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The Extinction of the Dinosaurs

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

2.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

3.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 increaseby 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 . 4.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.

5.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.

6.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