70 Centimetres and Rising
- The Earth: An Intimate History by Richard Fortey
Harper Perennial, 501 pp, £9.99, March 2005, ISBN 0 00 655137 8
Alfred Wegener, born in 1880, pioneered the use of balloons in meteorology, and in 1906 broke the endurance record by staying up in the air for 52 hours. He spent several years studying the weather on Greenland, crossing the island on foot. He died there in 1930, after getting lost in a blizzard while returning from a trip to relieve stranded colleagues. In 1910, he had mentioned in a letter to his wife how nicely the east coast of South America fitted under the bulge of West Africa, something that the explorer and natural philosopher Alexander von Humboldt had remarked on a century earlier. Humboldt believed the match showed that the Atlantic Ocean was a flooded valley; Wegener tried fitting the continents together. This revealed that many features – the hills of Newfoundland and the Scottish Highlands, for example – aligned like the pattern on a broken plate. The following year, he came across palaeontological papers describing how many fossil plants and animals from Africa were also present in the Americas.
In 1915, Wegener published The Origin of Continents and Oceans, in which he argued that the continents had once been joined together, before being separated by a process he called ‘continental displacement’, creating mountains at the collision zones where they rejoined. The book was ignored, dismissed or ridiculed by the great majority of geologists. One reason is that Wegener was a meteorologist, and all scientists look less kindly on ideas impinging on their field from outside. Another is that some of Wegener’s evidence looked as if it was based more on his wish to find it than on anything more substantial, and it was known that coastlines could change their form as sea levels changed. Most important, there was no known mechanism that could shift continents around the Earth. Wegener suggested that the Earth’s shape would cause the continents to slip south as it spun, and that tidal forces would also slide them west. Such woolly speculation did him more harm than good.
This isn’t to say that geologists of the time couldn’t envisage change; but they tended to view the Earth’s movements as having been up and down, rather than from side to side. To explain mountain-building, many thought that the Earth would have shrunk as it cooled from its molten origins: mountains were thus the puckering of rocks once stretched taut on the surface of a turgid globe. Some researchers – today they would be called biogeographers – suggested that plants and animals had travelled between continents over now submerged land bridges. The name Gondwana (which now describes a supercontinent that last existed about 200 million years ago) was borrowed by the Austrian geologist Eduard Suess from a region of India to describe a sunken land mass filling the gap between Africa and India. Suess also borrowed the name Atlantis to fill the gap between Europe and America.
One of the few geologists convinced by the evidence for continental drift was Arthur Holmes at the University of Durham. In 1927, he suggested that the radioactive elements heating the Earth’s interior could provide the power to move continents. The hot rocks would rise, then fall when they reached the cooler upper layers, creating a convection current that followed a circular motion – like the hot water in a saucepan. The crust would be riding along on the top of the rotation cycle. Holmes kept his place in the geological mainstream despite holding such unorthodox views, and even pushed the idea of continental drift in his 1944 textbook, Principles of Physical Geology, but few others were interested in pursuing Wegener’s ideas.
Things didn’t start changing until the 1950s, when ships used sonar to provide the first comprehensive picture of the topography of the ocean floor, revealing a submerged mountain range that runs up the middle of the Atlantic. They also retrieved samples from the seabed by drilling and dredging. None of the rocks proved older than about 150 million years – hard to explain in a supposedly unchanging environment. New techniques for measuring the orientation of magnetic particles in rocks showed that over time the continents’ positions had moved relative to the poles: so either the poles had wandered, or the continents had.
By the early 1960s, the old geological worldview was crumbling under the pressure of accumulative data. In a paper published in 1962, Harry Hess, a geologist at Princeton and former naval captain, proposed that the mid-Atlantic ridge was in the middle of the Atlantic because the continents were moving away from it at an equal rate: the ocean was getting wider and growing new crust at its centre. Evidence for this, again drawn from magnetic signatures in rocks, was published by two Cambridge geologists in the following year. Two years later again, Tuzo Wilson drew on his studies of earthquake faults to suggest that the crust was divided into plates, which could slide over and alongside one another, as well as diverging. In 1967, Jason Morgan and Dan McKenzie independently worked out how these plates could move about the Earth, driven by Holmes’s mantle convection.
Finally, in 1968, Xavier Le Pichon used this work, along with catalogues of the sites of earthquakes, to build a global map of plate motion, a picture which has remained more or less unchanged since. Where plates move apart, crust is created, as at the mid-Atlantic ridge; where one plate slides over another, crust is destroyed, as along the west rim of the Pacific; and where they slide laterally, neither happens, as at the San Andreas Fault. In less than a decade, a generation of brilliant scientists had produced a framework to explain the Earth’s structure and movement, and turned continental drift into plate tectonics.
Richard Fortey witnessed some of the ferment first-hand while studying for a PhD in palaeontology at Cambridge, and in the 1980s, he used the distribution of trilobite fossils from half a billion years ago to work out the positions of some of the land masses before they coalesced into Pangaea (this agglomeration of the continents was the latest coming together in the plate-tectonic dance). In the process, Fortey and his colleagues revealed a previously unknown ancient sea and struck a blow against those geologists who had put their faith in the magnetic evidence according to which this water should not have existed.
In his enjoyable new book, Fortey surveys the current state of knowledge about the Earth, and how we found it out. He visits current geological hotspots, such as Hawaii, and other places where geologists have long been active, such as the Grand Canyon and Naples, the stop on the Grand Tour where Charles Lyell set the Earth moving with his observation that rocks that were firmly on dry land in 1828 contained fossil seashells.
In The Earth Fortey aims to go beyond telling the story of geology. He calls plate tectonics the world’s ‘unconscious’, and argues that geological factors underpin both biology and history: rocks decide what you can build where; climate controls what you can grow where; mountains and seas create new species by dividing animal populations, and new societies by dividing human groups. There are good examples of this: the eruption of the volcanic island of Santorini in 1700 BC probably contributed to the fall of Minoan civilisation; the islands of Bali and Lombok are separated by only 25 kilometres of water, yet because they lie on different plates, their flora and fauna are strikingly different; New Guinea surely owes its thousand different languages to its mountainous terrain. And it remains to be seen what long-term effects the Indian Ocean tsunami will have on the political, social and economic structures of Asia and the world.
The tsunami showed that even those who don’t live close to fault lines or volcanoes are subject to the vagaries of plate tectonics. In 2001, researchers in London and California predicted that an eruption of the volcano Cumbre Vieja in the Canary Islands could trigger a landslide that would send 50-metre waves towards West Africa, Brazil, the Eastern US and Atlantic Europe. And the Earth has experienced volcanic upheavals on a scale that, were one to occur today, would probably wipe out humanity. Between 65 and 60 million years ago, about the time dinosaurs died out, volcanoes in what is now India produced more than a million cubic kilometres of lava, covering the area called the Deccan Traps in two kilometres of basalt. Beneath Yellowstone there is a super volcano that erupts approximately every 600,000 years, with 1000 times the force of the 1980 Mount St Helens eruption. The last eruption was 640,000 years ago, and the ground in Yellowstone has risen by as much as 70 centimetres in the past century; this may or may not be lava building up.
But the evidence is not always easy to interpret. DNA sequencing shows that leguminous plants in the Horn of Africa are more closely related to those living in similar climates in the Americas than to their neighbours on the African savannah or rainforest. Is this a strike against a plate-tectonic view of evolution, because the plants’ divisions don’t match those of the continents? Or does it count in its favour, because the positions of landmasses, seas and mountains control the climate? For such explanations to be meaningful, we need to unpack plate-tectonic theory to get at geological facts, undoing the work of Wilson, Morgan et al. It makes no more sense to call tectonic plates ‘the ultimate controls on the personality of the planet’, as Fortey does, than it would to argue that radioactivity controls this personality by moving plates, or that quantum physics does, because it explains radioactivity. Not all levels of explanation are equally useful.
All the same, historical or social geology, or whatever you want to call it, is worth exploring, since human history is also the story of our efforts to get round the limitations of rocks, climate and soil. Ever since the builders of Stonehenge carted Welsh bluestone to Salisbury Plain, we have sought to defy the ground beneath our feet as much as accommodate or exploit it. Manhattan is a good place to build tall buildings, because the island is made of a hard rock called schist; but if you are willing to sink concrete-pile foundations, you can build a skyscraper on marshy ground, as Canary Wharf demonstrates. And by employing various engineering devices, such as flexible foundations that absorb tremors, and reinforced concrete cores, you can build a skyscraper in an earthquake zone. California and Japan have stringent building codes; in poorer countries, where urbanisation and planning are not on good terms, new apartment blocks can be death traps.
We could do now with a geological conservation movement to match the one working to preserve animal species and their habitat. In fact the two are inseparable: the society that built Easter Island’s statues collapsed once deforestation and soil erosion made agriculture impossible. Our refusal to accept the limitations of climate and soil now threaten to repeat this on a global scale, as Jared Diamond argues in his new book, Collapse.[*] The fastest growing cities in the United States are in the desert; to someone flying over Phoenix, the bright green scars left on the Sonoran desert by the golf rush of the late 20th century are as shocking as the gold mines of Alaska were to travellers a century earlier. At last year’s meeting of the American Association for the Advancement of Science, soil damage was described as an environmental problem on a par with climate change. Deforestation and farming can lead to desertification: the Gobi desert is growing by more than 10,000 square kilometres each year. Irrigation has caused salinisation problems in Australia and the Middle East. The UN estimates that 100 million hectares of farming land, enough to feed Europe, has been lost through soil degradation. Although the evidence is debated and hard to measure, it’s thought that a third of the world’s agricultural land has been damaged in some way.