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  3. Earth Rocks on Most of the time, the ground feels solid beneath our feet. That’s comforting. But it’s also misleading becaus

Earth Rocks on Most of the time, the ground feels solid beneath our feet. That’s comforting. But it’s also misleading becaus

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问题                      Earth Rocks on
    Most of the time, the ground feels solid beneath our feet. That’s comforting. But it’s also misleading because there’s actually a lot going on underground. Masses of land (called plates) slip, slide, and bump against one another, slowly changing each other, slowly changing the shape of continents and oceans over millions and billions of years.
    Scientists know that Earth formed about 4. 5 billion years ago. They also know that our planet was hot at first. As it cooled, its outermost layer, called the crust (地壳) , eventually formed moving plates. Exactly when this shift happened, however, is an open question.
    Now, an international group of researchers has an answer. They’ve found new evidence suggesting that Earth’s crust started shifting at least 3. 8 billion years ago. The new estimate is 1. 3 billion years earlier than previous ones.
    Not long before 3. 8 billions years ago, lots of asteroids (小行星) were hitting Earth, keeping its crust in a hot, melted state. After the hard crust formed, much of it sank at various times into the planet’s hot insides. There, it melted before returning to the surface. In some places, however, the crust never sank. One of the oldest such places is in Greenland, in an area called the Isua Supra-crustal (上地壳) Belt. The rocky crust there is between 3. 7 and 3. 8 billion years old. The belt was once part of the seafloor, but now it is exposed to air.
    The researchers recently took a close look at the Isua supracrustal belt. They noticed long, parallel cracks in the rock that have been filled in with a type of volcanic rock.
    To explain this structure, the scientists proposed that tension in the crust caused the seafloor to crack open long ago. Hot, liquid rock, called magma (岩浆) , flowed up slowly from deep inside Earth to fill the cracks. Finally, the whole area cooled, forming what we see today.
    That explanation, plus chemical clues inside the rock, suggests that the Isua supracrustal belt was once part of a plate under the ocean, beginning around 3. 8 billion years ago.  
It took a long time for the melted earth to become hard again.
选项 A、Right
B、Wrong
C、Not mentioned
答案C
解析 文章没有谈到这方面的信息。文章第四段提到“After the hard crust formed，much of it sank at various times into the planet’s hot insides．There，it melted before returning to the surface．”当坚硬的地壳形成后，大部分都在不同时间沉入到了这个星球炙热的内部，在那里熔化后再回到地球表面。并没有提及熔化了的地壳回到地球表面耗费的时间是长还是短。
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