The article under review is titled "Repeated Blows: Did Extraterrestrial Collisions Capable of Causing Widespread Extinctions Pound the Earth Not Once, But TwiceùOr Even Several Times." It was published in the March 2002 issue of the Scientific American. In this article, Becker described and discussed the validity of the recent research efforts to determine whether the collisions between gigantic asteroid rocks and the Earth could have triggered dramatic biological crises that transformed the planet and led to the extinction of living species. Essentially, this article focuses on how researchers have sought to find physical evidence to prove that asteroid impacts caused the great catastrophes including the extinction of the dinosaurs 65 million years ago and the Great Dying in which 90 percent of the planet's species became extinct approximately 250 million years ago during the Permian period.
According to Becker, researchers studying craters and shattered rocks have accumulated both direct and indirect physical evidence that link them to asteroid impacts. During the 1980s, scientists found a high concentration of iridium within a thin layer of clay in Gubbio, Italy. Because iridium is an extremely rare element on Earth, but a common one in meteorites, the scientists concluded that the large amount of iridium was likely to be fallout of an asteroid impact on earth. Even more interestingly, the clay layer that was dated to 65 million ago fitted with the end of the Cretaceous period in which dinosaurs became extinct. Last year, the discoveries of helium and argon with similar compositions to meteorites and cosmic dust in fullerenes, a new form of carbon, from end Permian locations in China and Japan supported the association between asteroid impacts and the Great Dying (77-8).
These controversial research studies thus set the foundation for challenging the conventional belief that global volcanic activity had led to the...