In the late 1970s, geologist Walter Alvarez was studying limestone cliffs in the mountains near Gubbio, Italy. He was mapping the boundary between two geological ages: the Cretaceous, when dinosaurs ruled, and the Tertiary, when they were gone. He noticed something strange. In the lower, older rock, fossil shells were large and diverse. But above a thin, centimetre-thick band of red clay, they vanished completely. It was as if a curtain had fallen on life itself. This thin layer marked the exact moment the dinosaurs disappeared from the fossil record. The rock told a clear story of a sudden, catastrophic event, but the cause was a complete mystery. The clue, Alvarez suspected, was hidden within that thin band of red clay.
The Asteroid That Killed the Dinosaurs
A thin clay layer identified in 1980 explains why the dinosaurs died out 66 million years ago: a city-sized asteroid impact.
Chapter 1
The Red Layer
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Frequently Asked Questions
What was the Alvarez Hypothesis?
Published in 1980, the Alvarez Hypothesis proposed that the mass extinction of the dinosaurs was caused by a large asteroid impact. The key evidence was a thin layer of clay found worldwide that contained iridium at levels up to 160 times higher than the surrounding rock, an element rare on Earth but abundant in asteroids.
Where did the asteroid hit the Earth?
The impact occurred at Chicxulub on Mexico's Yucatán Peninsula. The crater is approximately 180 kilometres wide but is now buried under a kilometre of sediment, so it is not visible on the surface.
What kinds of animals survived the extinction event?
Survival favored small animals. Mammals that lived in burrows and ate seeds or insects were protected from the initial blast and subsequent cold. The only dinosaurs to survive were small, feathered avian dinosaurs, which are the direct ancestors of all birds today.
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