[32]The devils and the details

感谢收听,期待你对本期节目的评论留言哦～ 2017.3.25 Economist Science and Technology Geology The devils and the details Powerful whirlwinds旋风 explain a strange feature of the Atacama desert阿塔卡玛（智利北部）. The Salar de Gorbea戈尔韦阿盐湖, at the southern end of the Atacama desert, in Chile, is one of the most hostile[ˈhɒstaɪl]不利的 places on Earth. It receives virtually 实际上 no rainfall and the little water it does host is contained in ponds both acidic [ə’sɪdɪk] 酸性的 and salty. It therefore has no vegetation[ˌvedʒəˈteɪʃn]植物. It is, though, the site of some of the most extraordinary dunes[dju:n]沙丘 on Earth. Most dunes are made of sand: grains颗粒 of silica that are 2mm across, or less. There are exceptions. The White Sands National Monument白沙国家纪念碑 in New Mexico, for example, is so called because the ingredients组成部分 of its dunes are sand-grain-sized crystals结晶体 of gypsum [ˈdʒɪpsəm]石膏.But this exception proves the rule, because the point about a dune is that it is created by the wind, and when it comes to minerals[ˈmɪnərəl], the wind can generally pick up and move around only sand-sized objects. The dunes of Salar de Gorbea, however, are an exception that proves no rule at all. They, too, are white, because they are also made of gypsum. But the gypsum in question includes crystals more than 20cm long. How such dunes could form by wind action has long been a mystery. Kathleen Benison, of West Virginia University西弗吉尼亚大学,thinks, however, that she has solved it. Gypsum is a form of calcium[ˈkælsiəm] sulphate[ˈsʌlfeɪt] 硫酸钙created by the evaporation[ɪˌvæpə'reɪʃn]蒸发 of water laden[ˈleɪdn]负载的 with that substance物质. Dr Benison knew that gypsum crystals of the size found in Salar de Gorbea’s dunes form in ponds 5km from those dunes. She thus suspected that these ponds are the source of the dunes’crystals. This suspicion[səˈspɪʃn]怀疑 was reinforced [ˌri:ɪnˈfɔ:s] , she explains in a paper just published in Geology,when she compared the internal bands marking stages阶段 of the growth of crystals from the dunes with those of crystals from the ponds. They appeared identical. That suggested crystals are somehow being transported from the ponds to the dunes. She was able to rule out排除......的可能性 one mechanism机制 for such transport—that the crystals had been moved by long-vanished消失 streams or rivers—for several reasons. First, the Atacama is believed to have been too dry for streams to form for millions of years. Second, gypsum dissolves [dɪˈzɒlv] 溶解in water (this is, indeed, the reason dunes made of it are rare, for most deserts have at least some rainfall). And third, the faces表面 of crystals from the dunes were scored刮擦 in ways which indicated that they had been bashed [bæʃ]猛击 around by strong winds. The only inland winds obviously powerful enough to have done this are in the funnels [ˈfʌnl]漏斗of tornadoes. The Atacama desert does not, though, experience such storms. It does experience lesser whirlwinds, called dust devils. But the textbooks say that dust devils are not powerful enough to lift and carry objects the size of the crystals found in the dunes. Textbooks, however, are not always correct, so Dr Benison decided to check for herself. She went to Salar de Gorbea and monitored the dust devils there. She found that devils do regularly form in valleys along the edge of the region. Some then pass over the ponds where the gypsum crystals are growing, pluck拉 crystals out of those ponds, carry them the 5km to the dunes, and then dissipate[ˈdɪsɪpeɪt]消散,dropping their loads on the accumulating heaps堆. What she does not yet know is how they do it, for the textbooks are, in one sense, correct. The most powerful recorded dust devils have wind speeds of 70kph. This is indeed insufficient不足的 to carry mineral particles bigger than 2mm across. For the devils of Salar de Gorbea to be transporting large gypsum crystals they must be far more powerful than that. Dr Benison seems therefore to have substituted[ˈsʌbstɪtju:t]代替 one mystery for another. The devils clearly are responsible for Salar de Gorbea’s dunes. What is responsible for these devils’great powers remains to be found out. 歌儿:The flood----Joshua Hyslop