通过泛读提高雅思阅读水平

通过泛读提高雅思阅读水平,标签：雅思阅读评分标准与雅思阅读真题下载,http://www.xuehuiba.com

　　Studying the life cycle of vent organisms is difficult. Researchers have visited only a fractionof the ocean's hot spots. They have been able to observe vent life only by shining bright lights on creatures accustomed to inky darkness, and many specimens die quickly when removed from their unique environment. Underwater cameras are helping scientists make less intrusive observations, but diving expeditionsare still the most useful way to gather information. The1993 Alvin expedition to the East Pacific Rise was one in a series of dives to the area. The site was first visited in 1989, and scientists observed vent organisms thriving there. But when Alvin returned in April 1991, its flabbergasted occupants witnessed the birth of a hydrothermal vent. A recent volcanic eruptionhad spread glassy lava across the ocean floor, and the researchers measured temperatures up to 403C-the hottest ever recorded at a hydrothermal vent. The scientists dubbed the site Tube Worm Barbecue, because the worms they brought back to their ship had charred flesh.

　　“The most spectacular sight down there was this massive blinding snowstorm of bacteria,” says Rich Lutz, a marineecologist at Rutgers University, who led the expedition. On the ocean floor, the bacteria formed mats several inches thick, but the scientists saw no other living things.

　　Since the eruption, scientists have been able to watch several stages of colonization at the site. When they returned in March 1992, only a few bacterial mats remained. In their place were colonies of Jericho worms and a variety of small crustaceans. The scientists named the area Phoenix, because new life had arisen from the ashes of the eruption.

　　The scientists first observed the giant tube worms at Phoenix in December 1993. They also noticed a number of mineral deposits, some towering to heights of more than 30 feet. These structures form where hot vent water meets cold seawater, causing metal sulfides to precipitateout. The precipitating sulfides, which look like smoke, amass to form chimneys called black smokers. Like the vent fields, some smokers have names. Smoke and Mirrors, for example, has shelf-like overhangs that trap hot water rising from below, creating upside-down shimmering pools. The largest known black smoker is Godzilla, a 160-foot-tall structure off the coast of Oregon.

　　During a December 1993 dive to the Phoenix vent field, Alvin accidentally toppled a 33-foot-tall smoker. When the sub returned for a brief visit three months later, the chimney had already grown back 20 feet. Scientists were surprised by the speedy recovery, which seems to parallelthe rapid growth of tube worms and other organisms at the vents. The visits to the Phoenix site “give us a sense of how quickly these vents are colonized,” says Van Dover.

　　Another expedition is planned for November. By then, the community of organisms now prospering at the vents may already be a ghost town. When the flow of hot, sulfide-rich water slows to a trickle, death also comes quickly.

上一页  [1] [2] [3]