Mass ExtinctionsEarth's Biggest Crises for Life

Life on Earth has come close to being wiped out several times. The game covers six crises: Snowball Earth, the end of the Ordovician, the late Devonian, the end of the Permian, the end of the Triassic and the end of the Cretaceous. The worst was the end-Permian extinction, which killed about 96% of marine species. The most famous was the end of the Cretaceous, when an asteroid about 10 km wide hit the Yucatán Peninsula and wiped out the non-avian dinosaurs.

This page first lines up the six crises in one table, then covers how scientists worked out each one.

The six crises at a glance

The six crises had different causes: ice, oxygen loss, volcanoes, acidification and an asteroid. Mass extinctions are not the same disaster repeated.

Snowball Earth: a planet under ice

Rock layers in Namibia suggest that about 700 million years ago the Earth may have been almost entirely covered in ice (Hoffman et al., Science 1998). The paper's title is "A Neoproterozoic snowball Earth". Note the word "may": this is an inference drawn from the rocks.

Oxygen and its absence

The late Devonian crisis was tied to ocean oxygen loss, and the end of the Permian also saw the ocean acidify and lose oxygen. To understand what losing oxygen means, you need to know that oxygen in the air did not arrive all at once. It rose over a long, back-and-forth process (Lyons, Reinhard & Planavsky, Nature 2014).

End of the Permian: the worst one

Precise dating shows that the end-Permian extinction may have lasted only about 60,000 years, which fits the Siberian volcanic activity (Burgess, Bowring & Shen, PNAS 2014). Sixty thousand years is a blink in geological time. This crisis was both deep and fast.

End of the Cretaceous: from iridium to a crater

The end-Cretaceous story is one of the best-known detective cases in the history of science.

• The clue: rock layers from the end of the Cretaceous hold unusually high levels of iridium. Luis and Walter Alvarez concluded that an asteroid had hit the Earth (Alvarez et al., Science 1980).
• The scene: under Mexico's Yucatán Peninsula lies a crater about 180 km across, matching the asteroid that did it (Hildebrand et al., Geology 1991).

The two studies are more than ten years apart. One made the inference and the other found the evidence.

Sorting by cause

• Ice: Snowball Earth, with ice over almost the whole planet; the end of the Ordovician, when an ice age struck suddenly, sea level fell and many shallow-sea animals vanished.
• Oxygen loss: the late Devonian, when large parts of the ocean ran short of oxygen and the placoderms all disappeared; the end of the Permian, when the ocean acidified and lost oxygen.
• Volcanoes: the Siberian volcanoes erupted for a long time at the end of the Permian; at the end of the Triassic the Atlantic began to rift open, and volcanic activity changed the climate sharply, clearing the stage for the dinosaurs.
• Impact: at the end of the Cretaceous, an asteroid about 10 km wide hit the Yucatán Peninsula.

One crisis can fall into several categories. The end-Permian, for example, involved volcanoes, acidification and oxygen loss together. Put side by side, these categories show that mass extinctions were never caused by a single thing. The end-Triassic cleared the stage for the dinosaurs, while the end-Cretaceous killed all the non-avian dinosaurs.

Papers to search for

• Hoffman et al. (1998): "A Neoproterozoic snowball Earth"
• Lyons et al. (2014): "The rise of oxygen in Earth's early ocean and atmosphere"
• Burgess et al. (2014): "High-precision timeline for Earth's most severe extinction"
• Alvarez et al. (1980): "Extraterrestrial cause for the Cretaceous-Tertiary extinction"
• Hildebrand et al. (1991): "Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico"