Lesson 30 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 th
ought to the bottom confines of the oceans probably assumed that the sea bad was flat. Sir James Cla
rk Ross had obtained a sounding of over 2,400 fathoms in 1839, but it was not until of deep sounding
s 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 sub
ject 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 there 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 o
f 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 featu
res of the oceans. The continents form rugged tablelands which stand nearly three miles above the fl
oor 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 conti
nents. The real dividing line between continents and oceans occurs at the foot a steeper slope.
This continental slope usually starts at a place somewhere near the 100-fatheom mark and in the cour
se of a few hundred miles reaches the true ocean floor at 2,500-3,500 fathoms. The slope averages ab
out 1 in 30. but contains steep, probably vertical, cliffs, and gentle sediment-covered terraces, an
d near its lower reaches there is a long tailing-off which is almost certainly the result of materia
l transported out to deep water after being eroded from the continental masses.