Back in 1977, geologist Walter Alvarez returned from a scientific expedition to Italy with a peculiar rock sample, liberated from limestone that was once underneath a long-gone ocean. The rock’s older, bottom layers were full of fossils. But above them was a layer of clay that had none. That layer captured the aftermath of an event 66 million years ago, when something caused a mass extinction, slaying 75 percent of the species on the planet, including T. rex and triceratops.
When he showed it to his physicist father, Luis Alvarez, both became obsessed with studying this rock, convinced it held the answer to what was, at the time, a huge mystery: What killed the dinosaurs?
Over time, scientists would amend that query to “What killed the dinosaurs — and is it coming back?” The rock set in motion a series of scientific inquiries that would ultimately suggest that, like clockwork, Earth might experience a catastrophic housecleaning.
But how? Why? And really? Although the rock set off a course of events that led to the idea of cyclical mass extinctions, the concept would evolve over three decades into a heated debate that continues today.
Doom of the Dinosaurs
Walter and Luis noticed something strange about that rock when they analyzed its chemistry. The element iridium was trapped inside, right in the clay layer where the fossils flickered out — it was a trace, but more than might be expected. While there’s plenty of iridium in Earth’s core, there’s not much in our planet’s crust. One way it can land on the surface is to flutter down like ashes when tiny meteorites burn up in the atmosphere. Where did this anomalous amount come from, Luis Alvarez wondered, and what did it have to do with dinosaur death?