Wednesday, July 20, 2011

Volcanism and Biotic Extinctions

Extinction Events are increasingly seen as important factors in the history of life on Earth, and recent studies suggest catastrophic causes for at least some biotic mass extinctions. Two catastrophic processes that have been invoked are impacts of asteroids or comets and series of large volcanic eruptions. On one hand, the end-Cretaceous (65 Ma) mass extinction (the Cretaceous/Tertiary or K/T boundary) has been convincingly correlated with the impact of a 10-km-diameter comet or asteroid, and evidence of impact has been found close to the times of several other extinction events. On the other hand, the coincidence of the eruption of the Siberian flood basalt lava flow province and the even more severe end-Permian extinctions (250 Ma), and the near-coincidence of the Deccan flood basalt province (India) and the K/T extinctions, fostered speculations that flood basalt eruptions have contributed to a number of mass extinctions. Several workers compared the dates of extinction events of various magnitudes with dates of flood basalt episodes and found some significant correlations, supporting a possible cause-and-effect connection. Thus, it could be that extreme events of both extraterrestrial and terrestrial origin are responsible for many of the punctuation marks of the fossil record.

A major question regarding any possible relationship between flood basalt lava eruptions and extinction events involves the nature and severity of the environmental effects of the eruptions and their potential impact on life. Although the correlation between some flood basalt episodes and extinctions may implicate volcanism in the extinctions, it is also possible that other factors lead to the apparent association. Flood basalt episodes have been related to the inception of mantle plume activity, and thus may represent one facet of a host of geological factors (e.g., changes in seafloor spreading rates, rifting events, increased tectonism and volcanism, and sea-level variations) that tend to be correlated, and may be associated with unusual climatic and environmental fluctuations that could lead to significant faunal changes. It has also been suggested that a coincidence of both a large impact and a flood basalt eruption might be necessary in causing severe mass extinctions, and some workers have even proposed that large impacts might in some way trigger or enhance the volcanism.

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