The Long Snowfall
A deep and tremulous Earth-Poetry.
EVERY PART OF EARTH or air or sea has an atmosphere peculiarly its own, a quality or characteristic that sets it apart from all others. When I think of the floor of the deep sea, the single, overwhelming fact that possesses my imagination is the accumulation of sediments. I see always the steady, unremitting, downward drift of materials from above, flake upon flake, layer upon layer—a drift that has continued for hundreds of millions of years, that will go on as long as there are seas and continents.
For the sediments are the materials of the most stupendous ‘snowfall’ the earth has ever seen. It began when the first rains fell on the barren rocks and set in motion the forces of erosion. It was accelerated when living creatures developed in the surface waters and the discarded little shells of lime or silica that had encased them in life began to drift downward to the bottom. Silently, endlessly, with the deliberation of earth processes that can afford to be slow because they have so much time for completion, the accumulation of the sediments has proceeded. So little in a year, or in a human lifetime, but so enormous an amount in the life of earth and sea.
The rains, the eroding away of the earth, the rush of sediment-laden waters have continued, with varying pulse and tempo, throughout all of geologic time. In addition to the silt load of every river that finds its way to the sea, there are other materials that compose the sediments. Volcanic dust, blown perhaps half way around the earth in the upper atmosphere, comes eventually to rest on the ocean, drifts in the currents, becomes waterlogged, and sinks. Sands from coastal deserts are carried seaward on offshore winds, fall to the sea, and sink. Gravel, pebbles, small boulders, and shells are carried by icebergs and drift ice, to be released to the water when the ice melts. Fragments of iron, nickel, and other meteoric debris that enter the earth’s atmosphere over the sea—these, too, become flakes of the great snowfall. But most widely distributed of all are the billions upon billions of tiny shells and skeletons, the limy or silicious remains of all the minute creatures that once lived in the upper waters.
The sediments are a sort of epic poem of the earth. When we are wise enough, perhaps we can read in them all of past history. For all is written here. In the nature of the materials that compose them and in the arrangement of their successive layers the sediments reflect all that has happened in the waters above them and on the surrounding lands. The dramatic and the catastrophic in earth history have left their trace in the sediments—the outpourings of volcanoes, the advance and retreat of the ice, the searing aridity of desert lands, the sweeping destruction of floods.
The book of the sediments has been opened only within the lifetime of the present generation of scientists, with the most exciting progress in collecting and deciphering samples made since 1945. Early oceanographers could scrape up surface layers of sediment from the sea bottom with dredges. But what was needed was an instrument, operated on the principle of an apple corer, that could be driven vertically into the bottom to remove a long sample or ‘core’ in which the order of the different layers was undisturbed. Such an instrument was invented by Dr. C. S. Piggot in 1935, and with the aid of this ‘gun’ he obtained a series of cores across the deep Atlantic from Newfoundland to Ireland. These cores averaged about 10 feet long. A piston core sampler, developed by the Swedish oceanographer Kullenberg about 10 years later, now takes undisturbed cores 70 feet long. The rate of sedimentation in the different parts of the ocean is not definitely known, but it is very slow; certainly such a sample represents millions of years of geologic history.
Another ingenious method for studying the sediments has been used by Professor W. Maurice Ewing of Columbia University and the Woods Hole Oceanographic Institution. Professor Ewing found that he could measure the thickness of the carpeting layer of sediments that overlies the rock of the ocean floor by exploding depth charges and recording their echoes; one echo is received from the top of the sediment layer (the apparent bottom of the sea), another from the ‘bottom below the bottom’ or the true rock floor. The carrying and use of explosives at sea is hazardous and cannot be attempted by all vessels, but this method was used by the Swedish Albatross as well as by the Atlantis in its exploration of the Atlantic Ridge. Ewing on the Atlantis also used a seismic refraction technique by which sound waves are made to travel horizontally through the rock layers of the ocean floor, providing information about the nature of the rock.
Before these techniques were developed, we could only guess at the thickness of the sediment blanket over the floor of the sea. We might have expected the amount to be vast, if we thought back through the ages of gentle, unending fall—one sand grain at a time, one fragile shell after another, here a shark’s tooth, there a meteorite fragment—but the whole continuing persistently, relentlessly, endlessly. It is, of course, a process similar to that which has built up the layers of rock that help to make our mountains, for they, too, were once soft sediments under the shallow seas that have overflowed the continents from time to time. The sediments eventually became consolidated and cemented and, as the seas retreated again, gave the continents their thick, covering layers of sedimentary rocks-—layers which we can see uplifted, tilted, compressed, and broken by the vast earth movements. And we know that in places the sedimentary rocks are many thousands of feet thick. Yet most people felt a shock of surprise and wonder when Hans Pettersson, leader of the Swedish Deep Sea Expedition, announced that the Albatross measurements taken in the open Atlantic basin showed sediment layers as much as 12,000 feet thick.
If more than two miles of sediments have been deposited on the floor of the Atlantic, an interesting question arises: has the rocky floor sagged a corresponding distance under the terrific weight of the sediments? Geologists hold conflicting opinions. The recently discovered Pacific sea mounts may offer one piece of evidence that it has. If they are, as their discoverer called them, ‘drowned ancient islands,’ then they may have reached their present stand a mile or so below sea level through the sinking of the ocean floor. Hess believed the islands had been formed so long ago that coral animals had not yet evolved; otherwise the corals would presumably have settled on the flat, planed surfaces of the sea mounts and built them up as fast as their bases sank. In any event, it is hard to see how they could have been worn down so far below ‘wave base’ unless the crust of the earth sagged under its load.
One thing seems probable—the sediments have been unevenly distributed both in place and time. In contrast to the 12,000-foot thickness found in parts of the Atlantic, the Swedish oceanographers never found sediments thicker than 1000 feet in the Pacific or in the Indian Ocean. Perhaps a deep layer of lava, from ancient submarine eruptions on a stupendous scale, underlies the upper layers of the sediments in these places and intercepts the sound waves.
Interesting variations in the thickness of the sediment layer of the Atlantic Ridge and the approaches to the Ridge from the American side were reported by Ewing. As the bottom contours became less even and began to slope up into the foothills of the Ridge, the sediments thickened, as though piling up into mammoth drifts 1000 to 2000 feet deep against the slopes of the hills. Farther up in the mountains of the Ridge, where there are many level terraces from a few to a score of miles wide, the sediments were even deeper, measuring up to 3000 feet. But along the backbone of the Ridge, on the steep slopes and peaks and pinnacles, the bare rock emerged, swept clean of sediments.[16]
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Now that the sediments have been measured over much greater areas of the ocean floor, the reaction of oceanographers is one of considerable amazement— but their surprise concerns the fact that on the whole the mantle of sediments is so much thinner than related facts would lead them to expect. Over vast areas of the Pacific the average thickness of the sediments (unconsolidated sediments plus sedimentary rock) is only about a quarter of a mile. It is little thicker over much of the Atlantic. (These are average figures; some much deeper deposits of course exist.) In some areas there has been almost no sedimentation. A few years ago several oceanographers obtained photographs of manganese nodules lying on the floor of the Atlantic at great depths and of others on the Easter Island Ridge of the southeast Pacific. Sharks’ teeth dating from the Tertiary, hence possibly as much as 70 million years old, sometimes form the nuclei of these nodules. Certainly their growth, by deposit of successive layers around the nuclei, must be very slow. Hans Pettersson has estimated a growth of about 1 mm. per thousand years. Yet during the period these nodules have lain on the ocean floor, sediments deep enough to cover them have not been accumulated.
Some idea of the rate of sedimentation during post-glacial time has been gained by observation of the rate of radioactive decay of some of the components of the sediments. If this sedimentation rate had prevailed during the supposed life of the oceans, the average thickness of the sediments would be enormously greater than it now appears to be. Did much of the deposited sediments dissolve? Were most of the present land masses submerged for far greater periods than we now assume, with consequently long periods of slight erosion? These and other explanations of the mystery of the sediments have been suggested, but none seems wholly satisfying. Possibly the dramatic project of boring holes in the floor of the ocean down to the Mohorovicic discontinuity (Project Mohole; see Preface) will provide the explanation that is now lacking.