Catastrophic asteroid impacts are gaining a credible edge over violent volcanic eruptions as the greatest killers Earth has ever seen, according to two pieces of scientific detective work reported in tomorrow's issue of the journal Science. The first cataclysm in question occurred about 250 million years ago, when according to the fossil record more than 90 percent of Earth's marine species and 70 percent of life on land perished. The event is known as the Permian-Triassic (P-T for short) mass extinction, named because it falls on the boundary between the two geological eras. The second event, known as the Cretaceous-Tertiary (K-T) mass extinction, occurred about 65 million years ago, leading to the demise of the dinosaurs and most of the creatures and plants that lived with them. According to the geological record, bouts of extreme volcanism occurred around the same time as these mass extinctions and many scientists have suggested that the volcanic activity is directly responsible for the loss of life. However, the discovery and analysis over the past few decades of a crater from an asteroid impact about 65 million years ago, and of meteorite fragments from an apparent asteroid impact about 250 million years ago, is leading some scientists to believe that the impact events, not volcanism, were the primary cause of the extinctions. Two studies published in the November 21 issue of Science support these theories. One study presents further evidence for an impact event about 250 million years ago and the second study suggests that the volcanism around the K-T boundary was probably not a major contributor to the K-T mass extinction.
View: Full Article | Source: National Geographic
