Walter Alvarez
The geologist who found the iridium-rich clay at Gubbio and, with his father Luis, linked the end of the dinosaurs to an impact.
Key facts
- Born
- 1940
- Nationality
- American
- Field
- Geologist
Walter Alvarez is a geologist at the University of California, Berkeley, who has spent much of his career studying the rocks of Italy. In the 1970s he was working on the ancient limestones near the town of Gubbio, in the Apennine mountains, which record many millions of years of sea-floor sediment.
At one level in the rock, the tiny fossils that fill the limestone change suddenly. Below, there are many kinds; above, almost all have gone. Between them lies a thin layer of clay, about a centimetre thick. This marks the extinction at the end of the Cretaceous period, 66 million years ago, when the dinosaurs died out.
Walter wanted to know how long that clay had taken to settle. His father, the physicist Luis Alvarez, suggested measuring iridium, which arrives from space at a slow and steady rate. Instead of a small amount, they found a huge excess. Together with Frank Asaro and Helen Michel, they proposed in 1980 that a large asteroid impact had caused the extinction.
The iridium layer was soon found at the same boundary all over the world, together with shocked quartz and tiny glassy spherules from the impact. Walter Alvarez told the story in a popular book, T. rex and the Crater of Doom, and the hypothesis is now widely accepted.