Iridium is the element whose unusual abundance in the K–Pg boundary layer provides evidence for a large asteroid impact. The thin, worldwide layer formed around 66 million years ago, when the event associated with the end-Cretaceous mass extinction occurred.
Iridium is rare in Earth’s crust but comparatively common in many asteroids and meteorites. Scientists Luis and Walter Alvarez, with colleagues, argued in 1980 that the global iridium anomaly reflected material added by an extraterrestrial impact rather than ordinary surface geology.
The clue became much stronger after the Chicxulub impact crater was identified beneath the Yucatán Peninsula and Gulf of Mexico. Its age matches the boundary, and the crater’s size is consistent with an asteroid roughly 10 kilometers wide. The impact would have blasted dust and vapor into the atmosphere, disrupting sunlight, climate, and food webs.
Rhodium, platinum, and osmium are also platinum-group elements, so they can appear among the wrong options. However, the globally famous K–Pg geochemical signal is specifically the iridium anomaly.