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The research ship Atlantis set sail on July 16, 1947, in an attempt to uncover the secrets of the Mid-Atlantic Ridge, a mountain system submerged under the Atlantic Ocean that is the longest mountain system in the world. The ten scientists on board, including American geophysicist Maurice Ewing who helped found Columbia University's Lamont-Doherty Geological Observatory, had many questions to answer regarding the depth, age, and topography of the ridge. The expedition resulted in much new information, removing some of the mystery that had surrounded the Mid-Atlantic Ridge.
By Maurice Ewing
Professor of Geology, Columbia University
“We’re over the Ridge!”
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All hands were tense as the word spread through the little research vessel Atlantis, for it meant we had reached our goal. A mile or so beneath our keel stretched the gloom-shrouded peaks, valleys, and ridges of the longest mountain system on earth—the mysterious Mid-Atlantic Ridge, which we had come to explore.
From 300 to 600 miles wide, this mighty submarine mountain range extends nearly 10,000 miles from Iceland almost to the Antarctic Circle. It separates the Atlantic Ocean into eastern and western basins roughly three miles deep.
The range is probably continuous except for a narrow break at the Equator called the Romanche Trench.
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From its base on the ocean floor, at a depth of about three miles, the Ridge rears its rugged crest to an average height of 10,000 feet, or a mile below the surface. A few of its peaks actually emerge as the islands of the Azores, St. Paul Rocks (Rochedos São Paulo), Ascension, Tristan da Cunha, Gough, and Bouvet.
Ever since its discovery 75 years ago, this ocean-covered mountain range of continental size has stirred the imagination of men in many lands. Romanticists inevitably connect the Ridge with the legend of the lost Atlantis, the mythical Atlantic continent which Plato related sank beneath the waves 'in a single day and one fatal night.'
Though our ship was named Atlantis, we had no illusions of solving that age-old mystery story.
In an expedition sponsored jointly by the National Geographic Society, the Woods Hole (Massachusetts) Oceanographic Institution, and Columbia University, we hoped to pierce the veil of hundreds of fathoms of water with our deep-sea camera, probe this dark undersea world with new instruments, map its hidden geography, and bring up rocks and sediments eloquent of its structure and age.
Almost the only earthquakes in the Atlantic Ocean occur along the entire length of the Ridge. The crust of the earth is being deformed and broken on this line of submarine mountains, while the rest of the ocean basin remains undisturbed.
This is perhaps the most definite information we have about the Ridge. It comes from observations made on land thousands of miles away by the world-wide system of seismograph stations, developed during the last 40 years, which continually records and locates all the major earthquakes of the world.
Except for the soundings which have outlined its extent, the Ridge itself is unexplored territory in comparison with mountains on the continents, as is most of the ocean floor. This Jules Verne world under the sea forms one of the last great challenging frontiers of geography.…
As we sailed above the Ridge, I had the strange feeling of being an aviator flying high over an unknown planet tantalizingly hidden from view but outlined on his radar screen. Its peaks and valleys were revealed to us by our deep-sea recording Fathometer. This instrument measures the depth of the water by the time required for the echo of a sound signal—a 'ping' like the high-pitched note of a horn—to return from the ocean floor.
By drawing a continuous profile of the bottom at even the greatest depths on our course, our new and improved deep-water Fathometer gave us a great advantage over past oceanographic expeditions.…
What did these hidden mountains look like? Of what kinds of rocks were they made? What sediments covered them?
To answer such questions, our expedition had set sail in Atlantis on July 16, 1947.
Atlantis is the veteran research vessel of the Woods Hole Oceanographic Institution.…
She is a 146-foot steel-hulled ketch, built in Copenhagen in 1930-31 especially for oceanographic work. Diesel engine and 7,300 square feet of canvas give her a cruising range out of all proportion to her size. Her speed, however, is limited to about ten knots.
Two good-sized laboratories occupy the choicest space on the ship, for science comes first and comfort second. Living space, further restricted by the big winch in the hold and the smaller winch on deck, was even more crowded than that on a submarine. We carried a crew of 18 headed by Capt. A. K. Lane, late of the U. S. Coast Guard, and 10 assorted scientists, some of whom slept on deck in good weather to relieve the congestion below.…
On our second day out we made our first water temperature and salinity measurements, the 3,603d such Hydrographic Station made from Atlantis during her 17 years of oceanographic work.
Each is insignificant in itself, like individual weather-station observations, but all form part of a great picture which gives man better understanding of the waters, winds, and weather of his globe.
Bottles for collecting water samples for chemical analysis were fastened at intervals to a wire and lowered over the side. When the bottles were at the desired depths, a small weight called a messenger was sent sliding down the wire, causing all the bottles to close.
Deep-sea thermometers attached to each bottle were inverted at the same time, breaking the mercury thread in such a way that the water temperature could be read upon return to the surface.
A dozen or so bottles are usually lowered at a time to learn how cold and salty is the water at as many different depths.
At the same time we generally made net tows to learn the concentration, at various depths, of the tiny plants and animals called plankton.
Some of these organisms spend their life in perpetual twilight, going down by day to avoid the light and coming up near the surface at night. We towed our silken nets for them every night at 2 a.m.
So incredibly numerous are such sea creatures that this layer of ocean life actually returns an echo of the sound sent down by the Fathometer. The echo from this so-called 'scattering layer' is sometimes so strong that it causes navigators to think they are sailing over a shoal.
Five days after we left Woods Hole, Atlantis tied up to a mooring in St. George's Harbour, Bermuda, being denied the privilege of docking because of the ton of TNT we carried for scientific use. When this was removed to a storage magazine next day, Atlantis was welcomed into polite society.…
When at last we reached our working area over the Central Highland of the Ridge we were almost exactly in mid-ocean, 1,650 nautical miles east-southeast of New York City and 1,680 miles west of Casablanca, on the Moroccan coast.
We had chosen this area because charts showed the bottom to be about as rough as any on the Ridge and because it lies in the calm of the Horse Latitudes where good working conditions could be expected.
To get acquainted with the mysterious world of mountains beneath these waters, we first made a series of runs back and forth across the Ridge with our Fathometer probing its hidden contours.
Would the Ridge be just a chaos of peaks or would it follow some understandable pattern? Upon the answer to this question much of the success of our expedition would depend.
At first the topography seemed the wildest confusion, but as we studied more and more profiles a definite pattern began to emerge. We found that we were able to predict when certain types of bottom would be encountered. For instance, on the flanks of the Ridge strangely flat terraces were often followed by abrupt upward slopes.
A steep slope, where sediments could not accumulate, seemed the most promising place to get rocks.
For the first attempt I chose the slope of a steep hill which rose more than half a mile from a depth of 1,900 fathoms, or about two miles.
Docks had been cleared for action by throwing overboard the cramping deckload of now empty oil drums, and we unlimbered our 'big gun,' the deep-sea rock dredge.
Groping for rocks in deep water with a metal bag on the end of two or three miles of wire stands out as one of the hardest tasks of the submarine geologist, even when he attempts only to hit bottom at random.
Because of winds and currents affecting the ship, the wire does not go down vertically. Hence, a length of wire considerably greater than the depth of water must be used. How much is needed can only be estimated. There is urgent need, which we hope to meet, for a dredge and trawl cable containing an electrical conductor such as is used in 'logging' oil wells. This would enable the dredge or trawl to send up automatic signals telling how deep it is and when it hits bottom.
If too much wire is put out or if the right amount is put out too quickly, the slack on the bottom may cause kinking, breaking the wire and losing the instrument. If too little wire is put out, the dredge fails to reach bottom and all the time—at least three or four hours—is wasted.
Attempting to hit a target with the dredge greatly increases the difficulties, since the ship must remain stationary despite the currents and winds of the open sea.
The stories the rocks can tell are hidden unless we know the places and elevations from which they come. Accordingly, I decided to take what military men call a calculated risk and try 'pinpoint' dredging.
Well I knew that an error or wedging of the dredge in some rocky crevice below could mean loss of the equipment and a serious setback to the expedition.…
Lowering or raising the deep-sea instruments is a noisy as well as exciting process. The big winch makes a mighty rumbling, and the heavy cable snaps into hollows on the drum with loud reports that seem to shake the whole ship.
Although a gauge indicates the strain on the cable, the dredge's weight is so slight compared with that of two or three miles of wire that the gauge gives no clue as to when the dredge strikes bottom. The curving wire, miles long, strung out astern acts like a spring, and the jerks when the dredge hits bottom cannot be transmitted up it. One can only make an 'educated guess' in the light of Fathometer readings and previous experience.
In this case, when we raised the dredge it showed no sign of having touched bottom. All it contained was a doubtless surprised resident of the sea—one large red tunicate with an array of short rubbery tentacles. This seemed a rather slim reward for 4 hours and 17 minutes of effort!
Swallowing our disappointment, we went through the whole process again, except that this time we lowered a coring tube instead of the dredge, first on the top of the hill and then on the plain from which it rose. Both times the corer brought up only the soft cream-colored globigerina ooze.
A good breeze now came up, and for the next two days we saved precious oil by traveling under sail as our Fathometer constantly revealed new mountain profiles.…
As we crossed and recrossed the Ridge, our Fathometer outlined many a spectacular mountain and valley, but on August 7 it outdid itself. It showed that below us lay a sharp-bottomed valley about ten miles wide and an average of more than 2,350 fathoms deep, immediately followed by a mountain whose crest rose to 740 fathoms.
Thus the mountain rises some 9,700 feet from the trench at its foot to its crown, or higher than the mighty Matterhorn above Zermatt, Switzerland. Its slope had a gradient of roughly 1 in 6. Realizing that this was an extraordinary feature, we took a position above the valley at the foot of the slope where the depth was 2,600 fathoms, or almost three miles, and went to work.
First we sent down the Hough coring apparatus, but a heavy swell from a storm to the northward made it hard to judge when the corer hit bottom, and it came up empty. Later we sent down the heavier Stetson corer, but it, too, returned to the surface without having hit.…
At 2 a.m. we sent down the Stetson corer again, for we were determined to get results from this interesting spot on which we had already spent seven hours with no results.
This time the corer struck bottom. Though badly damaged from striking rock, it had not come back empty.
Under a core of only a few inches of sediment was a freshly broken bit of rock about one inch in diameter. This rock was plainly igneous—crystallized from a molten condition, like granite and many other familiar rocks. Geologists call this rock olivine gabbro.
The core above the rock was not the usual deep-sea sediment, but material resulting from chemical and mechanical breakdown of the gabbroic rocks.
The outside of the tube was deeply scarred along its lower four inches, and it had required great force to pull it clear of bottom. This probably means it had entered a crevice between boulders.
Now, having probed the deep abyss, we wanted a core from the top of the mountain which towered so high above it. Moving to a point above the summit, we sent down the Hough corer in some 800 fathoms. It came up empty and completely wrecked, apparently by striking rock. We decided to try the rock dredge.
Three hours of effort were rewarded by complete success. The dredge brought up from 850 fathoms about 100 pounds of greenish rocks which likewise had crystallized from a molten state. Prof. S. J. Shand, of Columbia University, has identified the rocks from various hauls as crushed anorthosite gabbro, with also much serpentine and basalt.
For comparison with the mountaintop rocks we next tried for some from the bottom of the abyss, sending down the dredge this time in 2,340 fathoms. It brought up 400 pounds of basalt rocks, many of which show glassy surface and the typical 'pillow' structure resulting from quick cooling of lava under water. This is one of the deepest hauls of such a size ever made.
Having been continuously on station for 24 hours, we decided to run north a day and then back to the trench, studying topography and giving our men a chance to rest. Before midnight, however, we made a TNT seismic test, which showed no sedimentary layer thick enough to be measured by that method.
The indicated absence of thick deposits here checked with results of our coring and dredging and gave us considerable satisfaction. Apparently the steepest slopes along the rugged spine of the Ridge were almost if not entirely free of sediment. Their rocks lay virtually bare for us to hit, fish up, and study.
Before our expedition, this had not been known, and the possibility had existed that the whole Ridge might be so deeply 'snowed under' with sediment that its rocks could not be reached and raised to tell their story.
The more we explored this mountain and trench the more their magnitude impressed us. The mountain measures 50 miles long, 10 miles from side to side, and nearly 2 miles high. Thus it is roughly comparable to the San Bernardino Mountains in southern California.
The combination of steep slope and deep ditch suggests that the feature may be a fault scarp and rift valley—a zone of slippage between earth masses—and that earthquakes may have occurred there in historic times.
The bottom on August 9 showed several flat stretches which we found typical of the flanks of the Ridge. These terraces were some 15 miles across and 2,000 fathoms (about 2 ¼ miles) deep, with higher rugged ground between rising to some 1,300 fathoms, or about a mile and a half.
In midafternoon we passed over a little hill rising to 1,400 fathoms and falling abruptly to 2,100 fathoms. Its steep slope was so tempting that, despite our exhaustion, we decided to dredge it.
During the lowering some of the men had a swim. The water was very clear, but cooler than that of Bermuda—76° compared to 84. We let only one man in at a time, so he could get out quickly if sharks appeared.
These ugly customers were frequently in evidence. Shortly after the last swimmer emerged one of the men lost a sneaker overboard. It was promptly rushed at by a large hammerhead shark, just as a man was getting ready to dive in after it.
As Henry McKean observed in his diary: 'The dredge made a great fuss coming up, bumping along the bottom and swinging the ship completely around several times. The strain on the wire went up by jerks as great as 1,000 pounds.'
The load consisted of several hundred pounds of rock, all heavily coated with the black manganese deposit frequently found in the deep sea, in striking contrast with the rocks of previous hauls, which had been perfectly clean.
A rather wild idea had led us to devote four hours to this particular rock dredging. Our hypothesis was that the long, level terraces, with sediments ranging up to 3,000 feet in depth, were submerged shore lines. If so, the steep cliffs rising from them should have boulders at their bases as do wave-cut cliffs on our shore lines today.
It is, of course, extremely radical speculation to identify these level stretches more than two miles below the sea surface as former beaches. Such a theory would require the obvious but almost incredible conclusion that the land here has subsided two miles or else the sea has risen by that amount.
Much work will have to be done before this startling theory can be proved or disproved. In any case, we were encouraged to find that at the bases of cliffs above such terraces rocks could readily be obtained.…
Our fuel was now running low, and instead of returning to Bermuda for more we decided to head for the Azores, examining the Ridge all the way.…
It seemed strange, after days of studying the Ridge at depths of one or two miles and more, to stand on one of its highest peaks, the island of São Miguel, largest of the Azores. Its volcanic nature was plain, and its rocks were akin to those our dredge had raised from its sister hills deep in the sea.
Diesel oil for our thirsty engines was unavailable. When we sailed on August 20, Atlantis was burning bunker oil and smoking like a coalburner. Everything and everybody soon became grimy. Our new white sails began to look like waste from the engine room.
Poor oil and head winds slowed us down and limited our time on the Ridge, but we wanted another chance or two at the big mountain and valley which stood out as the expedition's chief discoveries. As we steamed above them, the Fathometer again drew their huge and familiar features.
Rock dredging on the north slope of the gorge in 1,700 fathoms (about 2 miles) produced a wonderful haul—some 400 pounds of rock and clay. The clay was not a typical ocean-bottom deposit but contained many angular fragments, probably pulverized material resulting from the slipping of great rock masses along a fault, or crack, in the earth's crust.
These fragments tended to confirm our theory that this was a fault area, a center of earthquakes. The rocks—basalt, serpentine, and diabase—were all igneous and metamorphic (altered by heat, pressure, and water).
Following the deep gorge westward, we dredged again, this time in 2,300 fathoms (about 2⅔ miles). The haul was mostly serpentine, but it included a strange specimen, a mass of tremolite asbestos with strands six inches long.
This kind of asbestos is of different composition from the asbestos of commerce, which is mostly a fibrous form of serpentine. The fibers of tremolite asbestos are usually too short and weak to permit spinning or weaving for the manufacture of fabrics and packing. It is used, however, in molds or blocks, chiefly in the building trade. Such rock is generally considered typical of continents and not of ocean basins.
Our allotted time on the Ridge was now gone, and on August 30 we headed for Bermuda and home. In general, luck had been with us. Every 'tool' we had tried had worked.
In a total of 25 days on the Ridge we had cruised over approximately 10,000 square miles of these mid-Atlantic mountains, an area roughly equivalent to that of the State of Maryland.
We had found four distinct types of submarine geography in the part of the Ridge explored. These may now be summarized for the first time.
On the western side of the Ridge stretches the great plain of the American Basin. It is very level and 2,900 fathoms (about 3 miles) deep. Here no sedimentary layer could be detected by our bomb-and-echo tests.
The American Approach to the Ridge is rough, with gradual change in depth from 2,900 to 2,200 fathoms. Bomb tests showed a thick sediment layer—1,000 to 2,000 feet—in about three-fourths of the cases: none in the others.
On the American (western) Flanks of the Ridge lie level stretches, 2 to 20 miles broad, like terraces or beaches. We found these at six different levels, from 2,200 to 1,800 fathoms. Our bomb tests over such stretches always showed thick sediments, ranging up to 3,000 feet. Rough higher ground often separates successive terraces, and occasional isolated peaks punctuate this part of the Ridge.
The Central Highland of the Ridge ranges in depth from 1,800 to 700 fathoms (about two miles to four-fifths of a mile). Its topography is always rugged with never a flat stretch. Here, as on the great plain, no sedimentary layer was detected by bomb tests.
Most encouraging for the future was the fact that the Ridge had proved vulnerable to attack; it could be made to yield information. The deep-water rock dredge, for instance, had brought up about a ton of rocks from the Ridge, which previously had yielded no more than a pebble or two at the end of a sounding lead.
These telltale rocks told a story of formation of the Ridge by great heat and pressure. Once-molten rocks from the interior of the earth were seen to be overlain in some places by limestone formed from dead sea creatures.…
Our cores from the Mid-Atlantic Ridge, much farther from land, indicate that in the past, probably during the Ice Age, the water there was less warm than at present, but only slightly so.…
As we landed at last at Woods Hole, after 60 days on the Atlantic or its island mountain peaks, we were already talking of a new expedition to probe more of the secrets hidden in the dark world beneath its waters.
Source: Ewing, Maurice. “Exploring the Mid-Atlantic Ridge.” National Geographic, September 1948.
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East Pacific Rise; Mid-Atlantic Ridge; Plate Tectonics; Geology; Ocean and Oceanography; Eocene Epoch; Atlantic Ocean
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