BEFORE 1830, everyone was a catastrophist. The theory that most of the features in the Earth were produced by sudden, short-lived, worldwide events, tied in perfectly with the Christian fundamentalism that dominated thinking in the West.

Christians knew that the Earth had a turbulent history, including a planet-wide flood. Awkwardnesses, such as dinosaur fossils and seashells on lofty mountain peaks could be neatly explained away by reference to divine intervention in the geological record.

Then the Scottish geologist Charles Lyell came up with a radical theory, expounded in The Principles of Geology, published in 1830. Lyell said that the Earth's crust had been subjected not to a series of divine catastrophes, but instead to a slow and unending process of change and renewal.

The rivers wear the mountains away to dust. This dust is compacted into sediments, which eventually sink into the mantle, melt, and are thrust up again as new mountains. Then the whole process begins again: `'We find no vestige of a beginning, no prospect of an end,'' he wrote.

This idea - which came to be known as Uniformitarianism - dominated geology for 150 years. It seemed to tie in perfectly with Darwin's ideas of evolution by natural selection, which had no reliance on divine intervention. By the 1970s, Catastrophism, with its fundamentalist overtones, was pronounced as dead and buried as a slab of Jurassic limestone.

But there was a problem. At the end of the Cretaceous, some 65m years ago, the dinosaurs, the great marine reptiles, the flying pterosaurs and many other species all vanished, apparently in the space of a few centuries or less. Other mass extinctions had happened before in Earth's history. How could these sudden and dramatic events be explained in terms of Lyell's timeless concept of gradual change?

In 1980, father-and-son geologist team Walter and Luis Alvarez claimed that the end of the Cretaceous is marked by a thin clay layer rich in iridium. This layer was found in places as far apart as Denmark and New Zealand, implying a planet-wide origin.

The two geologists proposed that the Cretaceous extinctions had been caused by the impact of a six-mile-wide asteroid, which scattered iridium- rich dust throughout the atmosphere. Then in 1990, a 180km-wide impact crater was discovered in Mexico's Yucatan peninsula. The crater was of the right age and the right size to fit in with the Alvarez hypothesis.

After its death some 160 years before, Catastrophism was reborn as Neo-Catastrophism, shorn of its religious connotations. Many geologists now believe that Earth's history - including the history of life - has been profoundly influenced by cosmic impacts. We have no idea when the next impact will occur - earlier this month scientists thought for a few hours that an asteroid was on its way in just 30 years' time - but if something the size of the dinosaur- killer hits us, then arguing about geology will be the least of our worries.

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