Impacts and Mass Extinctions Meteorites and impact craters bear witness to the fact that large impacts occasionally occur on Earth. Meteor Crater in the northern Arizona desert of the United States formed about 50,000 years ago when a metallic impactor roughly 50 meters across crashed to Earth with the explosive power of a 20-megaton hydrogen bomb. Although the crater is only slightly more than one kilometer across, an area covering hundreds of square kilometers was probably battered by the blast and ejecta—the debris ejected or displaced during the formation of an impact crater. Far bigger impacts have occurred, sometimes with catastrophic consequences for life on Earth. The word “catastrophic” in the passage is closest in meaning to •A.widespread•B.disastrous•C.important•D.immediateWhile collecting geological samples in Italy in 1978, the father-son team of Luis and Walter Alvarez discovered a thin layer of dark sediment that had apparently been deposited 65 million years ago—at about the same time that the dinosaurs and many other organisms suddenly became extinct. Subsequent studies found similar sediment deposited at the same time at many sites around the world. Careful analysis showed this worldwide sediment layer to be rich in iridium, and element that is rare on Earth’s surface. But iridium is common in primitive meteorites, which led the Alvarezes to a stunning conclusion: the extinction of the dinosaurs was caused by the impact of an asteroid or comet. This conclusion was not immediately accepted and still generates some controversy, but it now seems clear that a major impact coincided with the death of the dinosaurs. While the dinosaurs were the most famous victims of this mass extinction, it seems that up to 99 percent of all living things were killed and that 75 percent of all species living on Earth were wiped out at that time. In paragraph 2, the author discusses the presence of iridium in the sediment layer discovered by the Alvarezes because •A.it supported their claim that toxic levels of iridium could have killed thedinosaurs•B.it led some scientists to question the validity of the Alvarezes’ study•C.it led the Alvarezes to conclude that a space impactor killed thedinosaurs•D.it showed that new discoveries were still being made about theextinction of the dinosaursWhich of the following statements about scientists’ theories on the extinction of the dinosaurs can be inferred from paragraph 2? •A.Some scientists do not believe that an asteroid or comet impact killedthe dinosaurs.•B.Some scientists think that iridium was abundant on Earth before theextinction of the dinosaurs.•C.Some scientists doubt that all of the dinosaurs died out 65 millionyears ago.•D.Some scientists believe that the dinosaurs became extinct only afterthe plants and animals they ate disappeared.How could an impact lead to mass ext...
