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 traveled 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 traveling 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.

【生词摘录】

1. makeup: n. [singular]a particularly combination of people or things that form a group or whole(人或物的)组成,构成
2. seismic: adj. technical connected with or caused by earthquakes or powerful explorations 【术语】地震的;地震引起的
3. vibration: n. [C,U]a continuous slight shaking movement 颤动,振颤,震 动
4. compression: n. an increase in the density of something 压缩
5. shear: n. 【地】切变
6. boundary: n. [C]the official line that marks the edge of a town, countryetc 分界线,边界[+between]
7. mantle: n. [C]technical the part of the Earth around the central core【术语】地幔
8. core: n. [C]the central part of the Earth or any other planet 地心;(天体的)核心
9. crust: n. [C,U]a thin hard dry layer on the surface of something(某物的)硬薄外层(the Earth’s crust 地壳)
10. molten: adj. molten metal or rock has been made into a liquid by being heated to a very high temperature 溶化的,熔融的(molten lava 熔岩) 11.lava: n. [U]hot liquid rock that flows from a volcano(火山喷出的)岩浆,熔岩

文波讲词：

lav:洗

lava[‘lɑːvə]n.  熔岩; 火山岩  volcano  crater   magma岩浆

lavish[‘lævɪʃ]adj.  非常大方的; 丰富的extravagant/  babyish/ childish

lavatory[‘lævət(ə)rɪ]n.  厕所; 洗手间; 盥洗室  toilet   rush/flush   double vanities

laundry[‘lɔːndrɪ]n.洗衣   laundry room

a drain on sth/the govenement’s budget— brain drain