In 1982 two paleontologists–scientists of ancient life–created a list of five major extinction events of global extent, to which I’ve added a sixth, #1 on the list. In the years since then, statistical methods have been applied to fossil diversity counts, which have muddied some of the distinctions between major and minor mass extinctions, but this is a good fundamental list for illustrating the concept.
An important aspect to keep in mind is that several of these span ten million years or more in extent. They represent not one cataclysm, but rather a very long trend of events, and frequently occurred in pulses, with smaller, more rapid extinction events strung together in a chain through many millions of years into one extended global crisis.
1) End of the Archean Era, 2.5 bya. Many species of ancient archean single-celled life forms produced oxygen as a waste product, and after more than a billion years of this, the decreased concentration of methane and increased concentration of oxygen in the atmosphere poisoned the archea, obligatory anaerobes (i.e. cannot tolerate oxygen) out of existence.
2) Ordovician-Silurian, 450-440 mya. 60-70% of all species were eliminated. The cause is not well known but current theory is intense volcanism and global warming.
3) Late Devonian, 375-360 mya. A long event spanning nearly 20 million years, driving 70% of species extinct. Sea level changes and oceanic anoxia are considered part of the explanation, but no firm theory exists to explain this event.
4) Permian-Triassic, 252 mya. The most major extinction: 90-96%, including 96% of marine and 70% of terrestrial, species disappeared, once called the “Great Dying”. The most common theory is the eruption of massive supervolcanoes in present-day Siberia—sources of the Siberian Traps formations–which alternately cooled and then warmed the planet and acidified the oceans. Other theories include multiple bolide (meteor) strikes and the sudden release of methane deposits in the ocean, leading to huge global warming.
5) Triassic-Jurassic, 201 mya. 70 to 75% of all species went extinct.
No clear cause is known. No crater large enough for an impact of global reach has been discovered, but a series of large volcanoes through equatorial Pangea could have been a significant part of the cause, leading to global cooling/warming and acidifying the ocean.
6) Cretaceous-Paleogene (formerly Cretaceous-Tertiary, or KT), 65.5 mya. About 75% of all species, including all non-avian dinosaurs, went extinct. The climate was already cooling but a meteor estimated at 10-15 km in diameter struck the present-day Gulf of Mexico and left a crater roughly 180 km in diameter. After the initial shockwave and firestorm, and the tremendous heat of crater ejecta falling back to earth, the billions of tons of ash blown into the sky would have sent the planet into wintry conditions for a year or more.
Tomorrow: the physics of water 1: phases of matter.
This post was previously published on dailykos.com and is republished on Medium.
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