新概念英語第四冊(含lrc文本)Lesson 30 Exploring the sea-floor

[ti:δ?] [ar:] [al:] [by:https://www.lrc.com.cn] [00:04.62]Lesson 30 [00:12.45]Exploring the sea-floor [00:13.12]How did people probably imagine the sea-floor before it was investigated? [00:19.99]Our knowledge of the oceans a hundred years ago [00:23.01]was confined to the 2-dimensional shape of the sea surface [00:27.62]and the hazards of navigation presented by the irregularities in depth [00:32.38]of the shallow water close to the land. [00:36.33]The open sea was deep and mysterious, [00:39.78]and anyone who gave more than a passing thought to the bottom confines of the oceans [00:45.15]probably assumed that the sea bed was flat. [00:49.31]Sir James Clark Ross had obtained a sounding of over 2,400 fathoms in 1839, [00:56.75]but it was not until 1869, [00:59.70]when H.M.S.Porcupine was put at the disposal of the Royal Society for several cruises [01:06.96]that a series of deep soundings was obtained in the Atlantic [01:11.55]and the first samples were collected by dredging the bottom. [01:16.24]Shortly after this the famous H.M.S.Challenger expedition [01:20.67]established the study of the sea-floor [01:23.07]as a subject worthy of the most qualified physicists and geologists. [01:28.66]A burst of activity associated with the laying of submarine cables [01:33.30]soon confirmed the Challenger's observation [01:36.11]that many parts of the ocean were two to three miles deep, [01:40.63]and the existence of underwater features of considerable magnitude. [01:45.94]Today,enough soundings are available to enable a relief map of the Atlantic to be drawn [01:52.32]and we know something of the great variety of the sea bed's topography. [01:58.09]Since the sea covers the greater part ofthe earth's surface, [02:01.44]it is quite reasonable to regard the sea floor [02:04.31]as the basic form of the crust of the earth [02:07.67]with,superimposed upon it,the continents, [02:11.05]together with the islands and other features of the oceans [02:15.34]The continents form rugged tablelands [02:18.73]which stand nearly three miles above the ocean. [02:23.88]From the shore line, [02:25.23]out to a distance which may be anywhere from a few miles to a few hundred miles, [02:30.91]runs the gentle slope of the continental shelf,geologically part of the continents [02:38.35]The real dividing line between continents and oceans [02:41.54]occurs at the foot of a steeper slope. [02:45.47]This continental slope usually starts at a place somewhere near the 100-fathom mark [02:52.00]and in the course of a few hundred miles [02:54.45]reaches the true ocean floor at 2,500-3,500 fathoms. [03:02.27]The slope averages about 1 in 30,but contains steep, [03:07.03]probably vertical,cliffs, and gentle sediment-covered terraces, [03:13.03]and near its lower reaches there is a long tailing-off [03:16.87]which is almost certainly the result of [03:19.32]material transported out to deep water after being eroded from the continental masses

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