how we know an asteroid killed the dinosaurs

The Mechanism

*Walter Alvarez* (born Berkeley, California, *3 October 1940*) was, in *1977*, a *36-year-old structural geologist* at the *University of California, Berkeley* who had spent his postdoctoral years (1971-77) at the *Lamont-Doherty Geological Observatory* of Columbia University working on the *paleomagnetism of Apennine carbonates* — using the magnetic-mineral grains in Italian limestone to reconstruct the rotation of the Italian peninsula relative to Europe over the past 100 million years. The work brought him repeatedly to a *gorge in the Umbrian-Marchean Apennines* called the *Gola del Bottaccione*, just outside the medieval hill town of *Gubbio*, where a continuous section of *Cretaceous-through-Eocene pelagic limestone* — the *Scaglia Rossa formation* — exposes a complete record of marine sedimentation across the *Cretaceous-Paleogene boundary* (then called the *Cretaceous-Tertiary* or *K-T* boundary; renamed *Cretaceous-Paleogene* or *K-Pg* by ICS in 2008), the geological horizon at which approximately *76% of all species on Earth* go extinct in the rock record, including all non-avian dinosaurs, all ammonites, all mosasaurs, all plesiosaurs, and most planktonic foraminifera. At Gubbio the boundary itself is marked by a thin (*about 1 cm*) layer of *bright reddish-brown clay*, lying conformably between the late-Cretaceous pink limestone below (full of the foraminiferal species *Globotruncana*) and the early-Paleogene grey limestone above (full of *Globigerina*). Walter Alvarez took specimens of the clay layer home to Berkeley in the summer of *1977* and asked an interesting question, prompted by a casual conversation with his father: *how long had this clay layer taken to deposit?* The question matters because *the extinction itself* — by the standards of the fossil record — occurs entirely *within* the clay layer. A clay-layer deposition time of *years* would imply a catastrophic event; *thousands of years* would imply a sudden but extended environmental crisis; *hundreds of thousands of years* would imply a gradual decline. *Luis Walter Alvarez* (born San Francisco, *13 June 1911*; died Berkeley, *1 September 1988*, aged 77), Walter's father, was a *Nobel-laureate experimental physicist* (1968 Nobel for elementary-particle research with hydrogen bubble chambers) at the *Lawrence Berkeley National Laboratory*. He proposed a method: measure the concentration of *iridium* in the clay layer. Iridium is one of the rarest elements in Earth's crust — average crustal abundance about *0.001 parts per billion by mass* — because it is a *highly siderophile* ("iron-loving") element that partitions strongly into molten iron and so was almost entirely sequestered into Earth's core during the planet's differentiation 4.5 billion years ago. The remaining iridium in the crust arrives almost exclusively as *cosmic dust* — *micrometeoritic infall* from interplanetary space, which deposits a small, *roughly constant* flux of iridium onto Earth's surface every year. The longer a clay layer takes to deposit, the more iridium-bearing cosmic dust it should contain. Luis recruited the Berkeley *Lawrence Berkeley Laboratory* *neutron-activation-analysis* facility — *Frank Asaro* and *Helen V. Michel*, the two analytical chemists who ran the iridium-detection programme there — to measure iridium concentrations in the Gubbio clay and in the limestones immediately above and below it. The detection limit was about *0.001 parts per billion*. The expected concentration, on a cosmic-dust deposition rate consistent with thousands-of-years clay-layer formation, was a few parts per *trillion* — at or just above the detection limit. The result, returned by the Berkeley facility in *1978*: the Gubbio clay layer contained iridium at *9 parts per billion* — *nine thousand* times the expected cosmic-dust background and *thirty times* the iridium concentration in the limestone immediately above and below. Iridium concentrations of that magnitude were *impossible* under any cosmic-dust-deposition explanation. The Alvarez team replicated the measurement at the *K-Pg boundary at Stevns Klint*, Denmark (a sea-cliff exposure of marine chalk) — iridium concentration *160 times* background. At *Woodside Creek*, New Zealand — 20 times background. Every K-Pg-boundary clay layer they could sample, on three continents, carried the iridium signature. The interpretation, worked out by the Alvarez team through 1978-79, was that the iridium had been delivered by a *single extraterrestrial impact*: an asteroid (or large comet nucleus) about *10 km in diameter* striking the Earth at *~20 km/s*, vaporising itself and a comparable mass of target rock, ejecting an iridium-rich vapour cloud into the high atmosphere that circulated globally and settled out as a thin worldwide clay layer over the months-to-years following impact. The pulverised target rock thrown into the stratosphere had darkened the atmosphere globally, shut down photosynthesis for years, and caused the extinction. The paper — Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., *"Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,"* *Science* 208(4448): 1095-1108 — appeared on *6 June 1980*. It was received with *furious skepticism* by the paleontology and geology communities, who argued for many years that the extinction was a gradual process driven by *Deccan Traps volcanism* (the 65-million-year-old Indian basalt province, which was indeed erupting at the time), and that the iridium spike could be alternative-explained by a slow accumulation in a sediment-starved interval. The Alvarez hypothesis received its definitive confirmation in *1991*, when *Alan Hildebrand* and colleagues identified a *180-km-diameter buried impact crater* — the *Chicxulub crater* — under the *Yucatán Peninsula* of Mexico, with an age (measured by argon-argon dating of crater-melt glass) of *66.043 ± 0.011 million years*, identical to the K-Pg boundary age. Drill cores from the Chicxulub peak ring, retrieved by an *IODP-ICDP joint expedition* in *April-May 2016*, showed a sequence consistent with a hypervelocity impact of a *10-12-km-diameter* projectile (now believed to be a *carbonaceous-chondrite asteroid*, rather than a comet, on the basis of the *chromium-isotope* signature of the boundary clay). The *2010* multidisciplinary review by *Schulte et al.* in *Science* (41 co-authors from across geology, paleontology, and geophysics) was the formal scientific-community consensus that the Chicxulub impact alone, without invoking the Deccan Traps as a primary cause, was sufficient to explain the K-Pg extinction. Luis Alvarez died in September 1988, having lived to see the iridium-anomaly result vindicated at every K-Pg section worldwide but three years before the Chicxulub crater itself was found. Walter Alvarez, in 2026, is *85*, emeritus at Berkeley.