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Moho and Mohole
Harry Hess wanted to drill down to the Mohol the base of the earth's crust. The venture failed, but technological innovations developed for the Mohole Project made the Deep Sea Drilling Project possible.
Impressions of a Scientist
As they say in America, if you dig a deep enough hole, you will reach China. Nobody ever seriously prepared to do so, but Harry Hess once did persuade the Congress of the United States to invest millions of dollars in a hole to the mysterious realm of Moho, ten kilometers below the ocean.
I first came across Harry Hess in the Prudential Insurance Building in Houston, Texas. That was February 1954. I was fresh out of graduate school, and had just pulled down my first job, with the Exploration and Production Research Laboratory of the Shell Oil Company. Hess was already a well-known geologist, then chairing the Geology Department of Princeton University. He came to Houston on a fund-raising tour. For his talk to the Princeton Alumni Association, he chose to speak on guyots—the newly discovered sunken flat-top mountains of the Pacific.
Harry Hess had been the young commander of the troop-transport M/S Cape Johnson of the U.S. Pacific Fleet during the Second World War. While ferrying troops for landings in the Marianas and the Philippines and on Iwo Jima across the Mid-Pacific, the echo-sounder of his vessel registered a seafloor topography that was far from monotonous. Undersea mountains appeared one after another on the echogram; they rose thousands of meters above the deep ocean floor (Figure 1.1). They all had steep sides and a flat top, looking somewhat like mesas on the Colorado Plateau of the western United States. Crisscrossing over the mountains revealed a circular outline. This morphology indicated to geologist-commander Hess that those underwater features were sunken volcanic islands with their tops chopped off. He called them guyots, in honor of Arnold Guyot, Princeton's first geology professor, who also lent his name to the building that houses the Princeton geology department. When the war ended, Hess could report the discovery of 160 of the drowned islands in the Pacific basin.
Oceanographic vessels returned to Hess's old hunting ground and were able to find more guyots. They also used a tool for dredging, a scissor-like hook at the end of a long steel cable, to grab and twist off pieces of rock from the steep sides of the sunken islands. They turned out to be mostly volcanic rocks, as Hess had expected.
Submarine volcanoes are not uncommon on the present-day seafloor; they are called seamounts. What made the guyots unusual were their flat tops. Hess assumed these were caused by erosion—the guyots were once volcanic islands, but their conical tops had been planed off by the pounding of ocean waves. The islands seemed to have stood firm while waves were doing their job of cutting down the parts above sea level. Then, for some reason, subsidence started, and the flat-topped islands sank three or four thousand meters to become guyots.
I was always quick to jump to conclusions. I had learned in school that volcanoes in the oceans should sink under their own weight, but the rate of subsidence is finite. When seamounts (underwater volcanoes) are active, molten liquids, or lavas, pouring out of the interior of the earth pile up so fast that subsidence cannot keep up with the build-up. The seamounts eventually rise above sea level, forming volcanic islands. However, volcanoes become inactive sooner or later. Then ever-present gravity does its work and pulls the volcanic islands back down to the abyss.
That a volcanic island should sink under its own weight in an ocean had been, in fact, a common assumption in geology. Charles Darwin invoked the idea to explain the origin of coral atolls. Therefore, I was a little surprised that Hess should be disturbed by the presence of guyots in the deep sea. Too shy to bring up the point during the discussions after the talk, I nevertheless wrote Hess a letter.
I received a prompt reply from Professor Hess, typed single-spaced on three sheets of stationery, apparently by himself. He apologized that he could not find time to give me a more detailed explanation; the usual activities of a newly begun semester had taken a heavy toll on his working hours. He was also apologetic about his less than professional skill in handling a typewriter. Just the same, he patiently explained to the slightly arrogant young man why the simple-minded scheme of subsidence under load cannot explain all the problems in connection with the origin of guyots.
The facet of the problem that bothered Hess most was the flat top. Waves did their work slowly. Why did the islands seem to have paused for such a long time, waiting until the waves had removed mountains and cut the volcanic islands down to sea level? When and why did the flat-topped mountains start their irrevocable journeys down to the abyss? After discussing the pros and cons of the various explanations at some length, Hess reiterated his puzzlement and questioned the possibility of a simple answer.
Years later both of us learned more about the oceans. Guyots did indeed sink under gravity. Hess's difficulty could be traced to his assumption that the flat top was due to erosion. When we look at the islands of the Pacific today, some with volcanoes rising hundreds of meters above sea level, it seems a formidable task indeed to cut those volcanic mountains down. However, flat-topped islands—such as Saipan, Tinian, and others that were used as air bases for U.S. B-24 bombers during the war against the Japanese—are not uncommon. They are flat not because waves pounded the mountains down, but because the topography has been evened out by the deposition of flat-lying sediments. Elsewhere, flat-topped islands may owe their lack of relief to horizontally accumulated lava flows or ash beds. If we accept Darwin's theory that atolls build on a sinking volcanic foundation, the sediments trapped in the lagoons of an atoll should gradually cover the tip of the volcano to make a flat top. Those atolls on which coral grows too slowly eventually sink to become guyots. From this point of view, guyots are simply coral atolls that died young. In the tropical Pacific, where coral growth could always keep pace with the sinking of the foundations, sunken volcanoes are now crowned by living reef corals, as Charles Darwin once theorized.
I shall come back to the story of guyots in a later chapter. What impressed me back in 1954 was not what Hess wrote, but that he did write. I was not convinced by his arguments then, but I did treasure the autograph from the famous man.
My second encounter with Harry Hess took place in Washington, D.C., a few years later. I was appearing before a distinguished national audience during the annual meeting of the American Geophysical Union to present my first professional lecture. It was scheduled as the first talk of the first session—a rather ticklish spot. Furthermore, I had sent in the theme in a moment of lightheartedness, and I came to realize only belatedly that I had chosen a poor subject for a start. Normally a young man beginning his professional career would choose to report on a fact-finding research project, in which he could make a good impression on his peers by demonstrating his intelligence and diligence. I had instead selected a theoretical subject—the origin of geosynclines.
The concept geosyncline was invented by James Hall, a nineteenth-century geologist, to designate ancient sites of sediment accumulation that seemed destined to become mountain chains. Hall was the State Paleontologist of New York, and he studied the Paleozoic rocks in the Appalachian Mountains and in the plateau country to the west of the mountains. The mountain rocks, which are deformed, are quite similar to those on the plateau, which are flat. However, Hall found that a rock formation of any given age is several times thicker in the mountains than on the plateau (Figure 1.2); strata seemed to have been crumpled where they are much thicker. Before the sedimentary formations were crumpled during the process of building mountains, they had been laid flat. The bottom of a sedimentary pile should thus be most depressed where the pile is thickest. Such a warped surface had been called a syncline by geologists, and James Hall added geo- to the expression to emphasize its imposing dimensions. European scientists noted, however, that crumpled strata in the mountains were not always thicker than flat strata in plateau country, but the former seemed to have been deposited in deeper water. They too used the expression geosyncline to designate a site that was eventually to become a mountain chain.
The term geosyncline was to become a catchword, and appeared in everyone's theories even though no one seemed to know what a geosyncline was. There were no modern analogues. Instead, people speculated, somewhat idly, on why geosynclines sank, permitting the accumulation of thick piles of sediments. In the short discourse I planned to present orally, I would try to demythify the concept. I had some simple arguments to show that geosynclines are simply depressions on the surface of the earth, such as ocean basins, continental margins, and so on. After sediments were accumulated, the seafloor would subside under the added load, just as guyots sink under the weight of added volcanic piles.
The idea was not new, although I did give it a new twist with some recent data. The idea was also not all that bad; what I said then is still valid today, although I did miss a point or two (see chapter 6). However, it was presumptuous for a young man to tell his seniors the obvious; I grew increasingly nervous as the time of the meeting approached.
At the appointed hour, I went to the General Services Administration Building in Washington, D.C., where the meeting was being held. I was early, half an hour early. Gradually people filed in, and many venerated scientists of the older generation took their seats in the front rows. Hess was to open the meeting and chair the first session. Eight o'clock struck, the hour I was to give my talk, but he was nowhere in sight. As the time passed, 8:05 ... 8:10 ... 8:15 ... 8:20, I sat there and became more and more edgy, gradually losing my nerve. Finally, at about half past eight, Hess ran in and climbed up to the rostrum. Wiping sweat off his brow, he murmured an apology—he had looked for the wrong GSA (Geological Society of America) Building, and nobody seemed to know where it was. (It was in New York City!) Catching his breath, he tried—without remarkable success—to pronounce my name, as he introduced the first speaker. By then I was completely overcome with stage fright. When I stepped before the microphone, I realized that I could hardly find my voice. I stuttered, and had to read my written script in broken sentences. I did finally stagger through, but was met by dead silence at the end of a miserable presentation. It was a "bomb," as they say in show business, and the lack of response was a manifestation of the hostility toward the brash young man. Hess, however, was genuinely sorry; he thought that people remained silent because they did not want to start discussions, as the session had been delayed by his late arrival. He apologized profusely to me after the meeting, when I was blaming myself for my less-than-brilliant "debut."
As the years went by, I learned some self-confidence, while Hess remained modest. He did much for the earth sciences. His idea of seafloor-spreading started a scientific revolution, and his quest for an imaginative research project opened a new era of ocean exploration.
The last time I saw Hess was in the spring of 1969. We had just completed a drilling cruise in the South Atlantic (chapter 5), and I had come to Princeton to discuss the cruise results with my shipboard colleagues. This cruise has achieved fame as the expedition that verified the predictions of the theory first elaborated by Harry Hess. However, Hess was so modest that he actually seemed embarrassed to be proven right. Toward noontime, Hess and I walked from Guyot Hall to the Princeton Faculty Club for lunch. He did not mention the subject of seafloor-spreading, nor did we even discuss our findings. Instead, we chatted about the difficulties of privately endowed universities in the days of high inflation. He had just stepped down as chairman of the department, but he still had many national and international "obligations." He was overworked and seemed tired.
Hess died of a heart attack shortly after my visit. He will always be remembered by us as the one who made the first move that led to one of the most successful undertakings in the earth sciences—the Deep Sea Drilling Project (DSDP) of the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES).
One of Hess's many "obligations" had been to serve on a committee to help select research projects worthy of financial support by the U.S. National Science Foundation. Back in the fifties, when physicists were asking for and getting more and more funds for bigger and better accelerators, geologists seemed to content themselves with pocket money for making geological maps. It was said that Hess and his committee were going over one research application after another on a spring day in 1957. They grew increasingly restless and irritated after finding nothing exciting in the proposals, and began to ask themselves if anything worthwhile in geology was still left to be done.
The heroic age of geology, spanning the last decades of the eighteenth and the first half of the nineteenth century, had long since become a nostalgic memory of a far distant past, an age in which James Hutton founded geology as an observational science, William Smith discovered the value of fossils in ordering strata in temporal sequences, Charles Lyell demythified the book of Genesis and preached uniformitarianism, and Charles Darwin came up with his theory of evolution. It seemed that few discoveries of comparable significance had turned up during the twentieth century. Progress was, however, being made in geophysics, such as the discovery of the Moho.
On 8 October 1909, Yugoslavia was struck by an earthquake whose epicenter was some 25 kilometers south of the village of Papuspsko, near Zagreb. A local geophysicist, Andres Mohorovicic, made a routine study of the time it took the first earthquake waves to reach various registering stations—the so-called first-arrival time.
The shock of an earthquake sends out several different kinds of waves. Some are transmitted by alternately compressing and extending an elastic medium, similar to the way sound waves travel in air. These are called compressional waves, or P-waves. They are the fastest and should be the first registered by a seismograph at any station. In a homogeneous medium, the speed of propagation of the compressional wave, Vp , should be constant. Consequently the first-arrival time t should be directly proportional to the distance between the epicenter and the registering station, S, or
t = S/Vp.
This is, of course, a simple linear relation in Newtonian kinematics that we all learned in middle school. Traveling to a station 200 kilometers from the epicenter should take the wave twice as long as traveling to a station 100 kilometers from the epicenter. Plotting the arrival time against the distance traveled for the stations within 300 kilometers of Zagreb, Mohorovicic obtained a straight line through the origin, illustrating that the simple predicted relation was indeed confirmed by the seismic records (Figure 1.3). As our middle-school physics teacher tells us, the slope of the straight line in this figure, or the travel time divided by distance traveled, is the inverse of the speed of the wave propagation. Mohorovicic's calculations showed that the compressional waves were traveling at a speed of 5 to 6 kilometers per second when they reached those stations.
Excerpted from Challenger at Sea by Kenneth J. Hsü. Copyright © 1992 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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|List of Figures|
|List of Plates|
|Preface to the American Edition|
|Preface to the Chinese Edition|
|Preface to the German Edition|
|Pt. 1||The Eve of a Revolution, 1963-1968|
|Ch. 1||Moho and Mohole||3|
|Ch. 2||Ice Age and LOCO||21|
|Ch. 3||The Challenger Goes to Sea: The Inauguration of Glomar Challenger||35|
|Ch. 4||The Earth Science Revolution||46|
|Pt. 2||The Breakthrough, 1968-1973|
|Ch. 5||A Game of Numbers||75|
|Ch. 6||Atlantic and Tethys||95|
|Ch. 7||Arc and Trench in the Mediterranean||108|
|Ch. 8||Swallowing Up of the Ocean Floor||130|
|Ch. 9||Marginal Seas||146|
|Ch. 10||Hope and Frustration in Nauru||162|
|Ch. 11||Hawaiian Hot-Spot||184|
|Ch. 12||India's Long March||198|
|Pt. 3||Exploring New Territories, 1973-1975|
|Ch. 13||Antarctic Adventures||219|
|Ch. 14||Mid-Cretaceous Anoxia||238|
|Ch. 15||When the Mediterranean Dried Up||255|
|Ch. 16||The Black Sea Was Not Always Black||274|
|Pt. 4||Seeding a New Revolution While Mopping Up, 1975-1983|
|Ch. 17||Getting Stuck in Ocean Crust||299|
|Ch. 18||Eating Peanuts on Ocean Margins||317|
|Ch. 19||What Makes the Ocean Run||338|
|Ch. 20||The Great Dying||355|
|Appendix A: Deep-Sea Drilling Legs||393|
|Appendix B: Bibliographical Notes||403|
Posted May 3, 2000
Dr. Hsu has written an exceptional account not only on his experiences on the Challenger but has compiled an outstanding introduction to understanding global tectonics and crustal evolution of the earth. Beneficial reading for undergrads, grads, and practicing earth scientists interested in tectonics, deep drilling for scientific purposes, and exploration and genesis of resources associated with plate tectonics.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.