英語(yǔ)流行話題閱讀:語(yǔ)境識(shí)詞4500 43 What Caused the K-T Mass Extinction?

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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 Earth's 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 Earth's atmosphere, increasing concentrations of sulfuric acid and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path. The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally.

Those organisms that could not adapt to the temperature and light changes would die out. Since plants' energy is derived from the sun, they would likely be the first to be affected by changes in climate. As plants died out, the Earth's oxygen levels may well have dramatically decreased, both on land and in the oceans, suffocating those organisms which were unable to cope with the lower oxygen levels.

Major changes in the food chain would result from all of these environmental upheavals. The herbivores (plant eaters) who ate those plants would starve soon after the plants died. Then, at the top of the food chain, the carnivores (meat eaters), having lost their prey, would have to eat each other, and eventually die out.