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题库解析   >   主旨题题型   >   RC21-24篇
    本题由lindy提供
885次学习
RC21-24篇

First identified in 1969, komatiites are Earth’s oldest known volcanic rocks and contain three times as much magnesium as do

most volcanic rocks. This chemical composition suggests that komatiites formed from the hottest lava known ever to have erupted: a high concentration of magnesium changes the physical properties of lava so that unusually high temperatures would be required for the lava to exist as a liquid.

Komatiites’ discovery was surprising in light of then-current geological theories about magmas, molten rock that forms in the Earth’s mantle (the layer beneath the crust) and composes volcanic lava eruptions. Prior to 1960, geologists Bowen and Hess disagreed over whether or not the very high temperatures needed to produce magmas rich in magnesium could have existed on Earth. Hess suggested that the presence of water, probably released from minerals decomposing in the Earth’s mantle, might have meant that a high-magnesium magma could have existed at a lower temperature. But Bowen showed experimentally that the high temperatures were indeed necessary. By 1960, it was generally accepted that volcanic rocks with such high levels of magnesium could not exist, and thus the discovery of komatiites changed geologists’ assumptions about the characteristics of the Earth’s mantle around the time of the formation of komatiites, between 2.5 and 4 billion years ago.

Which of the following most accurately states the main point

of the passage?


    A. Komatiites provide information about rates of volcanic eruption between 2.5 and 4 billion years ago.

    B. Komatiites provide information about how the physical properties of lava in the Earth’s past compare with those of current-day lava.

    C. Komatiites provide evidence that undermines Bowen’s experimental conclusions regarding the temperatures at which lava exists as a liquid.

    D. Komatiites provide evidence that has changed geologists’ ideas about the characteristics of the Earth’s mantle between 2.5 and 4 billion years ago.

    E. Komatiites provide evidence that water in the Earth’s mantle may have reduced the temperature required for lava to exist as a liquid.


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答案:
D
第1结构:
先提出了一个事实 :一个东西K这个火山岩,它的M组成含量特别高。这个事实意味着什么:他是由最hot的岩浆形成的。 第2结构: K这个火山岩的发现是surprising的。第1层次:在1960年之前是什么情况。第2层次: 直到1960年,是什么情况,发现了K,什么结果:change geologist's assumptions about the characteristics of ...years ago. main point就是文章主旨,文章主要讲的就是发现了K这个东西产生了历史性的change,选D 选项A rates of volcanic eruption文章未提及 选项B compare with those of current-day lava.文章中没有这种比较,无关比较 选项C undermines Bowen’s experimental conclusions 与文章内容不符 选项E water in the Earth’s mantle may have reduced the temperature required for lava to exist as a liquid.与文章内容不符

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