2019托福阅读精选素材：火山岩

>>2019年托福考试：2019托福阅读精选素材：火山岩

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Lava (火山岩)

Lava is magma that breaks the surface and erupts from a volcano. If the magma is very fluid, it flows rapidly down the volcano’s slopes. Lava that is more sticky and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe (pronounced pah HOH ee hoh ee) flows; while a a (pronounced ah ah) flows have a jagged surface composed of loose, irregularly shaped lava chunks. Once cooled, pahoehoe forms smooth rocks, while a a forms jagged rocks. The words pahoehoe and a a are Hawaiian terms that describe the texture of the lava. Lava may also be described in terms of its composition and the type of rock it forms. Basalt, andesite, dacite, and rhyolite are all different kinds of rock that form from lava. Each type of rock, and the lava from which it forms, contains a different amount of the compound silicon dioxide. Basaltic lava has the least amount of silicon dioxide, andesitic and dacitic lava have medium levels of silicon dinxide, while rhyolitic lava has the most.

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