托福阅读背景知识汇总之Lava

　　Lava

　　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 stickier and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe (pronounced path HOH eel hob eel) flows; while 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 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, andesitic, decides, and hyalite is 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 deictic lava have medium levels of silicon dioxide, while holistic lava has the most.