Iridium anomaly
A layer of rock with far more of the rare metal iridium than normal, often a sign that an asteroid impact scattered its debris worldwide.
Iridium is a heavy metal that is very scarce in Earth's crust, because most of our planet's share sank into the iron core long ago. Many meteorites, however, still carry it at much higher levels. So when geologists find a thin layer of sediment unusually rich in iridium, it hints that extraterrestrial material once rained down.
The most famous case is the clay at the K–Pg boundary, 66 million years old. In 1980 a team led by physicist Luis Alvarez and his son, geologist Walter Alvarez, reported that this layer at Gubbio in Italy, and at sites in Denmark and New Zealand, held far more iridium than the rocks above and below. They proposed that a large asteroid impact caused the mass extinction that ended the age of non-bird dinosaurs. The idea was strongly supported when the buried Chicxulub crater in Mexico, about 180 kilometres across, was identified around 1990–91.
An iridium anomaly cannot be seen with the eye; it is measured in the laboratory. In the field the boundary often appears as a thin clay band, sometimes with shocked quartz and tiny glassy spherules.
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