18. Manicouagan Impact Crater, Quebec, Canada

Manicouagan is one of the largest known terrestrial impact craters. It is 65 kilometers (40 miles) in diameter, but it is worth remembering that this is small compared with some of the larger lunar structures that measure more than 600 km (375 miles) across. Produced by an asteriod impact some 200 million years ago, the concentric structure of the Manicouagan Crater results from the shock waves transmitted by the impact event. These resemble somewhat the rings that result when a pebble is dropped into water.

The significance of asteroid impacts on the history of the Earth has only been fully recognized in recent years. The extinction of whole groups of animals such as dinosaurs and ammonites at the end of the Cretaceous Period some 60 million years ago is now believed to have been caused by an asteroid impact. The catastrophic collision raised a dust cloud that obscured the Sun and lowered temperatures dramatically. Fallout from the impact covered the entire world, and provided a key pointer to the impact event: The fallout is rich in iridium, an element abundant in meteoritic materials and rare in surface rocks on Earth. The presence of this element at the Cretaceous-Tertiary boundary in several important sections has convinced many scientists that asteroidal impact was responsible for the mass extinctions at the end of the Cretaceous, but others remain skeptical. It is possible that the asteroid responsible may not have been a great deal larger than that which excavated the Manicouagan Crater. No extinctions associated with the Manicouagan event have yet been demonstrated, however.

STS-9, November-December 1983. Picture #9-48-3139.

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