新托福听写训练 第20期（在线收听）

Listen to part of a lecture in a geology class.

Most of you are probably familiar with the flow of lava

that can result from a volcano eruption. Lava actually being a mixture of magma and convince to be produced underground. And many people think this eruption refers to only to an explosion. But in geology, an eruption is any release of magma. This can and often does common the form of the explosion, but sometimes the magma just blows over the mouth of volcano. In any event, let's take a look now at some of the other hazards caused by volcano events. Pyroclastics is the word used to describe fragmentary rocks rejected during the volcanic eruption. A Pyroclastics fall is the combination of rock fragments and fluid fire lava that build in the air. This project tiles can be just tiny piece of ash or they can be rock fragments that are typically the size of baseball or volleyball. Although some can be so enormous they weigh many tons, the most dangerous of all volcanic events is what we call a pyroclastics flow, in this explosion, there are a

tremendous released of pressure, and these result in avalanche of supper heated mixture of gas and rock. This glowing avalanche as it called can reach the temperature of 1000 degree centigrade and can travel as far as 600 kilometers in an hour. It can literally destroy everything as it passed. And that's what happened Mount Pelee erupted on Martinique island in 1902, within minutes after the volcano erupted, the town of saint Pierre was completely wiped out.