The Alvarez Hypothesis (1980): An Asteroid Killed the Dinosaurs

Luis and Walter Alvarez proposed in 1980 that an asteroid about 10 kilometers across killed the dinosaurs. With chemists Frank Asaro and Helen Michel, they found that iridium, a metal rare on Earth but common in meteorites, spikes exactly where the Cretaceous ends, tying the extinction to an impact.

What was known before 1980?

Rocks around the world record a sharp break at the end of the Cretaceous period, the boundary geologists then called Cretaceous-Tertiary. Below it, the fossil record includes the non-avian dinosaurs. Above it, they are gone. Why it happened was an open question for geology and paleontology, and Solar System science played no part in the story.

What the paper found

• An iridium spike at the boundary. In deep-sea limestones from Italy, Denmark and New Zealand, iridium rose about 30, 160 and 20 times above background, exactly at the extinction boundary.
• A source from space. Iridium is rare in Earth’s crust but common in meteorites, so the excess pointed to an extraterrestrial origin.
• An asteroid about 10 kilometers across. The team proposed that a body of that size struck Earth.
• A killing mechanism. Dust thrown into the atmosphere by the impact, they argued, caused the mass extinction, including the non-avian dinosaurs.

How they did it

The team combined unusual skills. Walter Alvarez, a geologist, knew the rocks that span the end of the Cretaceous. His father Luis, a physicist, and the nuclear chemists Asaro and Michel could measure trace elements at very low levels.

Iridium made a good tracer because Earth’s crust holds very little of it while meteorites carry much more. The team measured it in seafloor limestones at three widely separated sites, comparing the boundary layer with the rock above and below it, the “background.” The same sharp excess at the same moment in Italy, Denmark and New Zealand was the key: a local accident could not explain it.

From the amount of extra iridium they built a scenario. An asteroid roughly 10 kilometers wide hits Earth, its debris rises into the atmosphere as dust, and the mass extinction follows.

Why it mattered

The paper linked astronomy to the history of life on Earth. Afterward, an impact from space was a hypothesis with a chemical signature that geologists could test in rocks anywhere.

It also launched the modern study of impact hazards. If a 10 kilometer asteroid could end the age of dinosaurs, finding and tracking asteroids became a practical problem. The direct descendant is NASA’s DART test, which changed a small asteroid’s orbit in 2022.

What we know now

The missing piece was the crater. In 1991 the Chicxulub crater, about 180 kilometers wide and buried under the Yucatán Peninsula in Mexico, was identified as the impact site.

Impacts are now seen as a force across Solar System history, from the giant collision that likely formed the Moon to today’s hunt for hazardous asteroids. The newest survey telescope, the Vera C. Rubin Observatory in Chile, found more than 11,000 previously unknown asteroids, 33 of them near-Earth objects, in about a month and a half of early test surveys before its 10-year survey began in mid-2026.

Who wrote it

Luis Alvarez was a physicist who had won the 1968 Nobel Prize in Physics for work in particle physics. His son Walter was a geologist, and Frank Asaro and Helen Michel were nuclear chemists. The paper appeared in Science on 6 June 1980 and has about 3,700 citations in Semantic Scholar (October 2026).

Questions people ask

What killed the dinosaurs?

The leading explanation is an asteroid impact. In 1980 Luis and Walter Alvarez and their colleagues proposed that a body about 10 kilometers across struck Earth at the end of the Cretaceous period. The Chicxulub crater, about 180 kilometers wide under Mexico’s Yucatán Peninsula, was identified as the impact site in 1991.

What is the Alvarez hypothesis?

The Alvarez hypothesis is the idea that an asteroid impact caused the mass extinction at the end of the Cretaceous, including the non-avian dinosaurs. Physicist Luis Alvarez, geologist Walter Alvarez and chemists Frank Asaro and Helen Michel proposed it in 1980, based on unusually high iridium levels in boundary rocks.

What is the iridium layer?

The iridium layer is rock at the end of the Cretaceous that is enriched in iridium, a metal rare in Earth’s crust but common in meteorites. The 1980 Alvarez study found iridium about 20 to 160 times above background at sites in Italy, Denmark and New Zealand, pointing to an impact.

Where did the dinosaur-killing asteroid hit?

It hit what is now the Yucatán Peninsula in Mexico, where the Chicxulub crater lies buried. The crater is about 180 kilometers wide and was identified as the impact site in 1991, eleven years after the Alvarez team proposed that an asteroid caused the extinction.

Read the original

Alvarez, L. W., Alvarez, W., Asaro, F. and Michel, H. V. (1980). “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Science 208, 1095-1108. doi:10.1126/science.208.4448.1095

The journal version may require a subscription.