Part of a lecture about volcanoes — how they form and why people live

Section 4. You will hear part of a lecture about volcanoes — how they form and why people live near them. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good morning. Today we're going to look at volcanoes, how they form, the different shapes they take, and why, despite the obvious dangers, so many millions of people choose to live near them. Let's start with the basics, and with a distinction that often trips students up. Beneath the Earth's solid surface lies a layer of intensely hot rock, hot enough to be partly molten. While this molten rock remains below ground, we call it magma. The moment it escapes onto the surface, however, it takes a new name: we then call it lava. It is the same material; only the location differs. A volcano, put simply, is just an opening in the ground through which this molten rock, together with gas and ash, is able to reach the surface. MAN: Why do volcanoes appear where they do? The Earth's rigid outer shell is not one continuous piece; it is cracked into a number of enormous slabs, which geologists call plates. Most volcanoes are found not in the middle of these plates but along their edges, where neighbouring plates pull apart, or grind past, or collide with one another. That is why a great many of the world's volcanoes are strung around the rim of the Pacific Ocean, a belt so active it has earned the nickname the Ring of Fire. MAN: Now, volcanoes are not all the same shape, and the shape tells you how they behave. The first type is broad and gently sloping, built up slowly from runny lava that spreads a long way before it cools; this is the shield volcano, and its eruptions are usually gentle. The second is the steep, cone-shaped kind most people picture, built from alternating layers of lava and ash; these composite volcanoes can be violent. The third and smallest is the cinder cone, a simple steep mound thrown up from the loose fragments hurled out during an eruption. MAN: What actually decides whether an eruption is gentle or catastrophic? The single most important factor is the thickness of the magma, how runny or how sticky it is. As magma rises, dissolved gas tries to escape from it, much as bubbles form when you unscrew a bottle of fizzy water. If the magma is thin and runny, the gas slips out easily and the lava simply pours out. But if the magma is thick and sticky, the gas becomes trapped, the pressure builds and builds, and the result is a devastating explosion. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: Eruptions can be destructive in several ways. The lava itself is only part of the story. One of the greatest killers is ash, which can bury crops, collapse roofs under its weight, and ground aircraft across whole continents. Deadlier still is a fast-moving avalanche of scorching gas and rock that races down the slopes faster than a car can drive. MAN: And yet people stay. Why? The chief reason lies in the soil. As old volcanic ash slowly breaks down, it releases minerals that make the surrounding land exceptionally fertile; some of the finest farmland on Earth sits in the shadow of volcanoes. On top of that, the heat stored in the ground can be harnessed to generate electricity, a clean supply we call geothermal power, and the dramatic scenery draws tourists whose money supports whole communities. MAN: Finally, can we forecast eruptions? Not to the day, but the warning signs are real. Scientists track the tiny earthquakes that tend to precede an eruption, they measure the way the ground swells as magma pushes upward, and they monitor the gases seeping out. Together, these clues can buy a community precious time to escape. We'll stop there. That is the end of Section 4. You now have some time to check your answers.