[00:01.52]Lesson 30
[00:03.24]Exploring the sea-floor
[00:11.90]How did people probably imagine the sea-floor before it was investigated?
[00:20.08]Our knowledge of the oceans a hundred years ago
[00:23.21]was confined to the two-dimensional shape of the sea surface
[00:27.87]and the hazards of navigation presented by the irregularities in depth
[00:32.65]of the shallow water close to the land.
[00:36.48]The open sea was deep and mysterious,
[00:40.10]and anyone who gave more than a passing thought to the bottom confines of the oceans
[00:45.51]probably assumed that the sea bed was flat.
[00:49.62]Sir James Clark Ross had obtained a sounding of over 2, 400 fathoms in 1839,
[00:57.40]but it was not until 1869,
[01:00.25]when H.M.S.Porcupine was put at the disposal of the Royal Society for several cruises
[01:07.71]that a series of deep soundings was obtained in the Atlantic
[01:12.46]and the first samples were collected by dredging the bottom.
[01:17.08]Shortly after this the famous H.M.S.Challenger expedition
[01:21.56]established the study of the sea-floor
[01:24.00]as a subject worthy of the most qualified physicists and geologists.
[01:29.67]A burst of activity associated with the laying of submarine cables
[01:34.51]soon confirmed the Challenger's observation
[01:37.28]that many parts of the ocean were two to three miles deep,
[01:41.78]and the existence of underwater features of considerable magnitude.
[01:47.04]Today, enough soundings are available to enable a relief map of the Atlantic to be drawn
[01:53.68]and we know something of the great variety of the sea bed's topography.
[01:59.24]Since the sea covers the greater part of the earth's surface,
[02:02.97]it is quite reasonable to regard the sea floor
[02:05.79]as the basic form of the crust of the earth,
[02:09.05]with superimposed upon it the continents,
[02:12.47]together with the islands and other features of the oceans.
[02:16.79]The continents form rugged tablelands
[02:20.34]which stand nearly three miles above the floor of  the open ocean.
[02:25.58]From the shore line,
[02:26.87]out to a distance which may be anywhere from a few miles to a few hundred miles,
[02:32.53]runs the gentle slope of the continental shelf, geologically part of the continents.
[02:39.97]The real dividing line between continents and oceans
[02:43.41]occurs at the foot of a steeper slope.
[02:47.37]This continental slope usually starts at a place somewhere near the 100-fathom mark
[02:53.81]and in the course of a few hundred miles
[02:56.35]reaches the true ocean floor at 2, 500-3, 500 fathoms.
[03:04.26]The slope averages about 1 in 30, but contains steep,
[03:09.07]probably vertical, cliffs, and gentle sediment-covered terraces,
[03:15.38]and near its lower reaches there is a long tailing-off
[03:18.92]which is almost certainly the result of
[03:21.38]material transported out to deep water after being eroded from the continental masses.