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We review the results of time-series analyses of the dates of flood-basalt volcanism, mass extinctions, and impact cratering, as well as other geologic phenomena of the last 250 m.y. It is found in all cases that only one likely periodicity exists in the range of possible periods longer than a million years: 33 ± 3 m.y. This periodicity appears to be intrinsically semiregular, and the most recent epoch in the mean cycle is close to the present time.

Purely internal explanations that have been proposed cannot yet accurately predict the order of magnitude of this geologic quasi-period and do not account for the similar quasi-period apparently detected in impact cratering. A possible extraterrestrial mechanism, however, is provided by comet impacts. Direct effects of large comet impacts on the Earth involve shock waves and debris put into the atmosphere-ocean system. Indirect effects follow from deep excavation of the lithosphere and extensive decompression melting of mantle rocks if the impact site is a hot-spot swell capping a mantle plume. Physical and statistical data for hot-spot swells and for terrestrial and lunar impact craters suggest that impacts which produce craters with diameters >100 km can account for the frequency and geographic distribution of continental flood basalts. Other geologic phenomena that may ensue from large impacts on the Earth include mantle-circulation disturbances, rifting, tectonism, sea-level fluctuations, magnetic reversals, and mass extinctions.

A quasi-periodicity of ~33 m.y. in impact cratering could ultimately be caused by comet showers from the Solar System’s outer halo of comets, as a result of quasi-periodic gravitational encounters of the outer comet halo with massive interstellar dust clouds as the Solar System moves through the Milky Way Galaxy.

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