Title：The Extinction of the Dinosaurs
Geologists define the boundary between sediment layers of the Cretaceous period (144. C65 million years ago) and the Paleocene period (65. C55 million years ago) in part by the types and amounts of rocks and fossils they contain or lack。
Before the limit of 65 million years ago, marine strata are rich in calcium carbonate due to accumulations of fossils of microscopic algae deposited on the sea floor. Above the 65-million-year limit, sea-floor sediments contain much less calcium carbonate, and fossils of several families of mollusks are no longer found. In continental sediments, dinosaur fossils, though frequent before 65 million years ago, are totally absent.
By contrast, new families of mammals appear, including large mammals for the first time. Scientists wondered for many years about what could have caused the dinosaurs' rapid disappearance at the end of the Cretaceous period, coming up with a great variety of theories and scenarios. For some, it could have been due to unfavorable genetic changes triggered by a dramatic increase by a factor of 10, 100, 1, 000 in cosmic-ray particles reaching the Earth after a supernova explosion somewhere in the neighborhood of the solar system. For these high-energy particles to affect life, they would have to get through the protective barrier of the Earth's magnetosphere, the region of the upper atmosphere controlled by Earth's magnetic field. That could have happened if the cloud of particles from the supernova explosion reached the Earth during a period when the magnetosphere was weakened, something that may happen when the Earth's magnetic field changes direction. And we know that the magnetic north and south poles of the Earth switch on the average twice every million years. However, this is not the only possible explanation for dinosaur destruction.
Other theories have raised the possibility of strong climate changes in the tropics (but they then must be explained). Certainly, if climate changes, the changed distributions of temperature and rainfall modify the conditions that favor one ecosystem over another. The extinction of a particular family, genus, or species may result from a complicated chain of indirect causes and effects. Over thirty years ago, scientist Carl Sagan quoted one suggestion that the demise of the dinosaurs resulted from the disappearance of a species of fern plant that was important for dinosaur digestion. Other theories involved a worldwide cold wave following the spread of a layer of cold but not very salty water in the world's oceans, which floated on the surface because, with its low salinity, the water was less dense.
Proponents of another theory that remains under consideration today postulate that the extinction of the dinosaurs corresponds to a period of intense volcanic activity. It's not a question of just one or even of a thousand eruptions comparable to the explosion of Krakatoa in 1883, one of the largest volcanic events in modern times, but rather of a prolonged period of activity. On the Deccan plateau in India, basalt (volcanic) rocks cover more than 500, 000 square kilometers (nearly 200, 000 square miles), and correspond to massive lava outflows occurring precisely at the end of the Cretaceous. This sort of outflow could correspond to volcanic activity similar to the activity that drives sea-floor spreading, with lava emerging from elongated fractures in the crust rather than from craters.
The volcanic convulsion that buried the Deccan plateau in lava must also have changed the composition of the atmosphere and severely affected climate. Initially, there must have been strong sudden cooling resulting from the blocking of sunlight by sulfate aerosol veils in the stratosphere (part of the Earth's atmosphere). If strong cooling lasted a year after the formation of the aerosols, it would have been the death of tropical species unable to adapt to such a volcanic winter.
However, a long period of strong volcanic activity (again, remember thousands of Krakatoas) would at the same time have added a substantial amount of carbon dioxide to the atmosphere, reinforcing the greenhouse effect. This would gradually warm things up, ending the extended cold snap and producing global warming together with geographic shifts of humid and arid (dry) zones. Certainly things would change to upset living conditions, leading to the extinction of some species while others would profit, if only from the disappearance of predators.
1. According to paragraph 1, which of the following is true of Paleocene sediments
A. They lack fossils from some families of mammals found in Cretaceous sediments.
B. They contain fossils of dinosaurs.
C. They contain fossils of some animals that did not exist during the Cretaceous.
D. They contain fossils of more kinds of mollusks than are found in Cretaceous sediments.
2. The word unfavorable in the passage is closest in meaning to
A. Unusual B. Disadvantageous C. Sudden D. major
3. The word triggered in the passage is closest in meaning to
A. initiated B. intensified C. followed D. aided
4. Why does the author include the information that the magnetic north and south poles of the Earth switch on the average twice every million years
A. To provide evidence showing that Earth's protective barrier is difficult to get through
B. To show that it is reasonable to think that particles from a supernova explosion could have reached Earth
C. To explain why some scientists believe a supernova explosion may have occurred near our solar system
D. To help explain why some scientists have rejected the theory presented in paragraph 2
5. According to the theory discussed in paragraph 2，a weakening of Earth's magnetosphere may have
A. caused a supernova to explode near our solar system
B. allowed gene-altering particles to reach Earth's surface
C. forced Earth's magnetic field to change direction
D. allowed clouds of protective particles to escape from Earth's upper atmosphere
6. AII of the following are mentioned in paragraph 3 as possible causes for the extinction of the dinosaurs EXCEPT
A. a change in the diet of dinosaurs
B. a change in the climate of the tropics
C. a decrease in global temperatures
D. a decrease in deep ocean salinity
7. In paragraph 3，why does the author include the quotation by Carl Sagan
A. To explain the connection between dinosaur extinction and the extinction of other animal species
B. To support the claim about species extinction being due to indirect causes and effects
C. To show that scientists have revised their ideas greatly in the last thirty years
D. To identify the differences between the various theories for the extinction of dinosaurs
8. According to paragraph 4，what was one unusual aspect of the volcanic activity at the end of the Cretaceous
A. Some explosions were much larger than Krakatoa.
B. Eruptions occurred over a long period of time.
C. Active volcanoes were sometimes separated by many kilometers.
D. There were active volcanoes in the sea as well as on land.
9. Which of the following is presented in paragraph 4 as evidence that intense volcanic activity occurred at about the time that the dinosaurs became extinct
A. The size of the volcanic craters on the Deccan plateau
B. An increase in sea-floor spreading
C. The formation in India of large amounts of a type of rock associated with volcanoes
D. The occurrence of a thousand or more volcanic explosions the size of Krakatoa
10. The word severely in the passage is closest in meaning to
A. certainly B. consequently C. greatly D. permanently
11. The word reinforcing in the passage is closest in meaning to
A. making possible B. spreading C. introducing D. strengthening
12. According to paragraph 5，all of the following are theorized to have occurred as a result of volcanic activity EXCEPT A. a decrease in the amount of sunlight reaching Earth's surface
B. a reduction in the number of sulfate aerosol veils in the stratosphere
C. increased dryness in some areas that were once more humid
D. changes in the atmosphere's composition resulting in an increase in temperature
13. Look at the four squares that indicate where the following sentence could be added to the passage.
Thus, ecosystems following periods of extensive volcanic activity necessarily had different combinations of species than earlier ecosystems did. Where would the sentence best fit Click on a square to add the sentence to the passage.