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Life on Earth has suffered periods of mass extinction, when many of the living organisms were wiped out within a short period of time – perhaps a few thousand years. Many of these undoubtedly occurred too long ago to be recognized in the fossil record, but between 5 and 20 such events are thought to have taken place in the last 540 million years. (The precise number depends on the definition of “mass extinction.”)

Probably the most catastrophic of these events occurred about 250 million years ago. Sometimes known as the “Great Dying,” the extinction marks the boundary between the Permian and Triassic periods in geological history. It is estimated that about 60% of all land species and 90% of all ocean species abruptly died out at that time.

The most famous extinction took place 65 million years ago and was responsible for ending the age of the dinosaurs. The most recent major extinction took place at the end of the Eocene period, about 35 million years ago.¹²

The causes of these mass extinctions are not well understood, but a major impact, or periodic increases in the rate of asteroid or comet impacts have been put forward as explanations for at least some of them. The most likely example of this is thought to be the event which coincided with the demise of the dinosaurs, as well as many other species of flora and fauna.

In 1980, a team of geologists, led by Luis and Walter Alvarez, discovered a relatively high concentration of iridium in a strip of clay that runs through rocks around the world. Iridium is an element that is very rare on Earth but common in meteorites and asteroids. This clay‐iridium layer marks the end of the Cretaceous period in geological history.

Figure 3.36 Meteor crater (also known as Barringer crater) in Arizona is one of the most recent impact craters on Earth. It was created about 50,000 years ago, when an iron meteorite excavated a hollow some 1.2 km in diameter and 180 m deep. The crater has a simple basin shape with no central peak or rim terraces. Surrounding the basin is a wall of material 30–45 m high where the rocks have been uplifted and, in some cases, overturned. The 30‐m wide meteorite probably weighed about 100,000 tonnes and struck the surface at a speed of around 12 km/s. The energy released was equivalent to about 2.5 megatonnes of TNT.

(D. Roddy / USGS, Lunar and Planetary Institute)

Figure 3.37 A computer‐generated gravity map reveals the Chicxulub impact crater (center) buried beneath Mexico's Yucatan peninsula. The impact structure is widely thought to be the cause of a mass extinction 65 million years ago. Gravity measurements show a multi‐ring basin with a fourth, outer ring about 300 km in diameter. It is one of the largest impact structures on Earth.

(LPI/NASA)

Searches for likely impact candidates revealed a crater up to 320 km across, buried beneath the Yucatan peninsula of Mexico. Geophysical surveys indicate that the multi‐ringed Chicxulub crater was formed on the ocean floor. The explosive impact would have caused huge clouds of debris to blanket the planet and rain down from the sky, disrupting the climate and the food chain, while surrounding shorelines were devastated by tsunamis. This interpretation is supported by finds of shocked rocks, tektites (small pieces of natural glass), and widespread deposits of iridium.

Whether similar impacts are to blame for other extinctions remains uncertain. It has been suggested that most mass extinctions over the past 300 million years coincided with large‐scale volcanic activity. For example, at the time of the Permian extinction, major eruptions were taking place in Siberia, where 1.5 million cubic kilometers of lava flowed from huge fissures in the crust. Other possible contenders include high‐energy radiation from a nearby supernova and environmental changes brought about continental drift.

Even the impact theory for the Cretaceous event is disputed by some scientists, who point out that the extinction also coincides with the formation of the Deccan Traps in India, one of Earth's largest eruptions of fluid lava. Geologists have identified a 600‐m thick lava sheet that may have piled up in as little as 30,000 years – potentially causing a deadly shift in atmospheric gases and global climate (see Volcanoes).