How a cockroach can be a canary

The Earth - Lovelock's Gaia: Signs of Life

四月 2, 2004

Andrew Robinson investigates symptoms of our planet from its melting crusty hot spots to its cooling gassy algae.

In the 1960s there was a seismic change in the study of geology. The much-derided idea of continental drift, published as early as 1915 by Alfred Wegener, became widely accepted, after essential modification, as the theory of plate tectonics. The earth's crust, composed of perhaps a dozen rigid "plates", was understood to be in constant motion, with new crust being created by volcanic action at some plate boundaries and old crust being subducted at other boundaries. Soon, plate tectonics began to explain mountain ranges, ocean trenches, earthquakes, volcanoes and other key areas of earth's evolution and behaviour. It was an exciting time for a student of paleontology like Richard Fortey.

"For some years I have been thinking about how best to describe the way in which plate tectonics has changed our perception of the earth," Fortey kicks off his latest, highly ambitious book for the general reader.

"The world is so vast and so various that it is evidently impossible to encompass it all within one book. Yet, geology underlies everything: it founds the landscape, dictates the agriculture, determines the character of villages. Geology acts as a kind of collective unconscious for the world."

Whether one agrees with such grand claims or not - and Fortey makes a strong and stylish case for them - the author has set himself a huge challenge in trying to make plate tectonics explain all of geology. It cannot be done, rather as physics cannot be made to explain all of chemistry or biology, despite the claims of some physicists. The Earth: An Intimate History is an eye-opening tour of some of the planet's most contrasting landscapes, ranging from the Bay of Naples, the volcanoes of Hawaii and the San Andreas Fault to the Deccan Traps of India, the Rift Valley of east Africa and the Grand Canyon - together with a clear, historically grounded account of the development of plate tectonics. But the synthesis has a fault: the theory provides only intermittent insight into the landscape.

The reason is that plate tectonics has major inadequacies, only reluctantly confessed by the author. For a start, there is furious debate, and not much agreement, about what forces inside the earth could have created the particular, highly irregular plates and their observed movement (surely not a "dance", as suggested by Fortey). Then there is the theory's prediction that earthquakes and volcanoes should occur at plate boundaries, as they do in Japan and Indonesia, the Andes and California. But what about the number of earthquakes that occur far from plate boundaries, such as a massive one in Missouri in 1811-12, or the volcanoes of Hawaii that erupt in the middle of the Pacific plate? The procession of this plate northwestwards at the rate of about 10cm a year satisfactorily explains the long chain of volcanic islands stretching northwest from the Big Island and their increasing ages - but only if scientists postulate a static thermal plume arising from the earth's mantle and melting the crust like a blowtorch: this "hot spot" is thought to have been active since the time of the dinosaurs. Other hot spots are, too conveniently, said to explain the volcanoes in east Africa, also located away from a plate boundary.

Even at boundaries, in cases where the plates adjoin continents, the theory frequently flounders. In the eastern Mediterranean, for instance, the African plate adjoins the Eurasian; yet at least two microplates, the Adriatic and the Anatolian, with obscure boundaries, have had to be postulated, producing "a tectonic nightmare that is far from resolved", involving an "ever denser thicket of ad hoc modifications and amendments to the theory and practice of plate tectonics".

This admission comes not from Fortey but from the article on plate tectonics in the latest Encyclopedia Britannica . He does say that the geology of Italy is very complex and puzzling but still insists: "Always in this story - far beneath us and largely unacknowledged - there is the bidding of the plates." This sounds like plate-tectonic fundamentalism, reminiscent of its genetic equivalent that imagines that our genes are our masters.

For much of the book, old-fashioned descriptive geology, spiced with human associations, takes over from musings about plate tectonics. It can be tough going, unless you know your granite from your gneiss, your Cambrian from your Ordovician, quite well; but is often rewarding and revealing. One excellent chapter describes the author's mule trip down the Grand Canyon and the fascinating strata he passes. On the way, he notes that water gushing out, "because the Bright Angel Fault throws rocks of different permeability together", supplies the buildings at the rim of the canyon.

"It seems curious that in a mighty landscape eroded by water it should be necessary to pump upwards for 1,000m to get a decent drink." Another chapter is devoted to mineralogy and contains some lovely nuggets. The 16th-century silver miners in Joachimsthal (Czech Republic) who produced silver thalers - the origin of the word dollar - apparently viewed the cockroach favourably, because it could sense the slightest movement in the rocks and would scuttle away if there was a danger of collapse: a silver miner's canary.

Even then, while away from plate tectonics, some material is of dubious status. In the Deccan Traps, formed in western India in an awesome outpouring of lava 66 million years ago, Fortey visits the famous caves of Ajanta and Ellora. They were excavated from basalt and then painted with extraordinary murals (at Ajanta) or transformed into amazing rock-cut temples (at Ellora). To Fortey, Ellora demonstrates intimacy between geology and human history: "the most direct link between geology and culture that I know." But why? The caves and sculptures could have been carved from other rock; I know of no art historian who insists that the rock at Ellora had to be volcanic. Indeed, later Fortey compares Ellora with the natural carving of the Grand Canyon by the Colorado River. The canyon's rocks are sedimentary and metamorphic, not igneous as at Ellora.

There is no meaningful causality connecting the basalt of the Deccan Traps and the art that was created within it a millennium or two ago.

Yet, genuinely important topics are omitted. A footnote informs us that some geologists are now "questioning the reality of mantle plumes". This debate has been under way for more than a decade and deserves space. The section on the San Andreas Fault implies that accurate earthquake prediction is a reality. Such claims were first trumpeted in the 1970s as a result of plate-tectonic theory, but proved groundless. And there is no reference at all to James Lovelock and his Gaia theory - unlike in Fortey's Life: An Unauthorised Biography (1997), in which it receives a single unsympathetic mention. This may be because, for lack of space, he specifically excluded geochemical cycles and their role in earth systems.

Nevertheless, Gaia's presence is too well established - even rating a mention in earth science textbooks - to be so signally ignored.

It is the subject of Jon Turney's short, well-informed and intelligent introduction, Lovelock and Gaia , in a series called Revolutions in Science.

The name of the series rather begs the question: if Gaia really is a revolution in science, why haven't "mainstream" earth scientists such as Fortey felt obliged to respond? Plate-tectonic theory became the norm in geology within about five years of its proposal in 1965; Gaia, which proposes that the biosphere somehow regulates the earth's entire surface - for example, its temperature, atmospheric oxygen and ocean salinity - so as to keep the planet habitable, has been around since the 1970s but remains a minority view.

The answer is complex, at least partly because Gaia has acquired new-age overtones untypical of a scientific theory (except, perhaps, quantum theory). Turney outlines three possible explanations for the lack of general acceptance. There is the one favoured by Lovelock: "Blinkered defence of established disciplinary ways of looking at things." The one favoured by the microbiologist Lynn Margulis, Gaia's early scientific supporter: "Unacknowledged social and cultural influences that constrain thought." Or, says Turney, "you can simply see it as a natural reluctance to jettison old ideas that seem to work and adopt new ones whose pay-off is uncertain."

The book describes the origin of the Gaia hypothesis in Lovelock's 1960s work for Nasa on its Mars mission; its publication in the 1970s; the damaging critiques of it by biologists such as Richard Dawkins in the 1980s; Lovelock's modifications, and his invention of the computer model Daisyworld; and the first scientific conference on Gaia in 1988. Then comes the crucial period, in which a number of scientists have investigated potentially Gaian mechanisms of regulation.

The most intriguing involves the cycling of sulphur from ocean to land.

Lovelock suggested that dimethyl sulphide (DMS) gas emitted by certain marine algae might nucleate cloud formation: the sulphuric acid-based clouds would cool the atmosphere (as is known to happen after a large volcanic eruption), thus acting as a thermostat, and the rain would deposit sulphate on the land. But what Darwinian advantage could there be for algae in this planetary regulation? Why would they "bother" to produce DMS? Then in 1998, William Hamilton, a well-known evolutionary theorist, suggested that the updraughts created by DMS and clouds might disperse algae spores to the winds more efficiently. (He also, rather extravagantly, compared Lovelock to Copernicus; Wegener might be more appropriate.) Such ideas obviously require more research. Investigating them cuts across biology, chemistry, geology, oceanography, meteorology and other disciplinary fault lines, even astronomy (where Gaia was born). The worldwide concern about climate change has given such research a practical boost. Whether it is labelled Gaian or a part of the International Geosphere-Biosphere Programme, the influence on it of Lovelock's theory is already widely, and justly, accepted.

Andrew Robinson, literary editor of The Times Higher , is the author of Earthshock .

The Earth: An Intimate History

Author - Richard Fortey
Publisher - HarperCollins
Pages - 501
Price - £25.00
ISBN - 0 00 257011 4



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