This is a classic article from New Scientist’s archive, republished as part of our 50th anniversary celebrations
About 65 million years ago, at the end of the Cretaceous Period, a wayward Apollo-type asteroid ploughed into our planet and was responsible for wiping out the dinosaurs and many other forms of life on Earth. This hypothesis is proposed by Luis W. Alvarez and his colleagues from the University of California, Berkeley, in a paper delivered to this year’s meeting of the American Association for the Advancement of Science in San Francisco. The theory is supported by the discovery of large concentrations of iridium in rocks from the Gubbio Valley in Italy and near Copenhagen in Denmark.
Neutron activation analysis of 48 constituent elements in a thin layer of clay has revealed a 25-fold increase in the amount of iridium present, compared with that in the limestone rocks above and below. The layer coincides precisely with the time of the Cretaceous-Tertiary extinctions, when 75 per cent of all plant and animal species on Earth suddenly disappeared.
Iridium, absent from the Earth’s crust relative to its abundance in space, is a good indicator of an influx of extraterrestrial material. It is tempting to explain the increase by implicating a supernova, but judging by the amount of iridium present in the clay, this explosion would have had to occur only a tenth of a light year away, and that would have extinguished most life.
Alvarez favours an asteroid as the extraterrestrial source. Asteroidal impact would throw up a large volume of pulverised rock from the Earth’s surface in a gigantic 100 million megaton explosion, 20 per cent of which would stay, as a thick layer of dust, in the stratosphere for about 3 to 5 years. Distributed uniformly about the globe it would eventually fall out, forming the 1-centimetre thick clay layer all over the Earth’s surface. Surveys of rocks in the layer marking the Cretaceous-Tertiary interface dug in Spain, Holland and the US, and retrieved from deep-sea cores have identified the same iridium-rich clay layer, suggesting it is evenly distributed worldwide.
Alvarez estimates the size of the asteroid to be about 7-10 kilometres in diameter. This would make a 175 km diameter dent in the crust and material 100 times the object’s mass would be thrown skyward. The resulting darkness would suppress photosynthesis, and give rise to the pattern of extinctions observed in the fossil record. Alvarez speculates that diversity of life on the planet suddenly fell, although extinctions were more severe in the oceans. For example, he estimates that 3 per cent of freshwater and 19 per cent of terrestrial species may have disappeared, compared with 50 per cent in the oceans; 90 per cent of marine reptiles as against 26 per cent of their land-ranging cousins. Alvarez believes that in the seas the main food chains would have been completely cut, whereas plant life on the continents would have been merely interrupted. Nevertheless animals larger than 60 lb (27 kilograms) would have died out, leaving only the small foraging creatures, like the early mammals.
Alvarez further suggests that statistically the Earth would be hit by an asteroid every 100 million years, and that fact conveniently fits roughly with the interval between major extinctions. Chinese workers, looking at rocks from the Permian-Triassic boundary have identified a similar thin clay layer, and that transition from one era to another was a period of a mass extinction of even greater proportions. The Californian work represents the first non-palaeontological evidence to be linked with mass extinction.
First published in New Scientist in January 1980
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