The geological history of Earth is characterised by very long periods of stability, interrupted by short violent crises, during which volcanic eruptions, orogenesis, climatic changes, sea levels variations and inversions of the magnetic field occur, together with mass extinction of species.
Geology is not able, yet, to give an explanation for these recurrent crises; there are, of course, several hypothesis, but no certitudes. Many of the geological phenomena often occur at the same time, and therefore a relation of cause-effect has been suggested between them, but it's not always clear which is the cause and which the effect. In the last few years the idea that mass extinctions are the result of catastrophic impacts of asteroids or comets is taking ground, as for some of them the coincidence with the impact of a large asteroid has been ascertained.
It should be noted, however, that simultaneity has been ascertained also between mass extinctions and other geological phenomena, like volcanism, cooling, warming, regressions, transgressions and so on, which are maintained by many scientists as responsible for mass extinctions of species. But nobody was able to explain convincingly how such phenomena, whether alone or combined, could have provoked world-wide ecological catastrophes, both on land and on the oceans (...)
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