K–Pg boundary
The thin layer of rock that marks the end of the Cretaceous period 66 million years ago, when a giant asteroid impact helped wipe out the dinosaurs.
Geologists divide Earth's history into periods. The K–Pg boundary is the line between the Cretaceous (shortened to K) and the Paleogene (Pg). It used to be called the K–T boundary. Rocks below it hold fossils of non-bird dinosaurs, ammonites and many other creatures; rocks above it do not. Roughly three-quarters of all species disappeared at that time.
The cause was mostly worked out from the boundary layer itself. It is enriched in iridium, a metal common in meteorites but rare on Earth's surface, and contains shocked quartz grains and tiny glassy beads that only a huge impact could make. The crater was later found buried beneath the Yucatán Peninsula in Mexico: Chicxulub, about 180 kilometres wide. The asteroid was probably around 10 kilometres across. Massive volcanic eruptions in India, called the Deccan Traps, were also happening around then, and scientists still discuss how much they added to the crisis.
You can see the boundary at a few places, such as in cliffs and road cuttings in Italy, Spain and the western United States, as a thin, often clay-rich band. Respect site rules, as many are protected.
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Level: Explorer Ages 10 and up