AWC Pictures

The extinction of the dinosaurs has long been considered a crime committed by a lone gunman: an incoming asteroid that struck the earth 65 million years ago, filling the air with sun-blocking dust. Now, however, controversy is being stirred anew as evidence suggests that the asteroid might have had a partner in crime: volcanoes, massive ones, blasting clouds of toxic gas from the bowels of the earth and poisoning much of the planet’s life.

It was nearly 30 years ago that physicist Luis Alvarez and his son Walter, a geologist, proposed the giant-impact theory of dinosaur extinction. Their evidence was compelling: a thin layer of iridium in the earth’s sediment dating to about the time of the die-off. Iridium is rare on Earth but common in asteroids. The iridium layer, mapped by the Alvarezes in scattered sites around the world, suggested an asteroid that vaporized on impact, spreading a cloud throughout the stratosphere. The argument seemed sealed in the 1990s, when geologists realized that a huge crater centered near Chicxulub, Mexico, was almost certainly caused by a giant impact at just the time the extinctions occurred.
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