EXTINCTIONS × TECHNOLOGY

A Layer of Iridium

1980 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

A centimetre of clay at the Cretaceous boundary holds tens to hundreds of times more iridium than the rock immediately above and below it. The metal is scarce in the Earth's crust and common in what arrives from space. The proposal that followed, that a mass extinction had one sudden cause, was not welcome in a discipline that had spent a century insisting only slow processes count. The crater that fitted was found in Mexico eleven years later.

Why it matters

It made catastrophe respectable again in geology. Many of the entries on this timeline that name a sudden cause, an impact, an eruption, a flood, are written in a vocabulary that this centimetre of clay put back into circulation.

Dating & uncertainty

1980 belongs to the hypothesis, not to its acceptance. For eleven years it rested on chemistry alone, with no crater to attach it to; identifying one in Mexico is what turned a contested proposal into the standard account.

Sources

1. Alvarez et al. (1980), Science · Extraterrestrial cause for the Cretaceous-Tertiary extinction
2. Osinski et al. (2022), Earth-Science Reviews · Impact Earth: a review of the terrestrial impact record
3. Erickson et al. (2020), Nature Communications · Precise radiometric age establishes Yarrabubba as Earth's oldest recognised meteorite impact structure

Earth history as one day

23:59:59.9991875 µs before midnight

if Earth’s 4.54 billion years were compressed into a single day