A thin line in the rock
In the Bottaccione Gorge outside Gubbio, a road cut exposes layers of pinkish limestone called the Scaglia Rossa. This rock formed from sediment accumulating on a deep ocean floor during the Cretaceous period. Embedded in this sequence is a single, dark clay layer, about one centimeter thick. This band of rock is the Cretaceous-Paleogene (K-Pg) boundary. It records the instant, 66 million years ago, when a catastrophic impact event triggered a mass extinction that eliminated about 75% of all species on Earth, including the non-avian dinosaurs.
Before the 1980s, the cause of this extinction was a complete mystery. In 1977, geologist Walter Alvarez was studying these limestone layers to understand Earth's ancient magnetic field. He noticed that the fossil record of tiny marine organisms called foraminifera changed dramatically across this one clay layer. Below the clay, Cretaceous limestone teems with a wide variety of large foraminifera. Directly above it, in the Paleogene rock, most of these species are gone, replaced by a few small survivors. This clear evidence of a mass extinction prompted a new question: what could have possibly caused it?
The extraterrestrial element
To determine how long it took for the clay layer to form, Walter Alvarez consulted his father, Nobel Prize-winning physicist Luis Alvarez. They decided to measure the concentration of the element iridium. Iridium is extremely rare in Earth's crust but is much more abundant in asteroids and comets. The team hypothesized that a slow, steady rain of cosmic dust would give them a baseline iridium level, allowing them to calculate the deposition time. The results were shocking. The clay layer didn't just have a bit of iridium; it had a concentration hundreds of times greater than the surrounding rock.
In 1980, the Alvarez team published their hypothesis: the iridium spike was the fallout from a massive asteroid, roughly 10 kilometers in diameter, striking the Earth. The impact would have ejected enormous quantities of pulverized rock and dust into the atmosphere, blocking sunlight for years and causing a global collapse of photosynthesis. This "impact winter" explains the sudden disappearance of so many species in the fossil record. The theory was initially controversial, and the discovery of similarly iridium-rich layers at K-Pg sites worldwide, from Denmark to New Zealand, confirmed it was a global event. The final piece of evidence arrived with the identification of the 150-kilometer-wide Chicxulub crater, buried beneath Mexico's Yucatán Peninsula, which dates precisely to the extinction event. The thin layer of clay in Gubbio linked a major impact to a mass extinction.