概括：内容选自老托福听力，是练习托福听力的绝佳材料。


Hints:
Earth
P waves
S waves
I'm glad you brought up the question of our investigations into the makeup of the Earth's interior. In fact, since this is the topic of your reading assignment for next time, let me spend these last few minutes of class talking about it. There were several important discoveries in the early part of this century that helped geologists develop a more accurate picture of the Earth's interior. The first key discovery had to do with seismic waves, remember they are the vibrations caused by earthquakes. Well, scientists found that they travelled thousands of miles through the Earth's interior. This finding enabled geologists to study the inner parts of the Earth. You see these studies revealed that these vibrations were of two types: compression or P waves and shear or S waves. And researchers found that P waves travel through both liquids and solids, while S waves travel only through solid matter. In 1906 a British geologist discovered that P waves slowed down at a certain depth but kept travelling deeper. On the other hand, S waves either disappeared or were reflected back, so he concluded that the depth marked the boundary between a solid mantle and a liquid core. Three years later another boundary was discovered - that between the mantle and the Earth's crust. There's still a lot to be learned about the Earth. For instance, geologists know that the core is hot. Evidence of this is the molten lava that flows out of volcanoes, but we're still not sure what the source of the heat is.