Catastrophism and Mass Extinctions

From the middle of the nineteenth century until about 1980, most geologists and other earth scientists opposed the idea that impact events played any significant part in the history of life on earth.

In 1980 Walter and Luis Alvarez and their colleagues Frank Asaro and Helen Michel published an historic paper suggesting that an asteroid about 10 kilometers (6 miles) in diameter struck the earth sixty-five million years ago at the end of the Cretaceous. The resulting impact should have left a crater at least 160 kilometers (100 miles) in diameter. If the impact site were in the ocean, huge tsunami ("tidal waves") would rise several kilometers in height, sweeping hundreds of kilometers across the continents, sweeping away everything in their path. As hot material ejected from the impact rained back down, huge fires would start up all over the world. Dust thrown up by the impact would have spread out covering the entire world in darkness. Temperatures would have dropped precipitously. Plants would have failed to receive enough sunlight to allow photosynthesis to continue. After the plants died, plant-eating animals dependent on them would die, as would meat-eating animals once their plant-eating prey were gone. Conditions in the oceans would not be much better as massive acid rain buildup poisoned the water and destroyed shell-bearing creatures, disrupting the entire food chain. About 70% of all species died out at the end of the Cretaceous. This included the dinosaurs which had dominated the landscape for over 160 million years.

Since 1980, the theory that the impact of a large asteroid or comet brought about the demise of the dinosaurs and many other forms of life at the end of the Cretaceous (the "K-T boundary") has gained in popularity. Some scientists remain unconvinced by the evidence, however. The following web pages discuss this mass extinction event as well as others before and since.

The growing popularity of the impact theory as an explanation for the extinction at the Cretaceous-Tertiary boundary opened the door to a reexamination of the role played by impact processes in shaping Earth history. The following web pages show how such neo-catastrophist thought now influences the teaching of geology.

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Last modified by pib on July 6, 2003.