For example, the final Permian extinctions have been linked to huge volcanic eruptions in what is now Siberia. The levels of inland seas and oceans lowered significantly, eliminating or changing marine habitats. Other theorized causes for the Permian extinctions include a large meteor impact and a supernova that exploded near enough to the earth to bathe it in radioactivity that destroyed the ozone layer. The most popular theory of the final Cretaceous extinction is that one or more asteroids or comets hit the earth, lifting massive amounts of debris and sulfur in the air and blocking the sunlight from reaching the earth's surface. In 1980 Walter Alvarez of the Univ.
Iridium is rare on earth, but is concentrated in meteors and comets.
Some scientists believe global wildfires that incinerated as much as one quarter of the earth's vegetation followed the impact. There is also evidence of other asteroid collisions at about the same time. Another theory concerning the cause of the final Cretaceous extinction is that it resulted from the huge volcanic eruptions that created the lava flows of the Deccan Traps in what is now India. In addition to eradicating large percentages of both land and sea creatures, mass extinctions also opened new ecological niches, permitting surviving species to thrive in new habitats and encouraging diversity. The Columbia Encyclopedia, Sixth Edition.