Volcanic eruptions in Siberia 251 million years ago may have started a cascade of events leading to high hydrogen sulfide levels in the oceans and atmosphere and precipitating the largest mass extinction in Earth's history, according to a Penn State geoscientist. "The recent dating of the Siberian trap volcanoes to be contemporaneous with the end-Permian extinction suggests that they were the trigger for the environmental events that caused the extinctions," says Lee R. Kump, professor of geosciences. "But the warming caused by these volcanoes through carbon dioxide emissions would not be large enough to cause mass extinctions by itself." That warming, however, could set off a series of events that led to mass extinction. During the end-Permian extinction 95 percent of all species on Earth became extinct, compared to only 75 percent during the K-T when a large asteroid apparently caused the dinosaurs to disappear.