托福阅读背景知识之火山岩

　　托福阅读背景知识是考生托福阅读备考过程中必不可少的参考资料，托福阅读内容量大，取材丰富，话题面广，需要考生积累必要的背景知识。本文中为大家带来托福阅读背景材料：火山岩，希望对大家托福备考有所帮助。

　　Lava （火山岩）

　　Lava is magma that breaks the surface and erupts from a volcano. If themagma is very fluid, it flows rapidly down the volcano’s slopes. Lava that ismore sticky and less fluid moves slower. Lava flows that have a continuous,smooth, ropy, or billowy surface are called pahoehoe (pronounced pah HOH ee hohee) flows; while a a (pronounced ah ah) flows have a jagged surface composed ofloose, 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 thatdescribe the texture of the lava. Lava may also be described in terms of itscomposition and the type of rock it forms. Basalt, andesite, dacite, andrhyolite 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 compoundsilicon dioxide. Basaltic lava has the least amount of silicon dioxide,andesitic and dacitic lava have medium levels of silicon dioxide, whilerhyolitic lava has the most.