Page 36 - 新思维科学学生用书9 样章

1.4 Climate change

The dust in the air meant that less light reached the Earth’s surface. Plants could not photosynthesise,
so animals had less food. As well as the disruption to food chains, the Earth became much colder,

because less heat from the Sun could reach the surface.
Most scientists think that these changes in climate, caused by the asteroid impact, explain why

dinosaurs became extinct around this time. Not only the dinosaurs, but also many other species on
Earth, were destroyed as a result of the climate change following the asteroid collision. The asteroid
caused a mass extinction. Up to three-quarters (75%) of all the species on Earth that were alive at that

time are thought to have become extinct because of the asteroid collision.
There are about 175 known asteroid impact craters around the world. The crater shown in the picture

is in the USA. The crater is more than 1 km wide and 170 m deep. It is estimated that the asteroid
collision that formed this crater happened 50 000 years ago.

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