What causes mass extinctions?
Although the best-known cause of a mass extinction is the asteroid impact that killed off the non-avian dinosaurs, in fact, volcanic activity seems to have wreaked much more havoc on Earth's biota. Volcanic activity is implicated in at least four mass extinctions, while an asteroid is a suspect in just one. And even in that case, it's difficult to disentangle how much of the end-Cretaceous extinction was caused by the asteroid and how much was caused by the steady ooze of lava that was blanketing most of India at around the same time.
While multiple causes may have contributed to many mass extinctions, all the hypothesized causes have two things in common: they cause major changes in Earth systems its ecology, atmosphere, surface, and waters at rapid rates. Here are some hypothesized causes for each of Earth's biggest mass extinctions:
What doesn't cause mass extinctions?
Why do some catastrophic events trigger mass extinctions and others do not? The devil seems to be in the details particularly in the chain reaction of Earth systems disruptions that are triggered (or not) and in the rate at which those disruptions occur. Mass extinctions seem to occur when multiple Earth systems are thrown off kilter and when these changes happen rapidly more quickly than organisms evolve and ecological connections adjust. For example, the asteroid that triggered the end-Cretaceous extinction happened to hit carbon-rich rocks, which probably led to ocean acidification, and hence the disruption of reef formation and the oceanic food web. However, the asteroid that caused the Manicouagan did not hit carbon-rich rocks and so did not set off this chain reaction or such a significant disruption of Earth systems. The Karoo-Ferrar volcanic activity, on the other hand, was so large that it would certainly have disrupted Earth's atmosphere and oceans; however, in this case, the changes came about very slowly. The volcanic activity was spread over millions of years. For comparison, the volcanic activity that may have caused the end-Triassic mass extinction likely occurred in less than 100,000 years, leaving no time for evolution to take place as habitats changed and leading to widespread extinction.
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Chart information from Barnosky, A.D., N. Matzke, S. Tomiya, G.O.U. Wogan, B. Swartz, T.B. Quental, … and E.A. Ferrer. 2011. Has the Earth's sixth mass extinction already arrived? Nature 471:51-57.
Photo of Manicouagan crater courtesy of NASA/JPL; Ferrar flood basalts photo courtesy of Murray McClintock, Department of Geology, University of Otago, New Zealand [permission pending]
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