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 cluster 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.
One of the most fascinating fields for speculation is the age of the submarine mountains compared with that of past and present mountains of the continents. Looking back over the past ages of geologic time (see chart in The Gray Beginnings), we realize that mountains have been thrust up on the continents, to the accompaniment of volcanic outpourings and violent tremblings of the earth, only to crumble and wear away under the attacks of rain and frost and flood. What of the sea’s mountains? Were they formed in the same way and do they, too, begin to die as soon as they are born?
There are indications that the earth’s crust is no more stable under sea than on land. Quite a fair proportion of the world’s earthquakes are traced through seismographs to sources under the oceans, and, as we shall see later, there are probably as many active volcanoes under water as on land. Apparently the Atlantic Ridge arose along a line of crustal shifting and rearrangement; although its volcanic fires seem to be largely quiescent, it is at present the site of most of the earthquakes in the Atlantic area. Almost the whole continental rim of the Pacific basin is aquiver with earthquakes and fiery with volcanoes, some frequently active, some extinct, some merely sleeping a centuries-long sleep between periods of explosive violence. From the high mountains that form an almost continuous border around the shores of the Pacific, the contours of the land slope abruptly down to very deep water. The deep trenches that lie off the coast of South America, from Alaska along the Aleutian Islands and across to Japan, and southward off Japan and the Philippines give the impression of a landscape in process of formation, of a zone of earth subject to great strains.