively why the fossil bones of bears, hyenas and other animals, not found in Britain in modern times, were found there. North America had once been connected to Asia across the Bering Strait, with animals and Native Americans undoubtedly crossing there. Land bridges joining Africa and South America seemed less likely, but the eminent Austrian geologist Eduard Suess (1831–1914) had a go at arguing this in his massive five-volume work (published between 1883 and 1909) on the earth.

He said that the constant rising and falling of the surfaces of the earth during geological history made this possible. What was now sea-bed had once connected the two continents.
Not everyone was convinced, five volumes or not. Enter the German Alfred Wegener (1880–1930). Wegener was equally interested in the history of the earth’s weather and its geology. In 1912 he gave a lecture on his theory of the continents moving: what would become ‘continental drift’. The lecture became a book in 1915, and Wegener spent the rest of his life looking for further evidence. He died on the job, leading an expedition to Greenland to search for more clues to support his theory. Wegener’s radical proposal was that, around 200 million years ago, there was only one large continent, Pangaea, surrounded by a vast ocean. This enormous continent had gradually broken up, with pieces of it literally floating on the ocean, like icebergs breaking away and floating on the sea. Unlike icebergs, which can melt and fade, the pieces of Pangaea became the new continents. And it wasn’t over.

Wegener thought the land masses were still moving apart, about ten metres a year. This estimate was way too high – recent measurements suggest a movement of only a few millimetres each year.

But anything over a long enough period produces dramatic results. Wegener had a few supporters, mainly in his native Germany, but most geologists found his ideas too far-fetched – too much like science fiction. Then, during the Second World War, submarines began the serious exploration of the ocean floor. After the war they revealed a new underwater landscape with enormous ridges of mountains and valleys, and extinct (and even active) volcanoes.

Harry Hess (1906–69), a geologist working for the US Navy, traced these ridges and valleys and followed them on to the better-known dry land. He also followed the fault lines, those regions of the earth above and below water where earthquakes and volcanoes are common. What Hess discovered was that the land masses and the ocean floor were continuous, they ran into each other. The land didn’t float as Wegener had suggested. How then might land masses move?

Hess was joined by physicists, meteorologists (weather watchers), oceanographers (studiers of the sea), seismologists (specialists in earthquakes) and the traditional geologists. They all began to try to work out the history of our earth, using the tools of these different sciences. This was not easy. The interior of the earth quickly gets very hot. Not that far down, instruments melt. So a lot of what we know about the composition and structure of our world’s inner reaches had to be learned by indirect methods.
Science is often like that.

Volcanoes spewing their molten lava had long been interpreted as the earth getting rid of the excess heat that had accumulated below, and in one sense, this is true. But it’s not the whole picture.

The discovery that radioactive elements, such as uranium, naturally release a lot of energy when they decay, added another source of interior heat. But radioactivity is an ongoing heat-producing source, and this meant that the older idea that the earth had once been a very hot ball but was now gradually cooling, was too simple.

At least, it was too simple for the geologist Arthur Holmes (1890–1965). He said that the earth gets rid of most of its continously generated internal heat by the familiar process of heat transfer, convection. The important bit was Holmes’s realisation that it wasn’t in the earth’s upper crust – where we live – that things were happening, but in the next layer down towards the centre of the earth. This layer is called the mantle, and Holmes believed that the molten rocks there gradually move upwards, like the hotter water in your bath. As they move up and away from the hotter area, they cool, and sink down again, to be replaced by other molten rock, in a timeless cycle. It is some of this molten rock on the rise that spews out when volcanoes erupt. Most molten rock never makes it to the earth’s surface, but spreads out as it cools and sinks, providing a mechanism to shift the continents apart, millimetre by millimetre.

As the depths of the oceans and earth were explored, a new way of working out the age of the planet added real meaning to deep time. The technique of radiometric dating had emerged from physicists’ discovery of radioactivity (Chapter 31). Now it allowed the scientists to date the rocks they were studying by comparing the amounts of a radioactive element and its end product (uranium and lead, for example) in a rock sample. Using this technique, it was possible to know how old the rocks were, since after they are formed, no new material is incorporated in to them. Knowing the age of individual rock layers has in turn helped understand just how old the earth is. Rocks of more than four billion years have been found. Such old rocks are always on land. Those at the bottom of oceans are always newer. Oceans don’t last as long as continents, and are in fact always dying and being reborn.

This of course happens over a very long period of time, so don’t worry about next summer at the beach. (On the other hand, man-made global warming may well keep melting the polar icecaps and lead to a dangerous rise in sea-levels in the coming decades.) Rocks not only capture radioactive elements as they are formed, but also keep the magnetic orientation of their iron or other magnetically sensitive material. Like radioactivity, magnetism has helped earth scientists unravel the age of rocks. The earth’s magnetic pole has not been constant over the long period of the earth’s existence. North and south have flipped around on several occasions, so the north–south orientations can also provide evidence about when a rock was formed. Compasses will point north in our life- times and the lifetimes of our grandchildren, but things were not always so, and will not be so in the distant future, if the past is anything to go by.

Magnetism, convection, deep-sea landscapes and radiometric dating had revealed important clues about the ancient conditions of the earth. Taken together, they were enough to convince earth scientists that Wegener was almost right. Right, because continental movement did occur: sensitive measurements by satellites have confirmed the movement. But the drift or floating he suggested was wrong. Instead, John Wilson (1908–93) and others finished off the bold train of thought that Wegener had begun when they argued that the upper part of the earth’s mantle is made up of a series of giant plates. These plates fit together, covering the earth, crossing the boundaries of land and sea. But they don’t fit together perfectly, and it is at the joins that the fault lines appear.

Understanding what goes on when one plate rubs against another, when they overlay each other or collide, is called plate tectonics.

Think about the highest mountain on earth, Mount Everest in the Himalayas. Everest is as high as it is because the Himalayan Mountains were formed by two of these plates starting to collide with each other some seventy million years ago. There’s no Nobel Prize in geology, but maybe there should be. Plate tectonics explains so much about earthquakes and tsunamis, mountains and rocks, fossils and living plants and animals. Our earth is a very old, but a very special place.