TOEFL Listening: ETS-TOEFL听力机经 - 783P81W8MZ4UUKC3Q$

Lecture: The Bodele depression: Narrator: Listen to part of a lecture in a geology class. Professor: Okay, so we've been talking about the effects the Amazon rainforest has on other parts of the world. But today, I want to talk about a place that has an important effect on the Amazon even though it's thousands of miles away in Africa. It's a geographic region called the Bodele depression. The Bodele depression is in the eastern part of the Sahara Desert in Northern Chad and is located between two mountain ranges. As the wind blows through the narrow path between the mountain ranges it creates a wind tunnel which causes a great amount of dust to be produced in the Bodele. It's actually the dustiest place on Earth. There are massive dust clouds in the whole area. So that if you were standing there you would get covered in a light, powdery dust. And we know from satellite photographs that the area is active year round, and most active in the winter months, which is a little unusual for the Sahara. But back to the Amazon connection. The dust produced in the Bodele is a major source of minerals for the Amazon rainforest. During the winter months in the Sahara, the dust from the Bodele depression is carried by trade winds across the Atlantic to the Amazon. In fact, recently it's been shown that over half of the minerals that fertilize the rainforest come from Bodele dust that blows over to the Amazon in the winter. Now, uhh, the effect on the Amazon is surprising. Because the Bodele depression is quite small in area compared to the Amazon. And the Bodele is only a small part of the Sahara. Well, we've known for a while that this area is an important dust producer. The information from satellite photography provided some indication of the relative dustiness of the area during different months of the year. But a more accurate estimate of how much dust the Bodele depression produces and especially how it produces. It could only be obtained by collecting data on the ground. Which is quite a challenge because of the area's extreme heat and frequent dust storms. But, in 2004 some scientists did go and collect data. What did they find? Well, they already knew that the ground at the Bodele was made, not of sand like you might expect in the desert, but of a sediment called diatomite. Diatomite is the crushed up remains of freshwater creatures that lived in the area thousands of years ago when it was submerged under a huge lake. So the ground itself was diatomite. Now, the researchers also expected to find fine powdery deposits of diatomite covering the ground. Light enough to be whipped up into dust clouds. But instead, they found chunks of diatomite that were much larger, like gravel covering the ground. They were way too heavy for even the strongest winds to blow them into the air as dust clouds. So, where on Earth was all of this dust coming from? Well, one of the researchers had a hypothesis that at first seemed pretty farfetched and it had to do with the dunes in the Bodele. Now, normally we think of sand dunes right. Mounds of desert sand which might be a ground up rock like quartz. Well, the hypothesis was that the Bodele dunes were not made of sand but of diatomite. That is, those large chunks of diatomite were being blown across the ground and collected into dunes. And as the wind blew diatomite particles up the slope of a diatomite dune, the particles would rub and grind against each other which according to the hypothesis would break them up into particles light enough to be blown into the air as dust. Now this was a radical idea. No diatomite dunes were known to exist anywhere. But sure enough, when researchers went out to these dunes they found that they were made of chunks of diatomite of various sizes. And when the wind picked up, dust was clearly being blown off the top pf the dunes creating dust clouds, the air was clear whatever fewer dunes.