Earths Inner Core
Scientists have long struggled to understand what lies at the planets center. Direct observation of its center is impossible, so researchers must___ （1）___ to other evidence.
In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves ___（2）___ jolts （晃动） from one side of Earth through the center to the other side. Then in 1936, Danish geophysicist Inge Lehmann studied the waves ___（3）___ to determine that within Earths core of molten （熔化了的） iron lies a solid inner core － but ___（4）___ that core was made of eluded （难倒） her. Other geophysicists quickly determined that Lehmanns inner core was composed mostly ___（5___） iron. Since then, Lehmanns discovery has ___（6）___ conventional Earth science.
But now scientists are challenging traditional theory with new and radical ___（7）___. For example, Earths center could actually contain an inner core within the inner core, claim Ishii and colleague Adam Dziewonski.
Analyzing hundreds of thousands of earthquake wave ___（8___）, they maintain that the inner core has at its heart a tiny, even more solid sphere （球体）. This sphere may be the oldest fossil ___（9）___ from the formation of Earth, says Dziewonski.
Dziewonski and Ishii speculate that shortly ___（10）___ Earth formed around 4.8 billion years ago, a giant asteroid （小行星） smashed into the young planet and nearly melted it. But Earths center didnt quite melt; it ___（11）___ mass as the planet cooled. The core within a core may be the kernel （核心） that endured. Its presence could change our basic ideas about the ___（12）___ of the planet, Dziewonski says.
Dziewonskis idea is tame （温和的） compared to the ___（13）___ theories of independent geophysicist J. Marvin Herndon. Earths inner core is made not of iron, he claims, but a___ （14）___ of nickel and silicon. Herndon has a truly revolutionary notion: Within the nickel silicide （硅化物） inner core is also an inner inner core － an 8 km-wide ball of the element uranium. Uranium is radioactive. Herndon thinks the uranium releases heat energy as its atoms ___（15）___ fission-split and crash into one another in a chain reaction. In other words, we may live on top of a gigantic, natural nuclear power plant.
1 A try B leave C turn D point
2 A create B receive C feel D overcome
3 A work B solution C job D patterns
4 A whether B what C why D how
5 A from B within C of D to
6 A followed B dominated C restored D opposed
7 A ideas B demands C phenomena D movements
8 A things B acts C methods D records
9 A taken B benefited C left D kept
10 A after B before C since D when
11 A expanded B modified C gained D melted
12 A size B origin C structure D shape
13 A radical B traditional C classical D conventional
14 A system B copy C model D compound
15 A charge B last C experience D show
参考答案:
1. C 2. A 3. D 4. B 5. C
6. B 7. A 8. D 9. C 10. A
11. C 12. B 13. A 14. D 15. C
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