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National Geographic : 1987 Jan
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LONNIEG. THOMPSON Frozen archive of weather reports,the 50 meterface of the Quelccaya Ice Cap in Peru's Andes displays annual layers of snow separatedby dry-season dust. A 165-meter long ice core, taken at the ice cap's summit, holds 1,500 years of climatic data.Ice cores from Greenlandand Antarctica trace conditions as far back as 150,000years. But Quelccayaoffers a rarerecord of equatorial weather patterns,such as El Nino, which can wreak global havoc. multistriped ice fields along the Gulf of Alaska. Each black stripe of glacial debris is a side moraine from a different ice river. Count the stripes, and you know the number of glaciers feeding the final broad ice stream. Occasionally a glacier will "surge"-a technical term to glaciologists, translated in popular jargon to "galloping." The ice sud denly begins flowing far faster than normal, grinding and pushing downhill at speeds of tens, even hundreds of feet a day. I went to Alaska in 1982 because a well known glacier immediately adjacent to the Hubbard, the Variegated, was in early stages of a surge. Glaciologists from the Uni versities of Washington and Alaska, Cal tech, and the Swiss Federal Institute of Technology were camped on the glacier with ice drills, theodolites, and other gear to record the ice river's gallop forward. It had begun only a few weeks before. "Our thinking is that a glacier that surges has changed its internal plumbing system," explained Barclay Kamb of Caltech, a friendly, articulate ice scientist. "Meltwater at the base, blocked in its normal escape pas sages, builds up pressure until the friction between ice and rock is nearly overcome, al lowing the ice to slide downhill much faster [diagrams, pages 114-15]." Day by day I could sense it moving hear it and all but see it. Along the edges of the Variegated, creaking and groaning and booming of tortured ice gave loud proof that the frozen river on which the scientists were camped was indeed lurching downhill. "We can measure not only the surface mo tion-between three and twenty feet a day right now-but also what is more interesting to us: pulses of water pressure traveling un derneath. In a series of drill holes through the glacier, we are recording pressure waves moving as fast as 1,000 feet an hour." The surface of the ice was also rising and falling slowly, like the back of a massive cat erpillar, measured by precise instruments mounted on high promontories and cliff ledges above the glacier. Water flowed through deep crevasses in the glacier and gushed in a milky freshet from its snout. When British explorer George Vancouver sailed this coast in 1794, ice entirely blocked the mouths of several of today's deep, NationalGeographic,January 1987 102
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