Geophysical surveys reveal that the earth's magnetic fields have become reversed on a number of occasions. Some physicists believe that this would have caused the van Allen belt to collapse and allowed cosmic rays and other radiation, normally deflected by the poles, to strike the earth. This can be ruled out as a cause of dinosaur extinction, however, because many marine forms were also profoundly affected despite the fact that a thin layer of water will act as a shield from cosmic rays. The radiation hypothesis cannot, therefore, be substantiated because many of the wrong creatures died.
Several authors have suggested that radiation from the explosion of a supernova, mostly in the form of X-rays,would be absorbed by the ozone layer at the ionosphere of the earth's outer atmosphere. The turbulence generated would have altered dramatically the property of the atmosphere to retain heat. While the drastic effects of a nearby supernova explosion are not in doubt, there is no evidence that one did occur at the end of the Cretaceous period and no explanation of why, if it did, its effects should have operated in the selective manner necessary to produce the observed results. The same objections apply to the suggestion of Harold Urey in 1973 that collision with a comet could have been responsible.
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