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Volcanoes are the ultimate earth-moving machinery. Eruptions have rifted continents, raised mountain chains, constructed islands

admin2020-12-14  64
问题     Volcanoes are the ultimate earth-moving machinery. Eruptions have rifted continents, raised mountain chains, constructed islands and shaped the topography of the earth. The entire ocean floor has a basement of volcanic basalt.
    Volcanoes have not only made the continents, they are also thought to have made the world’ s first stable atmosphere and provided all the water for the oceans, rivers and ice-caps. There are now about 600 active volcanoes. Every year they add two or three cubic kilometers of rock to the continents. Imagine a similar number of volcanoes smoking away for me last 3,500 million years. That is enough rock to explain the continental crust.
    What comes out of volcanic craters is mostly gas. More than 90% of this gas is water vapor from the deep earth: enough to explain, over 3,500 million years, the water in the oceans. The rest of the gas is nitrogen, carbon dioxide, sulphur dioxide, methane, ammonia and hydrogen. The quantity of these gases, again multiplied over 3,500 million years, is enough to explain the mass of the world’s atmosphere. We are alive because volcanoes provided the soil, air and water we need.
    Geologists consider the earth as having a molten core, surrounded by a semi-molten mantle and a brittle, outer skin. It helps to think of a soft-boiled egg with a runny yolk, a firm but squishy white and a hard shell. If the shell is even slightly cracked during boiling, the white material bubbles out and sets like a tiny mountain chain over the crack—like an archipelago of volcanic islands such as the Hawaiian Islands. But the earth is so much bigger and the mantle below is so much hotter.
    Even though the mantle rocks are kept solid by overlying pressure, they can still slowly ’flow’ like thick treacle. The flow, thought to be in the form of convection currents, is powerful enough to fracture the ’eggshell’ of the crust into plates, and keep them bumping and grinding against each other, or even overlapping, at the rate of a few centimeters a year. These fracture zones, where the collisions occur, are where earthquakes happen. And, very often, volcanoes.
What accounts for a large proportion among those things which volcanic craters bring out?
选项 A、Water.
B、Gas.
C、Soil.
D、Ocean.
答案B
解析 细节题。原文第三段第一句，“What comes out of volcanic craters is mostlygas．”从火山喷发出来的主要是气体。“mostly”(大多)对应“account for a largeproportion”(占较大比例)。其余三个选项均为干扰项，水和土壤占据的比例较小．火山喷发并不是导致海洋形成的直接原因，它只是提供了海洋形成需要的水资源。
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