American geologist who, with his father, Louis, discovered evidence that a large impact was the most likely cause of the mass extinction of dinosaurs. Alvarez analyzed samples from the Italian mountains that were deposited at the boundary between the Cretaceous and Triassic periods (known to geologists as the "KT boundary") and noticed high levels of iridium, commonly found in meteorites. This led to his discovery that a large meteor or a small asteroid impact caused the dinosaurs' extinction.