高考英语《流行话题 语境识词4500》Unit43 What Caused the K-T Mass Extinction素材.docVIP

Unit 43　　What Caused the K-T Mass Extinction?　　The latter part of the Cretaceous Period was a time of continental drift and accompanying volcanic activity. The supercontinent was splitting up and the continents were taking on their modern-day forms. Many mountain ranges were formed. The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. The Earth was getting colder.　　About 65 million years ago, at the end of the Cretaceous, a large portion of plant and animal families suddenly went extinct. In this Cretaceous-Tertiary or K-T mass extinction -- K is for Kreide, meaning chalk in German, which describes the chalky sediment layer from that time; T is for Tertiary, the next geologic period -- all land animals over about 55 pounds went extinct, as did many smaller organisms. The K-T extinction eliminated the dinosaurs, some families of birds and marsupial mammals, many families of bony fishes, snails, sponges, sea urchins and others. This catastrophe eventually led to the Age of Mammals.　　There are a lot of theories about why this K-T (Cretaceous-Tertiary) extinction occurred, but a widely accepted theory (proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist), is that an asteroid 4~9 miles (6~15 km) in diameter hit the Earth about 65 million years ago. The impact would have penetrated the Earths crust, scattering dust debris into the atmosphere, and causing huge fires (generated by hot debris thrown from the crater tsunamis, severe storms with high winds and highly acid rain, seismic activity, and perhaps even volcanic activity. The impact could have caused chemical changes in the Earths atmosphere, increasing concentrations of sulfuric acid and fluoride compounds. The heat from the impacts blast wave would have incin