Scientists now know, however, that the 1. _______ of the Earth is not as permanent(永久的) as had been thought. Scientists explain that the suce of our planet is always in 2. _______. Continents move about the Earth like huge 3._______ 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(地壳) is 4. _______ as it rolls down into the hot area and melts again. Only since the nine-sixties have scientists begun to 5. _______ that the Earth is a great 6. _______ structure. Some experts say this new understanding is one of the most important revolutions in scientific thought. The revolution 7. _______ the work of scientists who study the movement of the continents—a 8. _______ called plate tectonics.