Meteorites and Volcanoes
Once in a while, a truly massive plume of hot rock from the Earth's mantle can erupt through the crust for centuries or even millenia, producing acid rains, destruction of the ozone layer from emissions of chlorine-bearing compounds, and a chill down in climate from the resulting increase in atmospheric dust and sulphur-based aerosols. The last such eruption occurred some 65 million years ago, created India's Deccan Traps, and -- in combination with the Chicxulub meteorite impact -- contributed to the extinction of the dinosaurs.
An earlier and even larger event in Siberia in combination with an impact by a meteorite that may have been slightly larger than the one associated with the Dinosaur extinction event -- that is, 3.7 to 7.4 miles wide (six to 12 km) and equivalent to a magnitude-12 earthquake -- may have caused the Permian-Triassic extinction 251 million years ago, when 90 to 95 percent of all species were wiped out.
Researchers suggest that perhaps in both extinctions, a plume of lava may have already risen close to the surface, but a massive meteorite impact turned a small eruption into a colossal one. As a result, within less than a million years, enough lava oozed out of the ground in Siberia to cover the entire planet 10 feet (three meters) deep. (One Permian-Triassic crater candidate is the 125-mile/200-km or more diameter Bedout Structure off the coast of northwestern Australia.
Revised from a page on the Earth.