墨尔本PTE素材库——Heat ID’d as Subtle Cause of Rockfalls

墨尔本PTE素材库——Heat ID’d as Subtle Cause of Rockfalls

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这段音频的主要内容，是关于石块掉落的研究。究竟石块是因为什么原因，才会掉落的呢？地震？还是别的什么原因？整段音频涵盖了几位研究人员对于Yosemite Valley中岩石的研究过程，在音频最后，给出了一个结果——那就是在天气特别热的时候，rocks更容易会掉落。而在天气变冷的时候，比如夜晚，比如冬天，石块间的缝隙会变小。

这段音频中的生词不多，如果是一位PTE目标为65+的学生，那么这段音频不应该有大部分的不理解，或者transcript中不应该有很多不认识的单词哦！尽管有一些专业相关的词汇，但是这些词汇并不难，平时在做阅读的时候也应该能够大致理解是什么意思哦！

澳大利亚语言学院会定期整理一些PTE的素材库，让各位PTE的考生，能够在平时练习的时候就能接触到一些和official questions criteria相近的话题素材，不但能够锻炼PTE考生的听力和复述的能力，也能在消化每篇素材的过程中提高自己的阅读能力以及词汇量。例如这篇关于Rockfalls的文章，大家可以试着re-tell这段音频你内容，并回答以下几个问题：

• Rocks更容易在什么时候掉落？
• 研究人员是用什么设备来做测量的？
• 究竟谁才是让石块掉落的罪魁祸首？

回答不出的小伙伴们，赶紧照着full transcript来学习啦！

【Key Vocabulary】

Earthquake: 地震

Crack:  破裂，裂缝

Cliff: 悬崖，绝壁

Crumble: 崩溃，破碎，粉碎

Granite: 花岗岩

Trigger:  触发

Rainfall: 降雨

Thawing: 融化

Culprit: 犯人

Geologist: 地质学家

Fracture: 破裂，断裂

Shrank: 收缩

Forth: adv. 向前，向外

Subcritical: 次临界的

Microscopic: 微观的

Slab: 厚板，平板，厚片

Sweltering: 闷热的，热的难受的

Detach: 分离

Hazard: 危险

Steep: 陡峭的

Rugged:崎岖的，高低不平的

Terrain: 地形

Mercury: 温度表

Full Transcript

Rockfalls without an obvious cause (like an earthquake or expanding ice) may be due to hot daily air temperatures expanding small cracks in cliff faces.

The floor of Yosemite Valley is littered with piles of rocks that crumbled off the park’s iconic cliffs. These rockfalls happen all the time, because Yosemite’s granite walls are riddled with cracks produced by geologic stress. Scientists know the rockfalls can be triggered by things like earthquakes, rainfall, and freezing and thawing ice. But many falls occur without an obvious cause. Now researchers think that heat may be the culprit.

“We noticed that there had been a bunch of rockfalls that were happening in the summertime, on particularly hot days.” Brian Collins, a research civil engineer with the U.S. Geological Survey. “And we noticed when we looked at the timing that they were happening in the afternoon, when we thought the temperatures were at their hottest.”

Collins and Greg Stock, Yosemite’s park geologist, wanted to know if small rock movements, induced by changing temperatures, might weaken cracks and contribute to rockfalls. So the researchers—who are both climbers—found a suitable fracture near the base of a 500-meter-tall cliff and installed instruments called crackmeters, which monitored the width of the crack over time.

The devices revealed that the crack grew almost a centimeter wider during the warmest part of the day. It shrank again when temperatures cooled off, for instance, at night and during the winter. But overall, the scientists found that the average width of the fracture grew over the course of a summer and over the entire three and a half-year study period, bringing the crack closer to breaking.

“We think that what happened was that every cycle — as the rock goes back and forth and back and forth — you’re getting to a part that we call subcritical crack growth. And that means that the crack where the rock is attached to the cliff is actually fracturing at a microscopic level. And so if you do that over the course of a year, then eventually you’re going to do some permanent damage to those points of attachment.”

The study is in the journal Nature Geoscience. [Brian D. Collins and Greg M. Stock, Rockfall triggering by cyclic thermal stressing of exfoliation fractures]

The slab of rock the scientists studied hasn’t fallen yet, and Collins doesn’t know how many cycles it will take before this or any other fracture finally breaks. When it does, the trigger might be a particularly sweltering day, when the partially detached slab is farthest away from the cliff. Or it could be another process, helped along by the fact that temperature changes already weakened the crack.

Either way, the results will help researchers assess the rockfall hazard in steep, rugged terrain. Because now they know when the mercury goes up, rocks are more likely to come down.

(source: http://www.scientificamerican.com/podcast/episode/heat-id-d-as-subtle-cause-of-rockfalls/)