Life › The age of dinosaurs › Chicxulub asteroid impact
Iridium layer at the K–Pg boundary
66 million years ago (found 1980)· Fossil or site· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
A thin clay layer, rich in the rare metal iridium, marking the K–Pg boundary worldwide; the first evidence of an impact.
See it on the Life map →What happened
At Gubbio, Italy, a clay layer about a centimeter thick separates Cretaceous limestone from younger rock. In 1980 the Alvarez team reported it held roughly 30 times the normal level of iridium, a metal common in asteroids but rare in Earth's crust because most of it sank into the core. The same spike has since turned up at more than 100 sites worldwide, on land and in deep-sea cores, often with shocked quartz and tiny glass beads.
Why it matters
It was the first hard evidence that a mass extinction had an extraterrestrial cause, and it launched the search that found the crater.
Think of it like
Like flecks of foreign paint at a crash scene: they do not show the car, but they prove something hit.
The evidence (fossils, genes, rocks)
Chemical measurements at Gubbio, Italy and Stevns Klint, Denmark, published in Science in 1980 by Luis and Walter Alvarez, Frank Asaro and Helen Michel, then repeated at sites worldwide.
Still debated
Some once argued volcanoes could supply the iridium, but its global spread, its ratios to other metals and the shocked minerals with it point to an impact.
Connections (2)
▸Iridium layer at the K–Pg boundary was the first clue to Chicxulub asteroid impactWell established
The connection
The iridium spike implied that an asteroid about 10 kilometers wide had struck Earth, but in 1980 no crater of the right age was known. The search narrowed when thick impact debris and tsunami deposits turned up around the Gulf of Mexico, leading to a buried crater at Chicxulub, confirmed in 1991.
When and how
From Gubbio, Italy (1980) to Yucatán, Mexico, where Pemex geophysicists Glen Penfield and Antonio Camargo had noted a buried ring in 1978; Alan Hildebrand's team confirmed it in 1991.
The evidence
Melt rock from the crater has the same age as the boundary layer, and impact glass beads found in Haiti match the crater's chemistry.
Debates and open questions
A few scientists argued for several impacts or volcanic iridium; these are now minority views, though smaller craters of roughly similar age, in Ukraine and off West Africa, are still examined.
▸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.