In the pink limestone gorges near the Italian town of Gubbio, there is a place where you can put your finger on the death of the dinosaurs. Between thick beds of ancient rock runs a thin, drab seam of clay barely a centimetre wide. Below it, the limestone teems with the fossils of tiny sea creatures. Above it, they vanish. That clay layer marks the boundary between two great chapters of Earth's history — the end of the age of dinosaurs and the dawn of the age of mammals — and for a long time nobody knew why life had fallen off a cliff at exactly that line.
In the 1970s a geologist named Walter Alvarez was studying those rocks and wondering how long the clay layer had taken to form. Was the extinction sudden or slow? He turned for help to his father, Luis Alvarez, a physicist who had won a Nobel Prize for work in particle physics and who never met a problem he did not want to attack with instruments. The father proposed an ingenious clock. Iridium, a metal in the platinum family, is extremely rare in the Earth's crust but rains down steadily from space in the form of micrometeorite dust. Measure how much iridium had accumulated in the clay, the reasoning went, and you could estimate how many years it represented.
An impossible reading
The result made no sense. The clay held far too much iridium — dozens of times more than the slow celestial drizzle could account for, and the same anomaly turned up in a matching clay layer at Stevns Klint in Denmark, then at sites around the world. Something had dumped an enormous quantity of iridium across the whole planet in a geological instant. Since iridium is abundant in asteroids and comets, the Alvarez team drew a startling conclusion: the boundary had been laid down by the impact of a huge extraterrestrial object, and that impact had killed the dinosaurs.
In 1980 they published the hypothesis. A body perhaps ten kilometres across, they argued, had slammed into the Earth around sixty-six million years ago with a violence beyond any volcano or war. The impact would have hurled so much pulverised rock and dust into the upper atmosphere that sunlight was choked off for months or years. Photosynthesis would have stalled, food chains collapsed, and the great cold and dark done the rest. Roughly three-quarters of all species on Earth died, including every non-bird dinosaur.
Many geologists hated it. The reigning view held that extinctions were gradual affairs, driven by slow changes in climate and sea level and by the era's vast volcanic eruptions in India, the Deccan Traps. A sudden death from the sky smelled of catastrophism, an old and disreputable idea. Critics also asked the obvious question: if an asteroid ten kilometres wide had hit the planet, where was the crater?
The smoking gun beneath the sea
The answer, it emerged, had already been found and forgotten. Oil-company geophysicists surveying the Yucatán Peninsula of Mexico had detected a huge buried structure near the town of Chicxulub, a near-circular ring some one hundred and eighty kilometres across, half on land and half beneath the Gulf of Mexico. In the early 1990s researchers dated it and found it matched the boundary precisely: about sixty-six million years old, exactly the right size, and formed in seconds by the impact of a body of exactly the scale the Alvarezes had predicted. Around the Gulf and Caribbean, geologists found tell-tale debris — shocked quartz grains, glassy droplets called tektites, and jumbled tsunami deposits — all pointing to a single day of apocalypse.
The Chicxulub crater turned a bold hypothesis into the mainstream account of the dinosaurs' end. Debate continues about the supporting role of the Deccan volcanism, and some scientists argue the eruptions and the impact together delivered the fatal blow. But the iridium layer — that unremarkable band of clay that Walter Alvarez first tried to use as a stopwatch — remains one of the most consequential discoveries in the history of the Earth sciences, a global signature of the worst day in the last hundred million years.
Luis Alvarez did not live to see the crater dated; he died in 1988. But the story he and his son pieced together from a smear of clay reshaped how we think about extinction itself — as something that can arrive not over millions of years but in an afternoon, from a direction nobody thought to watch.
Quiz nuggets
- The father-and-son team of physicist Luis Alvarez and geologist Walter Alvarez proposed the asteroid-impact theory in 1980.
- Their key evidence was an anomalous spike of iridium — an element rare in Earth's crust but common in asteroids — in the clay at the Cretaceous–Paleogene boundary.
- The impact is dated to about 66 million years ago and is thought to have killed roughly three-quarters of species, including all non-bird dinosaurs.
- The confirming crater, Chicxulub, lies on Mexico's Yucatán Peninsula and is about 180 kilometres across.
- A rival or contributing cause is the Deccan Traps volcanism in India around the same period.