From the roof of the world to the Deccan Traps

It was by tracking a collision not between a meteorite and the Earth but between two continents that we would flush out a new suspect that may well have caused the "Cenozoic massacres."

Early in 1980, a certain ebullience reigned at the Institut de physique du globe de Paris. Under newly signed agreements between Chinese and French research organizations, several teams would be allowed to study the Tibetan Plateau in the field. Off-limits to geologists (apart from Chinese ones, of course) for decades, the "roof of the world" was to many an object of wonder and inquiry. As long ago as the 1920s, the Swiss geologist Emile Argand had viewed this region as the result of a collision between the continental masses of India and Asia. He held that such reliefs could have arisen only by the crumpling together of what had once been several hundred kilometers of these two great continental assemblies. Heretical in a world of rigid uniformitarianism, where no one believed the Earth's crust could have undergone such major horizontal deformations, these ideas would not crop up again until more than three decades later, with the pioneering work of the British geophysicist Keith Runcorn and his colleagues at Newcastle.

The birth of plate tectonics (the modern version of Wegener's theory of continental drift) is often dated to the mid-1960s. But it was a good ten years earlier that the young Runcorn, a brilliant student of P. M. S. Blackett, had the idea of using a highly sensitive magnetometer, developed under his mentor's direction,1 to measure the magnetization of rocks in the British Isles, and later in India; as

1 Although it takes us rather far afield from the subject of this book, I cannot resist telling the reader that Blackett conceived this wonderful magnetometer, of a type called "astatic," to measure the magnetic field of a rotating copper sphere. Blackett believed that any rotating object will generate a magnetic field, for example as an electron does on the submicroscopic scale. The refutation of this hypothesis, published under the title Results of a negative experiment, was an important moment in the history of geomagnetism, and indeed of physics itself. 45

we will see, he subsequently deduced that India had drifted for thousands of kilometers since the Cretaceous Period. Runcorn was among the first to realize that the Earth's mantle is the seat of powerful convection currents, of which continental drift is only the surface expression. In the mid-1960s, the systematic exploration of ocean floors would confirm his ideas and give birth to plate tectonics.2

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