Extraterrestrial Cause for the Cretaceous–Tertiary Extinction
A thin clay rich in iridium reveals that an asteroid, not slow decline, ended the age of dinosaurs.
The rock that ended the dinosaurs left no eyewitnesses — but it left a chemical fingerprint, in a thin band of clay found on three continents.
The idea, unpacked
Between the rock of the dinosaur age and the rock above it lies a thin layer of clay. In 1980 a team led by Luis and Walter Alvarez found that this clay is loaded with iridium — a metal almost absent from Earth's surface rocks but common in asteroids and comets. The same iridium-rich band turned up in Italy, Denmark and New Zealand, at exactly the moment of the great extinction.
Their explanation: a mountain-sized asteroid, about ten kilometres across, slammed into the Earth 66 million years ago. The impact blasted enough pulverised rock into the high atmosphere to wrap the planet in dust for years. The sky went dark, plants and plankton stopped growing, and the food chains collapsed from the bottom up — taking the dinosaurs with them.
A father, a son, and a clock that broke
Walter Alvarez was a geologist studying the boundary clay at Gubbio, Italy; he wanted to know how many years it had taken to form. His father, Luis Alvarez, was a Nobel-winning physicist, and he suggested a trick: since space dust rains down at a steady rate, the iridium in the clay could act as a clock. Two Berkeley chemists, Frank Asaro and Helen Michel, made the measurement. The clock was useless — there was far too much iridium — but the broken clock was the discovery. That much iridium could only have arrived from space, all at once. For ten years the idea was attacked, especially by palaeontologists who read a slow decline in the fossils. Then, in 1991, the buried crater was found off Mexico: the right size, and exactly the right age.
Why it mattered
Before 1980, geology was governed by the slow-and-steady view inherited from Hutton and Lyell: the present is the key to the past, and great change comes gradually. The Alvarez paper showed that a single instant — a rock from space — could rewrite the history of life. It made catastrophe respectable again, but on hard evidence rather than myth. And it reframed our own existence: had that asteroid missed, mammals might never have inherited the Earth, and there might be no one here to read about it.
A page postmarked from the sky
Think of the rock layers as a stack of letters, each one a year of Earth's mail, the oldest at the bottom. Almost every page is ordinary limestone. But slipped into the stack, at one exact date, is a single sheet dusted with a metal that belongs to outer space — the same metal, in the same kind of dust, on three different continents. One letter, postmarked from the sky, slipped into the world's archive on the day the dinosaurs died.
Before and after
Hutton (1788) and Lyell (1830) taught that Earth changes slowly; Darwin (1859) built evolution on that same patient tempo. The Alvarez impact was the great exception — proof that contingency, an accident from space, can steer the history of life as forcefully as competition. It also opened a new field: geologists now find iridium and shocked minerals at other extinction boundaries, and astronomers map the asteroids whose orbits cross our own.
Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago.
Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record.