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 It is generally agreed that sea level was lowered during the existence of the great glaciers, for water was withdrawn from the sea and frozen in the ice sheet.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.[12]
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 found 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.[13] 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.[14]
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.[15]
12
In the ten years that have elapsed since this account of the canyons was written much more has been learned about them, but it may still be said that there is no general agreement about their origin. Many of the resources of the modern oceanographer have been brought to bear on the problem. Divers have engaged in direct exploration of the shallow heads of some of the California canyons, collecting samples of their walls and photographing them. Other canyons have been studied by oceanographers using deep-sea corers or dredges to obtain samples of rocks and sediments. Precision depth recorders have given much new information about their shapes. As a result of these studies it is now known that there are at least five types of canyons, so different in their characteristics that almost certainly they have different origins. No single theory may be expected to explain all of them. Professor Francis S. Shepard, the marine geologist who originally put forward the theory that the canyons had been cut by rivers and later submerged, now feels this explanation is adequate for some canyons but not for others. For example, some marine valleys, trough-shaped and straight-walled and occurring in areas where the earth’s crust is in a state of unrest, probably represent a fault or fracture of the rocky floor. The theory that some of the canyons have been cut by vast sediment flows called turbidity currents has gained support as a result of new concepts of dynamic activity on the floor of the sea. Further detailed study of all types of these extraordinarily fascinating features of the sea floor should not only clarify their own history but add greatly to our understanding of the history of the earth.
13
Somewhat greater depths have more recently been recorded in the Mariana Trench off the island of Guam, the trench into which the bathyscaphe
15
The supposition that the Atlantic Ridge may extend across the Arctic basin has been confirmed in exciting new developments in marine geology. Indeed, it is now suggested by some geologists that the whole mid-Atlantic ridge is part of a continuous range of mountains that runs for 40,000 miles across the bottom of the Atlantic, the Arctic, the Pacific, and the Indian Oceans (see Preface).
As for the exploration of the Arctic basin itself—the charting of details so long unknown and merely guessed at—the revolutionary development that made it possible to substitute fact for theory was the use of American nuclear-powered submarines to pass beneath the ice cover and directly explore the depths of this ocean. In 1957 the
Russian scientists, who have done rather extensive work in marine biology, obtained interesting data which seem to disprove Nansen’s earlier belief that the waters of the central Arctic are extremely poor in both plant and animal life. Data collected from the drifting station “North Pole” indicate that both plant and animal plankton in great variety exist in the region of the Pole. Little-studied organisms develop on the surface of the ice; these contain much fat and tint the ice shades of yellow and red. Diatoms are not found in the surface of the ice but develop (along with other plankton) in the lakes that form on the surface of the ice as it melts. By absorbing a great amount of energy from the sun, the abundant diatom colonies contribute to further melting of the ice cover. The wealth of plankton during the Arctic summer attracts numbers of birds and various mammals.