【英语时差8,16】地球的版块是固定不动的吗？

The first pictures of Earth taken from space showed a solid ball covered by brown and green land masses and blue-green oceans. It appeared as if the Earth had always looked that way -- and always would. Scientists now know, however, that the surface of the Earth is not as permanent as had been thought. Scientists explain that the surface of our planet¹ is always in motion². Continents move about the Earth like huge ships at sea. They float on pieces of the Earth's outer skin, or crust³. New crust is created as melted rock pushes up from inside the planet. Old crust is destroyed as it rolls down into the hot area and melts again. Only since the nineteen-sixties have scientists begun to understand that the Earth is a great, living structure. Some experts say this new understanding is one of the most important revolutions in scientific thought. The revolution is based on the work of scientists who study the movement of the continents - a process called plate tectonics. Earthquakes are a result of that process.

1 planet   

n.行星

参考例句：

• Neptune is the furthest planet from the sun. 海王星是离太阳最远的行星。
• Rubbish, however, is only part of the problem of polluting our planet. 然而, 垃圾只是我们这个星球的污染问题的一个方面。

2 motion   

n.打手势，示意，移动，动作，提议，大便；v.运动，向...打手势，示意

参考例句：

• She could feel the rolling motion of the ship under her feet.她能感觉到脚下船在晃动。
• Don't open the door while the train is in motion.列车运行时，请勿打开车门。

3 crust   

n.(一片)面包皮，硬外皮，外壳；地壳

参考例句：

• This article focuses on radiation present in the earth crust.本文重点介绍源自地壳的辐射。
• Earth quakes can result from stresses in the earth's crust.地壳内的应力可能引起地震。