Reading Comprehension (RC) | Since 1980, the notion that mass extinctions at the end of the

Since 1980, the notion that mass extinctions at the end of the Cretaceous period 65 million years ago resulted from a sudden event has slowly gathered support,although even today there is no scientific consensus. In the Alvarez scenario, anasteroid struck the earth, creating a gigantic crater. Beyond the immediate effects of fire, flood, and storm, dust darkened the atmosphere, cutting off plant life. Many animal species disappeared as the food chain was snapped at its base. Alvarez’s main evidence is an abundance of iridium in the KT boundary, a
thin stratum dividing Cretaceous rocks from rocks of the Tertiary period. Iridium normally accompanies the slow fall of interplanetary debris, but in KT boundary strata, iridium is 10–100 times more abundant, suggesting a rapid, massive deposition. Coincident with the boundary, whole species of small organisms vanish from the fossil record. Boundary samples also yield osmium isotopes, basaltic sphericles, and deformed quartz grains, all of which could have resulted from high-velocity impact.

Paleontologists initially dismissed the theory, arguing that existing dinosaur records showed a decline lasting millions of years. But recent studies in North America, aimed at a comprehensive collection of fossil remnants rather than rare or well-preserved specimens, indicate large dinosaur populations existing immediately prior to the KT boundary. Since these discoveries, doubts about
theories of mass extinction have lessened significantly. Given the lack of a known impact crater of the necessary age and size to fit the Alvarez scenario, some scientists have proposed alternatives. Courtillot, citing huge volcanic flows in India coincident with the KT boundary, speculates that eruptions lasting many thousands of years produced enough atmospheric debris to cause global devastation. His analyses also conclude that iridium in the KT boundary was deposited over a period of 10,000–100,000 years. Alvarez and Asaro reply that the shock of an asteroidal impact could conceivably have triggered extensive volcanic activity. Meanwhile, exploration at a large geologic formation in Yucatan, found in 1978 but unstudied until 1990, has shown a composition consistent with extraterrestrial impact. But evidence that the formation is indeed the hypothesized impact site remains inconclusive.