Nowhere off the Pacific coast of the United States is the continental shelf much more than 20 miles wide—a narrowness characteristic of coasts bordered by young mountains perhaps still in the process of formation. On the American east coast, however, north of Cape Hatteras the shelf is as much as 150 miles wide. But at Hatteras and off southern Florida it is merely the narrowest of thresholds to the sea. Here its scant development seems to be related to the press of that great and rapidly flowing river-in-the-sea, the Gulf Stream, which at these places swings close inshore.
The widest shelves in all the world are those bordering the Arctic. The Barents Sea shelf is 750 miles across. It is also relatively deep, lying for the most part 100 to 200 fathoms below the surface, as though its floor had sagged and been down-warped under the load of glacial ice. It is scored by deep troughs between which banks and islands rise—further evidence of the work of the ice. The deepest shelves surround the Antarctic continent, where soundings in many areas show depths of several hundred fathoms near the coast and continuing out across the shelf.
Once beyond the edge of the shelf, as we visualize the steeper declivities of the continental slope, we begin to feel the mystery and the alien quality of the deep sea—the gathering darkness, the growing pressure, the starkness of a seascape in which all plant life has been left behind and there are only the unrelieved contours of rock and clay, of mud and sand.
Biologically the world of the continental slope, like that of the abyss, is a world of animals—a world of carnivores where each creature preys upon another. For no plants live here, and the only ones that drift down from above are the dead husks of the flora of the sunlit waters. Most of the slopes are below the zone of surface wave action, yet the moving water masses of the ocean currents press against them in their coastwise passage; the pulse of the tide beats against them; they feel the surge of the deep, internal waves.
Geographically, the slopes are the most imposing features of all the surface of the earth. They are the walls of the deep-sea basins. They are the farthermost bounds of the continents, the true place of beginning of the sea. The slopes are the longest and highest escarpments found anywhere on the earth; their average height is 12,000 feet, but in some places they reach the immense height of 30,000 feet. No continental mountain range has so great a difference of elevation between its foothills and its peaks.
Nor is the grandeur of slope topography confined to steepness and height. The slopes are the site of one of the most mysterious features of the sea. These are the submarine canyons with their steep cliffs and winding valleys cutting back into the walls of the continents. The canyons have now been found in so many parts of the world that when soundings have been taken in presently unexplored areas we shall probably find that they are of world-wide occurrence. Geologists say that some of the canyons were formed well within the most recent division of geologic time, the Cenozoic, most of them probably within the Pleistocene, a million years ago, or less. But how and by what they were carved, no one can say. Their origin is one of the most hotly disputed problems of the ocean.
Only the fact that the canyons are deeply hidden in the darkness of the sea (many extending a mile or more below present sea level) prevents them from being classed with the world’s most spectacular scenery. The comparison with the Grand Canyon of the Colorado is irresistible. Like river-cut land canyons, sea canyons are deep and winding valleys, V-shaped in cross section, their walls sloping down at a steep angle to a narrow floor. The location of many of the largest ones suggests a past connection with some of the great rivers of the earth of our time. Hudson Canyon, one of the largest on the Atlantic coast, is separated by only a shallow sill from a long valley that wanders for more than a hundred miles across the continental shelf, originating at the entrance of New York Harbor and the estuary of the Hudson River. There are large canyons off the Congo, the Indus, the Ganges, the Columbia, the Sāo Francisco, and the Mississippi, according to Francis Shepard, one of the principal students of the canyon problem. Monterey Canyon in California, Professor Shepard points out, is located off an old mouth of the Salinas River; the Cap Breton Canyon in France appears to have no relation to an existing river but actually lies off an old fifteenth-century mouth of the Adour River.
Their shape and apparent relation to existing rivers have led Shepard to suggest that the submarine canyons were cut by rivers at some time when their gorges were above sea level. The relative youth of the canyons seems to relate them to some happenings in the world of the Ice Age. It is generally agreed that sea level was lowered during the existence of the great glaciers, for water was withdrawn from the sea and frozen in the ice sheet. But most geologists say that the sea was lowered only a few hundred feet—not the mile that would be necessary to account for the canyons. According to one theory, there were heavy submarine mud flows during the times when the glaciers were advancing and sea level fell the lowest; mud stirred up by waves poured down the continental slopes and scoured out the canyons. Since none of the present evidence is conclusive, however, we simply do not know how the canyons came into being, and their mystery remains.*
The floor of the deep ocean basins is probably as old as the sea itself. In all the hundreds of millions of years that have intervened since the formation of the abyss, these deeper depressions have never, as far as we can learn, been drained of their covering waters. While the bordering shelves of the continents have known, in alternative geologic ages, now the surge of waves and again the eroding tools of rain and wind and frost, always the abyss has lain under the all-enveloping cover of miles-deep water.
But this does not mean that the contours of the abyss have remained unchanged since the day of its creation. The floor of the sea, like the stuff of the continents, is a thin crust over the plastic mantle of the earth. It is here thrust up into folds and wrinkles as the interior cools by imperceptible degrees and shrinks away from its covering layer; there it falls away into deep trenches in answer to the stresses and strains of crustal adjustment; and again it pushes up into the conelike shapes of undersea mountains and volcanoes boil upward from fissures in the crust.
Until very recent years, it has been the fashion of geographers and oceanographers to speak of the floor of the deep sea as a vast and comparatively level plain. The existence of certain topographic features was recognized, as, for example, the Atlantic Ridge and a number of very deep depressions like the Mindanao Trench off the Philippines. But these were considered to be rather exceptional interruptions of a flat floor that otherwise showed little relief.
This legend of the flatness of the ocean floor was thoroughly destroyed by the Swedish Deep-Sea Expedition, which sailed from Goteborg in the summer of 1947 and spent the following 15 months exploring the bed of the ocean. While the Swedish Albatross was crossing the Atlantic in the direction of the Panama Canal, the scientists aboard were astonished by the extreme ruggedness of the ocean floor. Rarely did their fathometers reveal more than a few consecutive miles of level plain. Instead the bottom profile rose and fell in curious steps constructed on a Gargantuan scale, half a mile to several miles wide. In the Pacific, the uneven bottom contours made it difficult to use many of the oceanographic instruments. More than one coring tube was left behind, probably lodged in some undersea crevasse.
One of the exceptions to a hilly or mountainous floor was in the Indian Ocean, where, southeast of Ceylon, the Albatross ran for several hundred miles across a level plain. Attempts to take bottom samples from this plain had little success, for the corers were broken repeatedly, suggesting that the bottom was hardened lava and that the whole vast plateau might have been formed by the outpourings of submarine volcanoes on a stupendous scale. Perhaps this lava plain under the Indian Ocean is an undersea counterpart of the great basaltic plateau in the eastern part of the State of Washington, or of the Deccan plateau of India, built of basaltic rock 10,000 feet thick.
In parts of the Atlantic basin the Woods Hole Oceanographic Institution’s vessel Atlantis has fou
nd a flat plain occupying much of the ocean basin from Bermuda to the Atlantic Ridge and also to the east of the Ridge. Only a series of knolls, probably of volcanic origin, interrupts the even contours of the plains. These particular regions are so flat that it seems they must have remained largely undisturbed, receiving deposits of sediments over an immense period of time.
The deepest depressions on the floor of the sea occur not in the centers of the oceanic basins as might be expected, but near the continents. One of the deepest trenches of all, the Mindanao, lies east of the Philippines and is an awesome pit in the sea, six and a half miles deep.* The Tuscarora Trench east of Japan, nearly as deep, is one of a series of long, narrow trenches that border the convex outer rim of a chain of islands including the Bonins, the Marianas, and the Palaus. On the seaward side of the Aleutian Islands is another group of trenches. The greatest deeps of the Atlantic lie adjacent to the islands of the West Indies, and also below Cape Horn, where other curving chains of islands go out like stepping stones into the Southern Ocean. And again in the Indian Ocean the curving island arcs of the East Indies have their accompanying deeps.
Always there is this association of island arcs and deep trenches, and always the two occur only in areas of volcanic unrest. The pattern, it is now agreed, is associated with mountain making and the sharp adjustments of the sea floor that accompany it. On the concave side of the island arcs are rows of volcanoes. On the convex side there is a sharp down-bending of the ocean floor, which results in the deep trenches with their broad V-shape. The two forces seem to be in a kind of uneasy balance: the upward folding of the earth’s crust to form mountains, and the thrusting down of the crust of the sea floor into the basaltic substance of the underlying layer. Sometimes, it seems, the down-thrust mass of granite has shattered and risen again to form islands. Such is the supposed origin of Barbados in the West Indies and of Timor in the East Indies. Both have deep-sea deposits, as though they had once been part of the sea floor. Yet this must be exceptional. In the words of the great geologist Daly,
Another property of the earth is its ability … to resist shearing pressures indefinitely … The continents, overlooking the sea bottom, stubbornly refuse to creep thither. The rock under the Pacific is strong enough to bear, with no known time limit, the huge stresses involved by the down-thrust of the crust at the Tonga Deep, and by the erection of the 10,000-meter dome of lavas and other volcanic products represented in the island of Hawaii.*
The least-known region of the ocean floor lies under the Arctic Sea. The physical difficulties of sounding here are enormous. A permanent sheet of ice, as much as fifteen feet thick, covers the whole central basin and is impenetrable to ships. Peary took several soundings in the course of his dash to the Pole by dog team in 1909. On one attempt a few miles from the Pole the wire broke with 1500 fathoms out. In 1927 Sir Hubert Wilkins landed his plane on the ice 550 miles north of Point Barrow and obtained a single echo sounding of 2975 fathoms, the deepest ever recorded from the Arctic Sea. Vessels deliberately frozen into the ice (such as the Norwegian Fram and the Russian Sedov and Sadko) in order to drift with it across the basin have obtained most of the depth records available for the central parts. In 1937 and 1938 Russian scientists were landed near the Pole and supplied by plane while they lived on the ice, drifting with it. These men took nearly a score of deep soundings.
The most daring plan for sounding the Arctic Sea was conceived by Wilkins, who actually set out in the submarine Nautilus in 1931 with the intention of traveling beneath the ice across the entire basin from Spitsbergen to Bering Strait. Mechanical failure of the diving equipment a few days after the Nautilus left Spitsbergen prevented the execution of the plan. By the middle 1940’s, the total of soundings for deep arctic areas by all methods was only about 150, leaving most of the top of the world an unsounded sea whose contours can only be guessed. Soon after the close of the Second World War, the United States Navy began tests of a new method of obtaining soundings through the ice, which may provide the key to the arctic riddle. One interesting speculation to be tested by future soundings is that the mountain chain that bisects the Atlantic, and has been supposed to reach its northern terminus at Iceland, may actually continue across the arctic basin to the coast of Russia. The belt of earthquake epicenters that follows the Atlantic Ridge seems to extend across the Arctic Sea, and where there are submarine earthquakes it is at least reasonable to guess that there may be mountainous topography.*
A new feature on recent maps of undersea relief—something never included before the 1940’s—is a group of about 160 curious, flat-topped sea mounts between Hawaii and the Marianas. It happened that a Princeton University geologist, H. H. Hess, was in command of the U.S.S. Cape Johnson during two years of the wartime cruising of this vessel in the Pacific. Hess was immediately struck by the number of these undersea mountains that appeared on the fathograms of the vessel. Time after time, as the moving pen of the fathometer traced the depth contours it would abruptly begin to rise in an outline of a steep-sided sea mount, standing solitarily on the bed of the sea. Unlike a typical volcanic cone, all of the mounts have broad, flat tops, as though the peaks had been cut off and planed down by waves. But the summits of the sea mounts are anywhere from half a mile to a mile or more below the surface of the sea. How they acquired their flat-topped contours is a mystery perhaps as great as that of the submarine canyons.
Unlike the scattered sea mounts, the long ranges of undersea mountains have been marked on the charts for a good many years. The Atlantic Ridge was discovered about a century ago. The early surveys for the route of the trans-Atlantic cable gave the first hint of its existence. The German oceanographic vessel Meteor, which crossed and recrossed the Atlantic during the 1920’s, established the contours of much of the Ridge. The Atlantis of the Woods Hole Oceanographic Institution has spent several summers in an exhaustive study of the Ridge in the general vicinity of the Azores.
Now we can trace the outlines of this great mountain range, and dimly we begin to see the details of its hidden peaks and valleys. The Ridge rises in mid-Atlantic near Iceland. From this far-northern latitude it runs south midway between the continents, crosses the equator into the South Atlantic, and continues to about 50° south latitude, where it turns sharply eastward under the tip of Africa and runs toward the Indian Ocean. Its general course closely parallels the coastlines of the bordering continents, even to the definite flexure at the equator between the hump of Brazil and the eastward-curving coast of Africa. To some people this curvature has suggested that the Ridge was once part of a great continental mass, left behind in mid-ocean when, according to one theory, the continents of North and South America drifted away from Europe and Africa. However, recent work shows that on the floor of the Atlantic there are thick masses of sediments which must have required hundreds of millions of years for their accumulation.
Throughout much of its 10,000-mile length, the Atlantic Ridge is a place of disturbed and uneasy movements of the ocean floor, and the whole Ridge gives the impression of something formed by the interplay of great, opposing forces. From its western foothills across to where its slopes roll down into the eastern Atlantic basin, the range is about twice as wide as the Andes and several times the width of the Appalachians. Near the equator a deep gash cuts across it from east to west—the Romanche Trench. This is the only point of communication between the deep basins of the eastern and western Atlantic, although among its higher peaks there are other, lesser mountain passes.
The greater part of the Ridge is, of course, submerged. Its central backbone rises some 5000 to 10,000 feet above the sea floor, but another mile of water lies above most of its summits. Yet here and there a peak thrusts itself up out of the darkness of deep water and pushes above the surface of the ocean. These are the islands of the mid-Atlantic. The highest peak of the Ridge is Pico Island of the Azores. It rises 27,000 feet above the ocean floor, with only its upper 7000 to 8000 feet emergent. The sharpest peaks of the Ridge are the clust
er of islets known as the Rocks of St. Paul, near the equator. The entire cluster of half a dozen islets is not more than a quarter of a mile across, and their rocky slopes drop off at so sheer an angle that water more than half a mile deep lies only a few feet off shore. The sultry volcanic bulk of Ascension is another peak of the Atlantic Ridge; so are Tristan da Cunha, Gough, and Bouvet.
But most of the Ridge lies forever hidden from human eyes. Its contours have been made out only indirectly by the marvelous probings of sound waves; bits of its substance have been brought up to us by corers and dredges; and some details of its landscape have been photographed with deep-sea cameras. With these aids our imaginations can picture the grandeur of the undersea mountains, with their sheer cliffs and rocky terraces, their deep valleys and towering peaks. If we are to compare the ocean’s mountains with anything on the continents, we must think of terrestrial mountains far above the timber line, with their silent snow-filled valleys and their naked rocks swept by the winds. For the sea has an inverted ‘timber line’ or plant line, below which no vegetation can grow. The slopes of the undersea mountains are far beyond the reach of the sun’s rays, and there are only the bare rocks, and, in the valleys, the deep drifts of sediments that have been silently piling up through the millions upon millions of years.
Neither the Pacific Ocean nor the Indian Ocean has any submerged mountains that compare in length with the Atlantic Ridge, but they have their smaller ranges. The Hawaiian Islands are the peaks of a mountain range that runs across the central Pacific basin for a distance of nearly 2000 miles. The Gilbert and Marshall islands stand on the shoulders of another mid-Pacific mountain chain. In the eastern Pacific, a broad plateau connects the coast of South America and the Tuamotu Islands in the mid-Pacific, and in the Indian Ocean a long ridge runs from India to Antarctica, for most of its length broader and deeper than the Atlantic Ridge.