EXTINCTIONS × TECHNOLOGY

A Layer of Iridium

1980 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

A centimetre of clay at the Cretaceous boundary holds far more iridium than the Earth's crust ever does, and about as much as an asteroid. 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. Half the fiches on this timeline that name a sudden cause, an impact, an eruption, a flood, are written in a vocabulary that this centimetre of clay made available again.

Dating & uncertainty

The impact hypothesis was published in 1980 from a metal anomaly in a centimetre of clay. The crater that fitted it was identified in Mexico eleven years later, which is what turned the hypothesis 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