托福阅读背景材料之火山岩

　　托福阅读的关键是在于多看多练，为了帮助广大考生更好的复习，小编为大家整理提供托福阅读背景知识，以供各位考生复习参考，生活中有很多事情需要大家细心观察和发现，阅读文章不仅可以锻炼能力还能了解各方面的知识。小编也会经常找些文章让大家参考的，下面开始今天的托福阅读知识库扩充吧。

　　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.

　　以上就是今天的托福阅读背景知识，每天熟悉一部分托福常考知识点，坚持下来，一定会有不小的进步，了解背景知识的同时不要忘记，找些相关知识的单词造句试试，以便考试的时候能派上用场，英语不是一朝一夕的事，所以大家一定要勤加练习