【英语时差8,16】地球的版块是固定不动的吗？（在线收听）

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 planet is always in motion. Continents move about the Earth like huge ships at sea. They float on pieces of the Earth's outer skin, or crust. 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.