托福阅读真题+题目+答案：The Behavior of Magma

托福阅读真题+题目+答案：The Behavior of Magma

Why do some volcanoes explode violently while others erupt gently? Why do some kinds of magma (molten rock) solidify within Earth to form hard rocks, and others rise all the way to the surface to erupt from volcanoes as lava? To answer these questions, we must consider the properties and behavior of magma. Once magma forms, it rises toward Earth's surface because it is less dense than surrounding rock. As it rises, two changes 0ccur. First, it cools as enters shallower and cooler levels of Earth. Second, pressure drops because the weight of overlying rock decreases. Cooling and decreasing pressure have opposite effects on magma: Cooling tends to solidify it, but decreasing pressure tends to keep it liquid.

1.It can be inferred from paragraph 1 that high pressure has which of the following effects on magma?

O It does not allow magma to escape from deeper and hotter levels of Earth.

O It makes magma solidify.

O It makes magma rise toward Earth's surface.

O It cools magma.

Whether magma solidifies or remains liquid as it rises toward Earth's surface depends on the type of magma.⬛Basaltic magma commonly rises to the surface to erupt from a volcano. ⬛In contrast, granitic magma usually solidifies within Earth's crust (outer layer). ⬛Granitic magma contains about 70 percent silica, whereas the silica content of basaltic magma is only about 50 percent.⬛In addition, granitic magma generally contains up to 10 percent water, whereas basaltic magma contains only 1 to 2 percent water.

As in silicate minerals, silicate tetrahedral (four-faced solids, similar to triangular pyramids) in magma link together to form chains, sheets, and framework structures. They form long chains if silica is abundant in the magma but shorter chains if less silica is present. Because of its higher silica content, granitic magma contains longer chains than does basaltic magma. In granitic magma, the long chains become tangled, making the magma stiff or viscous. It rises slowly because of its viscosity and has ample time to solidify within the crust before reaching the surface. In contrast, basaltic magma, with its shorter silicate chains, is less viscous and flows easily. Because of its fluidity. It rises rapidly to erupt at Earth's surface.

2.According to paragraph 3, which of the following is NOT true of basaltic magma compared to granitic magma?

O Basaltic magma has shorter silicate chains.

O Basaltic magma is much less likely to solidify before reaching the surface.

O Basaltic magma's internal structure is less tangled.

O Basaltic magma is stiffer.

3.In paragraph 3, why does the author provide information about the formation of silicate chains in different types of magma?

O To support the claim that granitic magma contains more silica than basaltic magma does

O To argue that fast-rising magma does not have enough time to form long silicate chains

O To explain why granitic magma is more viscous than basaltic magma

O To illustrate how silicate tetrahedral link together

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