Heat ID'd as Subtle Cause of Rockfalls
高温是落石发生的微妙原因

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. 
无明显原因（例如地震或冰的扩张）的落石可能是由白天高温空气造成了悬崖表面的小裂纹扩张。

撰文：茱莉亚·罗斯（Julia Rosen）
翻译：陈旭琦
审校：乔虹

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.”
约塞米蒂谷（Yosemite Valley）的地表散落着一大堆从公园标志性悬崖剥落的岩石。这种落石总是发生，因为约塞米蒂的花岗岩墙体遍布着地质压力造成的裂痕。科学家们知道落石可能是由地震、降雨、冰冻以及融化的冰引起的。但是许多落石的发生常常没有明显的原因。现在研究人员认为热量也许是幕后元凶。“我们注意到有很多的落石发生在夏季，特别是高温天。”美国地质调查局（U.S. Geological Survey）的一位土木研究工程师布莱恩·柯林斯（Brian Collins）说，“同时我们注意到它们发生的时间在下午，我们想这是一天当中最热的时候。”

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.
柯林斯和约塞米蒂谷公园的地质学家格雷格·斯托克（Greg Stock）想知道气温变化引起的小岩石的移动是否会使裂缝变得脆弱并造成落石。所以研究人员们——同时也是攀岩者——在500米高的悬崖边找到了一个合适的破裂处，并安装了名为超声波探伤仪（crackmeters）的仪器，这些仪器可以一直监控裂缝的宽度。

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.
这些设备告诉我们在一天中最温暖的时候岩石的裂缝大概会增加一厘米。当气温下降，裂缝又会再次收缩，例如，在晚上和冬天。但是总体上，科学家们发现从整个夏天的时间以及三年半研究周期的时间跨度来看裂缝的平均宽度都有增加，这使得它们更容易破裂。

“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.”
“我们认为在一个岩石来回往复运动的周期中，会出现被我们称做亚临界裂纹增长的现象。并且这意味着，实际上，岩石与悬崖连接处的裂缝在非常细微地增加。所以，如果这样的细微破裂持续增长，那么一年后这些连接点就会出现永久的损坏。”

The study is in the journal Nature Geoscience. [Brian D. Collins and Greg M. Stock, Rockfall triggering by cyclic thermal stressing of exfoliation fractures]
这项研究发表在《自然地球科学》（Nature Geoscience）杂志。[布莱恩·D·柯林斯（Brian D. Collins）和格雷格·斯托克（Greg M. Stock），在岩石裂缝处由周期性热应力所引发的落石（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.
科学家们研究的一整块岩石还没有掉落，同时柯林斯并不知道在多少个周期后这块岩石会掉落，或者在它掉落之前还有多少条裂缝会产生。当这真的发生时，部分脱落的岩石会远离这个悬崖，这件事的触发者可能是特别闷热的天气。又或者会是另一种过程，气温的变化已经使这个裂缝变得脆弱。

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.
不管是哪种途径，最后的结果都会帮助研究人员评估陡峭、崎岖不平的地带中落石的危险。因为现在他们知道，当温度上升时，岩石更有可能落下。