2014届高考英语一轮复习话题阅读素材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.

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