Exploring the sea-floor

海底勘探

Our knowledge of the oceans a hundred years ago was confined to the two-dimensional shape of the sea surface and the hazards of navigation presented by the irregularities in depth of the shallow water close to the land. The open sea was deep and mysterious, and anyone who gave more than a passing thought to the bottom confines of the oceans probably assumed that the sea bed was flat. Sir James Clark Ross had obtained a sounding of over 2,400 fathoms in 1839, but it was not until 1869, when H.M.S. Porcupine was put at the disposal of the Royal Society for several cruises, that a series of deep soundings was obtained in the Atlantic and the first samples were collected by dredging the bottom. Shortly after this the famous H. M. S. Challenger expedition established the study of the sea-floor as a subject worthy of the most qualified physicists and geologists. A burst of activity associated with the laying of submarine cables soon confirmed the challenger's observation that many parts of the ocean were two to three miles deep, and the existence of underwater features of considerable magnitude.

Today, enough soundings are available to enable a relief map of the Atlantic to be drawn and we know something of the great variety of the sea bed's topography. Since the sea covers the greater part of the earth's surface, it is quite reasonable to regard the sea floor as the basic form of the crust of the earth, with, superimposed upon it, the continents, together with the islands and other features of the oceans. The continents form rugged tablelands which stand nearly three miles above the floor of the open ocean. From the shore line, out a distance which may be anywhere from a few miles to a few hundred miles, runs the gentle slope of the continental shelf, geologically part of the continents. The real dividing line between continents and oceans occurs at the foot of a steeper slope.

This continental slope usually starts at a place somewhere near the 100-fatheom mark and in the course of a few hundred miles reaches the true ocean floor at 2,500-3,500 fathoms. The slope averages about 1 in 30. but contains steep, probably vertical, cliffs, and gentle sediment-covered terraces, and near its lower reaches there is a long tailing-off which is almost certainly the result of material transported out to deep water after being eroded from the continental masses.T.F.GASKELL Exploring the Sea-floor from Science Survey

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100年前，我們只知道海洋是二維平面形的，以及靠近陸地淺水區(qū)的深淺不一能給航行帶來危險。無邊無際的海洋深邃而又神秘，凡是稍稍想過大海海底的人大概都會認為海底是平坦的。1839年，詹姆斯.克拉克.羅斯爵士曾測得海水深度超過2,400英尋;但直到1869年，皇家學(xué)會用英國“豪豬”號艦艇進行了幾次巡航后，才在大西洋測得一個海水深度，同時能過挖掘海底，取得了研究海底的首批樣品。此后不久，英國著名的“挑戰(zhàn)者”號艦艇對海底的那次考察，把對海床的研究確立為一個值得一流物理學(xué)家和地質(zhì)學(xué)家從事的研究課題，鋪設(shè)海底電纜的熱潮很快證實了“挑戰(zhàn)者”號的觀察結(jié)果：海洋中很多地方可深達兩三英里，水下特征差異極大。

現(xiàn)在已有足夠的水深測量數(shù)據(jù)來繪制一張大西洋洋底地形圖，而且我對海底地形的千變?nèi)f化也有了一定的了解。既然海洋覆蓋著地球的大部分表面，因此完全有理由把海床看作地殼的基本模殼，上面附加著大陸以及島嶼和海洋的其他形態(tài)。大陸是崎嶇不平的高地，高出遼闊的海洋海底近三英里。從海岸線向大海延伸幾英里到幾百英里的區(qū)域是大陸架慢坡，從地質(zhì)學(xué)上來說，它是大陸的一部分。大陸和海洋的真正分界線是在陡破腳下。

大陸架慢坡一般是從差不多100英尋水深的地方開始的，一直延伸到幾百英里遠深達2,500至3,500的地方，那里才是真正的海底。坡度平均約為1/30，但其中包括陡峭的、乃至垂直的峭壁和沉積物覆蓋的緩和的階梯地帶，在這個地帶的低處是很長的一段尾沙地段，基本上可以斷定這個地段是大陸塊體上侵蝕下來的物質(zhì)被水沖到深水處形成的。

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