新概念英语4册第30课

Exploring the Sea-Floor

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 1836 but it was not until 1800, 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 may 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 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 to 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 ice-fathom mark and in the course of a few hundred miles reaches the true ocean-floor at 2,5000 - 3,000 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.

一百年前我们对海洋的认识局限在二维的海平面和对陆地周围不规则的浅水的探险活动。广阔的海洋深邃且神秘。任何不仅仅是简单思考的人都可能认为海底是平坦的。James Clark Ross先生1836发明了可探测2400英寻(测量水深的单位, =6英尺)的探通术。但直到1869年H.M.S.Porcupine将这个方法应用在一系列皇家科学巡航上，大西洋底部才首次被测量。在这之后很段的时间里H.M.S的探险队建立起来了可以和物理学与地质学相提并论的以海床为主题的学科。一系列的活动随着水下电缆证实了挑战者观察到海洋的许多地方有二到三英里深。而水面下存在着许多显著特征。

今天测量术已经足可以描绘出大西洋的地貌图，并且我们知道辽阔的海床的一些地貌变化。因为海洋覆盖了地球大部分面积因为把海床，和在它上面的大陆以及岛屿和一部分海洋作为地壳的一种基本形式是合理的。大陆可被认为是海床上面大约三英里的崎岖不平的高原。从海岸线延伸到数英里以至数百英里是大陆架的缓缓斜坡。而海洋从陡峭的斜坡开始。大陆斜坡通常在某个ice-fathom mark开始，并延伸数百英里到达的2000-3000英寻的真正的海底。斜率平均为1/30。可能会更陡峭一些，甚至是垂直的悬崖，也可能是有沉积物的低天。而在它的低端有一条长长的尾部。大陆上被腐蚀下来的物质都被运送到这里。