The Geography of the Pacific
THE LARGEST AND DEEPEST OF ALL OCEANS
By Dr. E. S. C. SMITH, Professor of Geology, Union College, Schenectady, N. Y.
Delivered on January 6, 1942, during the General Electric Science Forum program from Schenectady, N. Y., over WGY
Vital Speeches of the Day, Vol. VII, pp. 282-284.
IN November of the year fifteen hundred and twenty, Ferdinand Magellan, the Portuguese navigator sailed out of the storm tossed channels of the straits that still bear his name into the quieter waters of a great new ocean, how great, Magellan himself had no conception. He named this peaceful sea the Pacific Ocean, grim misnomer for the moment. Although he did not live to complete his voyage, we pay homage to Ferdinand Magellan as the leader of the small but courageous band that first circumnavigated the globe, thereby proving incontestably that the earth is round.
Magellan was not, however, the first white man to set eyes on the waters of the Pacific. Seven years earlier, in fifteen thirteen, Vasco Balboa crossed the Isthmus of Panama and took possession of what he called the Gulf of San Miguel for the Spanish Crown, little realizing that the greatest of oceans spread itself before him, and that the long sought Indies were still ten thousand miles away.
It remained for Cook, Tasman and all that host of adventurous spirits to make known the truly vast expanses of the Pacific Ocean. This vastness can be well shown on anordinary geographic globe. If we slowly rotate such a globe on its axis we then see the several continents and seas in their proper relationship one to another. Here we see Asia and the Indian Ocean, then Europe and Africa; there is the Atlantic, and now the Americas are in sight. Turn the globe more slowly and look, here is the Pacific Ocean covering nearly a whole hemisphere as presented to our eyes. With the Hawaiian Islands in the center of your view, look at the earth from somewhat below the equator, say about twenty degrees, and you will see at once that one is quite justified in speaking of the earth as made up of a water hemisphere and a land hemisphere.
Rather constricted at the north between Asia and Alaska where it is called the Behring Sea, the Pacific widens out to eight thousand miles in breadth between Panama and Manila, and to about the same distance between Chile and Australia. Southward it merges into the Antarctic Ocean at about 60 degrees south latitude. Thus outlined the Pacific Ocean occupies some sixty-three million square miles. All of the land areas of the world, fifty-five million squaremiles, could be put into the Pacific and have millions of square miles of water surface left over. As calculated, the Pacific embraces nearly one-third of the surface area of the entire earth.
A few distances may help us to gain an idea of the grand scale upon which the Pacific is constructed. From San Francisco to Honolulu is just over two thousand miles, about the same distance as from New England to the Rocky Mountains. From Panama to the Hawaiian Islands it is almost five thousand miles, forty-seven hundred to be exact. From Honolulu to Tokio is three thousand three hundred and seventy-nine miles, and from Honolulu to the Philippines is four thousand eight hundred miles. From the Hawaiians to New Zealand is seventy-five hundred miles and from Panama to Sydney, Australia is seventy-seven hundred miles. These latter figures suggest the magnificent distances of the South Pacific. If one recalls that from New York City to Los Angeles, California is about three thousand miles, one then has a mental measuring stick which will give some notion as to the dimensions of the Pacific Ocean region.
If we now examine the eastern and the western portions of the Pacific area a number of striking features become apparent, for example, the almost unbroken west coast line of the Americas has no counterpart on the Asiatic side, instead, archipelagos of countless islands stretch from the peninsula of Kamchatka on the north of New Zealand in the far south. Passing eastward, the islands become sparser, and as we near the shores of the Americas the few islands encountered, like the Galapagos and Juan Fernandez groups become notable.
In discussing the islands of the Pacific we find that we must at once separate them into two categories, first, those islands which may be said to be continental in character, and second, those which are true oceanic islands. The continental islands are those whose rock formations, structures and physiographic features show them to be genetically related to the nearby continents, as for example, the Japanese Islands, the Philippines, the New Hebrides and New Zealand. The oceanic islands on the other hand are in no way related to the continents, but are of volcanic origin, having been built up from the sea floor as a result of numberless submarine eruptions. The Hawaiian Islands are typical of this sort of island, rising, as they do, some thirty thousand feet above the floor of the Pacific.
Many of the islands known to be of volcanic origin are, in the warmer seas, ringed or fringed with coral reefs. Some are so thickly covered with growths of coraline limestone that no evidence of the volcano remains, yet by analogy those coral islands must have been built up on the stumps of deeply eroded and long since extinct volcanoes. Guam and Midway Islands are examples.
The lavas of these volcanic islands have been carefully studied at many different localities, and all show a remarkable similarity, suggestive of a common origin far beneath the ocean's floor. They are all dark colored, heavy, dense lavas, rich in iron and magnesium, the type which the geologist calls basalt. This fact alone suggests that the material under the floor of the Pacific is basalt, but a further and interesting line of evidence points also in this same direction. Where no volcanic islands rise to the surface, over some of the deepest parts of the Pacific it has been possible to test the pull of gravity, and hence, to determine the density of the sub-oceanic rocks. Everywhere the story is the same. The material below the ocean floor has a density of about three times that of water; in other words, almost exactly the average density of the known basalts the world over. The average density of the continental rocks, which are largely similar to that familiar rock granite, is about two and seven-tenths times that of water, and to date nota single fragment of rock of the continental type has been yielded to the geologist's hammer at the surface or to the dredge in the depths. This leads to possible answers to the important questions, what holds the continents up to elevations of thousands of feet above the sea? Why do not the mountain ranges slump down to a low general level? It has been calculated that at a depth of about forty miles within the earth's crust the weight of the sub-oceanic rocks and the millions of cubic miles of sea water reposing upon them is nearly equal to the weight of the adjacent continents plus the sub-continental rocks. Thus the continents seem to be made up of higher, lighter blocks of earth stuff, balanced by the lower and heavier sub-oceanic blocks. The term for this condition of approximate balance is the word isostasy, derived from the Greek which means "equal standing", and the principle of isostasy, that every part of the earth's crust tends to be in equilibrium with every other part is one of the accepted tenets of modern geological doctrine.
But to return to the Pacific: Not only is it the largest of the oceans, but also it is by far the deepest. The depth and extent of these "deeps" as the depressions in the ocean floor are called, was made known by the first great deep sea exploring expedition that was ever undertaken. I refer to the famous cruise of the H.M.S. Challenger from 1872 to 1876, which covered 69,000 miles and in 1875 discovered and charted a depth of 26,850 feet off the Marianna Islands in 145 degrees east longitude, 18 degrees north latitude. Shortly before, however, in 1874, the U.S.S. Tuscarora had discovered a deep just off the Japanese Islands reaching a maximum depth of 27,972 feet. For a long time this remained the greatest recorded deep. The deepest known depression of today is in the Mindanao Deep, just west of the island of that name in the Philippines. Here a depth of 34,218 feet has been recorded, almost exactly six and a half miles. Other deeps of no mean proportions are: the Tonga Deep off the Tonga Islands in 174 degrees west longitude and 20 degrees south latitude, which is 30,132 feet deep, and the Kermadec Deep off the Kermadec Islands in 176 degrees west longitude and 30 degrees south latitude, which is 30,920 feet deep.
While many more soundings are needed before we can get a detailed picture of the configuration of the floor of the Pacific Ocean as a whole, yet with the data at hand we conclude that it is by no means a monotonous plain of mud. There have been discovered and charted, numerous ridges, technically called rises or swells, some of which actually come to the surface and express themselves as islands, and there are trough-like depressions, many of which contain the deeper of the great deeps previously mentioned. The majority of the important deeps lie in the West or North-west Pacific. In the North-eastern Pacific, that is toward North America, with but one exception, the soundings average about 15,000 feet. This exception is the series of deeps just south of the Aleutian Islands where depths of over 20,000 feet have been found. This series of deeps is important because it sheds some light perhaps, on the origin of all deeps. These Aleutian deeps can be traced eastward along great earth fractures, called faults, to Yakutat Bay, Alaska, site of a well known and recently active earth fracture. There seems, then, reason to believe that these deeps and perhaps all deeps may be broken and subsided earth blocks on a grand scale. Additional interest attaches to the fact that almost invariably the deep on the east or southeast is complemented by an up-thrown block on the west, as in the Tonga, Kermadec, Marianna, Philippine and Japanese Islands as well as many others.
There are no known deeps off the west coast of the Americas, the continents plunge directly into the abyssal depths of the Pacific with scarcely a trace of a continentalshelf so evident on the Atlantic side of both continents. Two shallow areas or rises are to be found, however, one stretches from the Isthmus of Panama southwestward to a point opposite the Coast of Ecuador where its visible summits form the Galapagos Islands. It was once thought to be connected with the rise on which the famous Easter Island is located, but this has been shown not to be the case. The other rise is off the coast of Chile, south of Valparaiso, called the Chilean rise. It is 1,500 miles long, a thousand miles wide, and from it rises the Juan Fernandez Islands.
Finally we have just time to touch on the grandest of all Pacific problems. How came this great basin to occur in the first place, this depression whose area is one-third the surface of the whole sphere? The French mathematician Poincaré and the English scientist George H. Darwin independently suggested on mathematical grounds that in thevery earliest stages of the earth's existence, when it was still at least partly in a liquid condition, a great mass was torn or thrown off into space which became our satellite, the moon. Thus the Pacific Ocean basin becomes the birth scar of the moon. This hypothesis is neither susceptible of proof, nor is it to some, particularly convincing, but it is true that the surface of the moon has just about the reflecting power, albedo as the astronomers call it, of granite, the very material which is missing from the surface of the earth in the Pacific region.
Here, then, are only too sketchily outlined, some of the problems of the Pacific. These together with questions of currents, tides and bottom deposits, belong to a new, young science, Oceanography, which is already rapidly taking its place side by side with the other fields that make up Earth Science as a whole.