Stephen P. Broker
All species of life on Earth are faced with the inevitibility of extinction, either through evolution into new life forms or replacement by successful competitors. Average duration of species is estimated to be between one and 10 million years. Compilations of family, genera, and species extinctions over the past 590 million years (the Phanerozoic) suggest that approximately 40% of all losses occur as a background extinction level of less than 5% of contemporary life forms. 55% of extinctions occur at a higher, “pulsed extinction” rate, and only 5% of all extinctions occur during mass extinction events, when at least 60% of all life forms disappear quickly, as evidenced by the fossil record.
Using the percentages shown above, paleontologists recognize seven major mass extinction events occurring during the Phanerozoic (which is divided into the Paleozoic, Mesozoic, and Cenozoic). They are the Early Cambrian, Late Ordovician, Late Devonian, Late Permian, Early Triassic, Late Triassic, and end-Cretaceous extinctions. The greatest of these occurred 250 million years ago at the end of the Permian, when 96% of marine organisms died out. The causes of the Permian extinction are still being sought. Less severe but still considerable was the mass extinction occurring 65 million years ago at the boundary between the Cretaceous and Tertiary. Best known as the period of extinction of the dinosaurs, the K-T extinction also included the disappearance of 63-77% of all species, among them many marine foraminifera and land plants in western North America and eastern Asia. Two groups which survived the extinction were the flowering plants and the small mammals.
Of the major mass extinctions, there is strong evidence of extraterrestrial causes for two of these sudden interruptions in the history of life. Crater ages and mass extinction dates correspond for the Devonian extinction of 368 million years ago and the 52 km diameter Siljan crater. The Chicxulub crater of the Yucatan corresponds perfectly with the Late Cretaceous extinctions of 65 million years ago. A third mass extinction, that of the Late Triassic 210 million years ago, has a possible fit with the 100 kilometer Manicouagan crater in Canada.
The first persuasive evidence for an extraterrestrial cause for the K-T extinctions came with the 1980 discovery of a one centimeter layer of clay (initially found in northern Italy, and subsequently in nearly 100 sites around the globe) containing iridium, a platinum-group metal which is extremely rare in Earth’s crust but comparatively abundant in meteorites. The iridium is believed to have been laid down by the impact of a 10 km diameter asteroid, the resultant formation of a global dust cloud from the impact, and settling to Earth’s surface of the iridium carried in by the asteroid. This “Alvarez” hypothesis of asteroid extinction, named for research team leaders Luis and Walter Alvarez, now is widely accepted as the best explanation for the K-T extinction.
The study of impact craters on Earth indicates that impact events have been common over geological time scales and that some of these impacts potentially have been catastrophic. Tables comparing crater diameters with their frequency of occurrence on Earth and average time intervals of occurrence indicate that 150 km diameter craters occur every 100 million years, 100 km craters every 50 million years, 50 km craters every 6 million years, and 10 km craters every 10,000 years. Only the largest of these craters would account for a mass extinction.
After the asteroid extinction proposal was made, a widespread search was undertaken for possible places on Earth where evidence remained of a huge asteroid impact. The probable site of this collision of Earth with an asteroid 65 million years ago has been located in the Yucatan Peninsula. The crater was detected belowground through identification of magnetic anomalies which describe a pattern of four concentric circles, and at ground level by distributions of cenotes (limestone sinkholes), on the Yucatan landscape. Cenotes correspond closely with magnetic anomalies.