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.
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.
GRE数学考点之Coefficient
GRE数学高频词汇A-B
GRE数学符号英文表达
GRE数学:五个注意
GRE数学考试就是与时间赛跑
GRE数学机经你会使用吗?
GRE数学:代数常考题型
GRE数学中容易混淆的词汇
GRE数学专用术语介绍(1)
英文版GRE数学考试重要考点(2)
GRE数学如何冲刺
举例说明GRE数学遇到不懂单词后怎么办
十道典型GRE数学排列组合、概率题目
GRE数学解题方法
GRE数学:做题方法精粹
GRE数学专项考试解读
如何彻底克服GRE数学粗心问题
GRE数学:有关“圆”的练习题
GRE数学复习参考书推荐
GRE数学满分秘诀:细心
GRE数学考试算术练习题
GRE数学必备基础词汇
GRE数学基本概念
GRE数学考试难词不认识怎么办?
GRE数学数据充分性题目解题方法
GRE数学机经怎么合理利用
GRE数学考试题型及其常见问题汇总
想要GRE数学考试题上手快就要多练习
GRE数学高分备考策略
GRE数学高频考点之Algebraic Fraction
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |