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Lesson 30 |
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Exploring the sea-floor |
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How did people probably imagine the sea-floor before it was investigated? |
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Our knowledge of the oceans a hundred years ago |
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was confined to the two-dimensional shape of the sea surface |
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of the shallow water close to the land. |
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The open sea was deep and mysterious, |
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and anyone who gave more than a passing thought to the bottom confines of the oceans |
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probably assumed that the sea bed was flat. |
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Sir James Clark Ross had obtained a sounding of over 2, 400 fathoms in 1839, |
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but it was not until 1869, |
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when H.M.S.Porcupine was put at the disposal of the Royal Society for several cruises |
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that a series of deep soundings was obtained in the Atlantic |
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and the first samples were collected by dredging the bottom. |
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Shortly after this the famous H.M.S.Challenger expedition |
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established the study of the sea-floor |
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as a subject worthy of the most qualified physicists and geologists. |
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A burst of activity associated with the laying of submarine cables |
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soon confirmed the Challenger's observation |
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that many parts of the ocean were two to three miles deep, |
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and the existence of underwater features of considerable magnitude. |
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Today, enough soundings are available to enable a relief map of the Atlantic to be drawn |
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and we know something of the great variety of the sea bed's topography. |
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Since the sea covers the greater part of the earth's surface, |
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it is quite reasonable to regard the sea floor |
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as the basic form of the crust of the earth, |
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with superimposed upon it the continents, |
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together with the islands and other features of the oceans. |
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The continents form rugged tablelands |
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which stand nearly three miles above the floor of the open ocean. |
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From the shore line, |
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out to a distance which may be anywhere from a few miles to a few hundred miles, |
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runs the gentle slope of the continental shelf, geologically part of the continents. |
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The real dividing line between continents and oceans |
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occurs at the foot of a steeper slope. |
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This continental slope usually starts at a place somewhere near the 100-fathom mark |
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and in the course of a few hundred miles |
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reaches the true ocean floor at 2, 500-3, 500 fathoms. |
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The slope averages about 1 in 30, but contains steep, |
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probably vertical, cliffs, and gentle sediment-covered terraces, |
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and near its lower reaches there is a long tailing-off |
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which is almost certainly the result of |
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material transported out to deep water after being eroded from the continental masses. |