A thin layer of clay in the Italian countryside helped Walter Alvarez explain how the dinosaurs disappeared. When I photographed him in Berkeley on a beautiful fall day in 2024, I had just finished his book, T. Rex and the Crater of Doom. I arrived with the pleasure of having followed a scientific mystery to its conclusion and then being invited to spend a day with the man who helped solve it.
Walter welcomed me into his warm, wood-paneled home, where I met his wife and traveling companion, Milly. They loved trains enough to have built a replica of a railway seating area, complete with timetables and a working train whistle. Even at home, there was a place to imagine the next journey.
Over a caprese salad in the garden, we talked about extinction and the detective work of geology. Rocks preserve time unevenly. Millions of years can be compressed into a few inches of stone. Walter wanted to understand what one thin layer of clay could tell him.
Near Gubbio, Italy, he had been studying limestone that formed on an ancient seabed. A thin seam of clay marked a striking change in the fossil record. Below it were abundant fossils of tiny marine organisms. Above it, many of those species were gone. The boundary recorded the mass extinction that ended the Cretaceous period, 66 million years ago.
Walter and his father, Luis Alvarez, a Nobel Prize winner in physics, wanted to know how long the clay had taken to accumulate. They enlisted nuclear chemists Frank Asaro and Helen Michel to measure its iridium, an element rare in Earth’s crust but more abundant in meteorites. Small amounts continually arrive in cosmic dust. Perhaps that steady delivery could serve as a clock.
Instead, they found far more iridium than expected. What had begun as an effort to measure time became evidence of a catastrophe.
In 1980, the four scientists proposed that a large asteroid had struck Earth. Dust thrown into the atmosphere could have blocked sunlight, interrupting photosynthesis and devastating the food chains that depended on it. The extinction wiped out the dinosaurs apart from the lineage that gave us birds, along with many other forms of life. The later identification of the Chicxulub impact crater beneath Mexico’s Yucatán region supplied crucial evidence for their explanation.
I had read this story before our visit, but hearing Walter describe the work brought its uncertainties closer. The answer had emerged through a geologist’s observations, a physicist’s questions, and measurements precise enough to detect an extraordinary trace in ordinary-looking clay.
Later, we walked through the Berkeley campus and stopped at the office where his father had worked. Walter spoke with deep admiration about him and about their collaboration. I had come to hear how he helped solve an ancient mystery. Standing there, I was also listening to a son remember his father.
