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Luis & Walter Alvarez
impact hypothesis 1980· Scientist· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
Physicist Luis Alvarez and his geologist son Walter proposed in 1980 that an asteroid impact killed off the non-bird dinosaurs.
See it on the Life map →What happened
Walter Alvarez was studying a thin clay layer at Gubbio, Italy, that marks the end of the Cretaceous, 66 million years ago. He, his Nobel-winning father and chemists Frank Asaro and Helen Michel found it held about 30 times the normal level of iridium, a metal rare in Earth's crust but common in asteroids. The same spike turned up in Denmark and New Zealand. In 1980 they proposed that an asteroid about 10 kilometers wide had struck, darkening skies worldwide.
Why it matters
It revived the idea that sudden catastrophes shape life's history, and showed how physics and chemistry could crack a fossil puzzle.
Discoveries and legacy
Their 1980 paper in Science; iridium spikes later found at over 100 boundary sites worldwide; and the predicted crater, identified at Chicxulub, Mexico, in 1991.
Still debated
How much the enormous Deccan Traps volcanic eruptions in India also contributed to the extinction is still argued.
Connections (2)
▸Luis & Walter Alvarez interpreted Iridium layer at the K–Pg boundaryWell established
The connection
Geologist Walter Alvarez asked his father, Nobel physicist Luis Alvarez, how to measure how long the Gubbio clay took to form. They used iridium, which rains down steadily in meteorite dust, as a clock. The level was far too high for slow dust, so in 1980 they argued an asteroid impact had delivered it.
When and how
University of California, Berkeley, 1977–1980, with nuclear chemists Frank Asaro and Helen Michel; published in Science, June 1980.
The evidence
Their measurements were repeated at boundary sites around the world, and the later discovery of the crater confirmed the interpretation.
Debates and open questions
The impact idea was fiercely resisted for a decade, partly because geology favored gradual change; some still argue volcanism played a large role.
▸Luis & Walter Alvarez predicted Chicxulub asteroid impactWell established
The connection
An asteroid big enough to spread iridium worldwide should have left a crater roughly 100–200 kilometers wide, yet none of the right age was known. Shocked quartz, tsunami deposits and glass beads found around the Gulf of Mexico narrowed the search. In 1991 a buried structure under Mexico's Yucatán coast was identified as the crater, matching the prediction.
When and how
Geophysicists Glen Penfield and Antonio Camargo, working for Mexico's state oil company Pemex, noticed the buried ring in 1978; Alan Hildebrand and colleagues confirmed it as an impact crater in 1991.
The evidence
Argon dating shows the crater and the boundary clay are the same age, about 66 million years; cores drilled in 2016 recovered shocked and melted rock.
Debates and open questions
Whether the impact alone caused the extinction or worked together with India's Deccan Traps eruptions.