TOEFL Listening: ETS-TOEFL听力机经 - 95JIM2KT2AE53370

Lecture: Hoodoos at Bryce Canyon: Narrator: Listen to part of a lecture in a geology class. Professor: Today I want to show you a picture that I think quite dramatically illustrates the rock sculpting power of nature – the geologic formation. It's called a hoodoo. Here's one at Bryce Canyon. Bryce Canyon is a national park in Western Utah in the Utah desert, just a couple hundred kilometers from campus actually. And it's filled with hoodoos. There are hundreds of these skinny irregularly shaped rock spires. They range in size from my height to taller than a 10-story building. Uh, Karen? Karen: What causes all those different shades of colors? Professor: Well, about 30 to 40,000,000 years ago, most of Western Utah, the whole Bryce Canyon area was underwater. It was covered by a gigantic lake. Well, eventually the lake dried up and the hoodoos we see today were actually carved out of that ancient dried-up lake bed. Color differentiations in hoodoos result from different minerals that had accumulated in sediments at the bottom of the lake. And these sediments built up and solidified under pressure. Now after the lake dried out, the lake bed, the rock was exposed to the elements to weathering. Can anyone guess which weathering processes we work here? Karen: Um, it's really dry and windy in Utah. It's a desert environment. Maybe the wind eroded the rock. Professor: Wind is an effective form of erosion for many locations, but not for Bryce Canyon. Actually the hoodoos are formed by both physical weathering and chemical weathering processes. The first and most prevalent process is what's called frost wedging. Every winter of Bryce Canyon, there's something like 200 freeze-thaw cycles. During the day, some of the snow there melts and seeps into cracks in the rock. At night, this water freezes again. Because water expands by nearly 10% when it freezes, the ice eventually pries open the cracks wider and wider, and finally breaks the rock, creating hoodoos over time. And you shouldn't underestimate the power of frost wedging. It's the same process that creates potholes in the street, you know, potholes, those gaping holes that can damage your tires if you drive over them. That's frost wedging for you. In springtime at Bryce Canyon, the primary physical weathering process switches from frost wedging to erosion. Streams of water from snowmelt carry away loosened rock, much of which was exposed and weakened by all that frost wedging. And in summertime, there are monsoon like thunderstorms which add to the erosion process. Okay. Next is chemical weathering. Chemical weathering happens when minerals in a rock get altered or dissolved by chemical processes. And the chemical weathering at Bryce Canyon comes mainly from acid rain. Acid rain which ... Yes, Karen? Karen: But, wait. Last week you said acid rain's caused by air pollution when stuff like sulfur oxide in the atmosphere turned the rain acidic, but Bryce Canyon has really clean air. Professor: So there's no air pollution at all in the desert? Karen: Yeah. Professor: Well, okay, the air at Bryce Canyon is relatively clean. Nevertheless, there's enough pollution overall in the region to turn the rain slightly acidic. It contains a weak carbonic acid. This carbonic acid slowly dissolves away the limestone in a hoodoo, which is more vulnerable to chemical weathering than siltstone and mudstone, which are also found in hoodoos. So within a single hoodoo, you got this siltstone and mudstone interspersed with limestone and each rock type dissolving at a different rate which contributes to the hoodoos lumpy profiles. And some hoodoos are capped with a magnesium-rich limestone called dolomite, which dissolves at a slow rate from acid rain. While mudstone dissolves quickly and forms the narrowest portion of the hoodoos. Okay. So hoodoos are created by weathering processes, but these processes are relentless. They continue to affect the hoodoos. Um, we estimate that Bryce Canyon's hoodoos are shrinking by up to 1.3 meter every hundred years. So we are pretty lucky to catch Bryce Canyon in this unusually beautiful state. And millions visit the park every year, but they are confined to established trails. People aren't allowed to get too close to hoodoos. Just walking up to one, this would actually weaken its foundation.