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Hess, Harry Hammond


(b. New York City, 24 May 1906; d. Woods Hole, Massachusetts, 25 August 1969)

geology, geological, mineralogy oceanography.

Hess was the son of Elizabeth Engel Hess and of Julian S. Hess, a member of the New York Stock Exchange. He attended Asbury Park High School in New Jersey, where he failed to filed to distinguish himself academically. Nevertheless, he entered Yale University in 1923, planning to become an electrical engineer. He became bored with electrical engineering.however, and, 1925 switched to geology. He took courses from Alan Bateman, Adolph Knopf, Chester Longwell, and Carl Dunbar. After receiving his B.S. degree in 1927, Hess spent two years in Rhodesia as an exploration geologist for Loangwa Concessions, Ltd., an experience, he later reported, that gave him a profound respect for fieldwork. In 1929 he returned to the United States to attend graduate school.

Hess eventually decided to study at Princeton. His major professors there were A.F. Buddington (petrology), A.H. Phillips (mineralogy), R.M. Field (oceanic structure), and Edward Sampson (mineral deposits). He was quite close to Buddington and, except for Sampson, he eventually coauthored articles with all of them. While still a sgraduate student he worked with the renowned Dutch geophysicist Felix Vening Meinesz, helping him obtain gravity measurements in the West Indies and the Bahamas. Vening Meinesz taught him the rudiments of geophysics, and they became lifelong friends. Hess obtained the Ph.D. from Princeton in 1932 with a dissertation the serpentinization of a large peridotitic intrusive located in Schuyler, Virginia.

Hess taught at Rutgers during the 1932–1933 academic year and spent several months in 1933 and 1934 at the Geophysical Laboratory of the Carnegie Institution of Washington, then returned to Princeton in 1934 to teach in the geology department. On 15 August 1934 Hess married Annette Burns, daughter of George Plumer Burns, a professor of botany at the University of Vermont; they had two sons.

A reserve officer in the navy at the time of the attack on Pearl Harbor, 7 December 1941. Hess initially was stationed in New York City, where he headed an operation charged with estimating the daily positions of German submarines in the North Atlantic. He volunteered for active sea duty and eventually took over the command of the attack transport U.S.S. Cape Johnson. He took part in four major combat landings, and at the rank of commander. Hess remained active in the navy reserve and was on call for advice during the Cuban missile crisis, the loss of the submarine Thresher, and the Pueble affair. At his death he held the rank of rear admiral.

Except for visiting professorships at Capetown University in South Africa (1949–1950) and Cambridge University (1965). Hess remained at Princeton, chairing the geology department from 1950 to 1966. In 1964 he was appointed to the Blair professorship of geology. Hess received numerous scientific honors, and he devoted a considerable amount of time to scientific organizations. In 1952 he was elected to the National Academy of Sciences. and in 1960 to the American Philosophical Society, and in 1960 honorary foreign member of the Geological Society of London the Geological Society of South Africa and the Sociedad Venezolana de Geólogos. In 1966 he received the Penrose Medal of the Geological Society of America and the Feltrinelli Prize of the Accademia Nazionale dei Lincei. He was president of two sections of the American Geophysical Union. Geodesy (1951–1953) and Tectonophysics (1956– 1958); of the Mineralogical Society of America (1963). In 1969 he was awarded (posthumously) the Distinguished Public Service Award of the National Aeronautics and Space Administration.

Hess was one of the major figures in the American Miscellaneous Society, an informal group of scientists from various fields formed to consider new ideas that might be worth considering. He played a crucial role in bringing to life one of these ideas, the drilling beneath the ocean into the mantle. Labeled’ Project Mohole, ’ this project was originally suggested to Hess by Walter Munk in 1957. Hess pushed for the project, and the National Science Foundation supported it from 1958 to 1966. He chaired the panel charged with determining where to drill, and the first core sample was obtained in 1958.

From 1962 until his death, Hess chaired the Space Science Board of the National Academy of Sciences, the function of which was to advise the National Aeronautics and Space Administration on its scientific program. He was chairing a Space Science Board conference at Woods Hole. Massachusetts, organized by him to reformulate the scientific objectives of lunar exploration, when he consulted a doctor about chest pains he had experienced and died in the doctor’s office.

Hess was a devoted family man. Although somewhat reserved, he possessed a forceful personality and was known for his courage-as naval officer, department chair, or defender of scientists or ideas, Throughout his career he was not afraid to hypothesize solutions to major problems and was usually the first to find fault with his earlier solutions.

The range of Hess’s reserach accomplilshments was extraordinary. He wrote detailed mineralogic studies about pyroxenes, devoted much of his life to the origin and significance of peridotite, and combined his work on peridotite with his treatment of large-scale problems about the origin of island arcs and oceanic trenches, ridges. and crust, in which be continually showed his ability to utilize data from exploratory geophysics to develop hypotheses about their origin. These concerns eventually led to his hypothesis of seafloor spreading-the most impotant conceptual innovation leading to the plate tectonics revolution in the earth sciences during the late 1960’s and early 1970’s.

From 1932 through 1938 Hess published a number of papers in which he offered solutions to several diverse problems: the presence and formation of magnetic serpentine belts in island arcs and Alphinetype mountain ranges, the presence of gravity anomalies near trenches in island are regions, and the formation of island arcs and mountain ranges. By 1937 he had developed a unifying solution to all of these problems. He (along with Vening Meinesz and others)supposed downbuckling of the earth’s crust, which resulted in the formation of negative gravity anomolies around trench regions, accompanying island arcs containing serpentinized peridotite intrusions of magma, and eventual alteration of sediments squeezed upward through continued action of the downbuckle. Hess supported this downbuckling hypothesis throughout his early career, and he extended its problem-solving effectiveness in 1940 when he offered an explanation for the generation of the Hawaiian Islands and accompanying swell that was derivative from the downbuckling hypothesis. In 1950 he explilcitly coupled the downbuckling hypothesis with convection currents in the earth’s mantle in order to explain how the crust could remain downbuckled for an extended period of time, and he utilized the conjunction to explain the rather complicated pattern of deep-foci earthquakes typically located on the continental side of island arcs.

Hess found a way to combine defense of his country with his ongoing research. While commanding the U.S.S. Cope Johnson, he managed to take numerous soundings of the Pacific seafloor. Through these soundings he discovered a number of submerged. reefless, flat-topped seamounts, which he named guyots in honor of the Swiss geologist Arnold Guyot, the first professor of geology at Princeton. In 1946 Hess presented aningenious hypothesis for their development and formation. The unique and puzzling characteristic of guyots is their absence of reefs; consequently he had to construct a solution for their origin and development that would prohibit reef development. Hess argued that guyots originally were Precambrian islands that, by the time lime-secreting organisms had emerged, were too far below sea level for those organisms to survive. Central to his analysis was the claim that sea level has risen with respect to oceanic structures because of the continual deposition of continental sediment upon the seafloor. Thesediments have raised the seafloor; and therefore, assuming a relative constancy of oceanic water since the Precambrian era, sea level has risen.

In 1953 and 1955 Hess wrote speculative papers in which he expanded his downbuckling theory; suggested a new solution to the origin of guyots; and devoted much of his attention to the nature, formation, and development of oceanic ridges. All of these pursuits were undertaken in light of his new analysis of the oceanic crust and its layer of covering sediments, which was based upon new studies of the ocean floor by Maurice Ewing and his co-workers at the Lamont Geological Observatory that had begun after the end of World War II. Hess’s new model of the oceanic crust, like many others at that time, was as follows: The oceanic crust was taken to be only 5 kilometers thick with an average of 0.7 kilometer of unconsolidated sediments above it, and the mantle was considered to be made of peridotite. Major differences between this model, were elimination of any granitic material from the oceanic crust and a drastic thinning of the basalt layer along with the consequent raising of the Moho discontinuity. Moreover, the 0.7 kilometer of unconsolidated sediments reduced by a factor of three to five times the size of former estimates, which had been based upon extrapolation of present rates back to the Precambrian.

With this model of the oceanic crust. Hess turned to the problem of the origin of oceanic ridges. The relevant, new ridge data he had to explain were that almost all ridges were associated with basalt volcanism, had no sediments older than Cretaceous atop them, appeared to be ephemeral, and exhibited little folding in their formation. In 1953 Hess, beginning with a thin basaltic crust and peridotite upper mantle, envisioned the formation of a less dense layer of crest through magmatic intrusion of basalt mixed with peridotite. Since the basaltic intrusing would be less dense than the surrounding peridotite, the column would rise as a result of isostatic adjustment. All this could occur without crustal folding; and the resulting surface materials, basalt mixed with peridotite, matched most of the samples that had been collected from the Mid-Atlantic Ridge. The subsequent cooling of the rising magma and cessation of the convection currents provided Hess with a solution to the problem of how ridges decrease in height after initial formation.

In 1955 Hess rejected his 1953 hypothesis for the origin of oceanic ridges. The genesis of this new model is found in his continued concern with serpentinite. Central to his 1995 hypothesls was the reaction.


Both olivine rock and serpentinite are forms of the approximately 500°C the reaction proceeds to the left, while below 500°C it moves to the right. Because the reactive equivalents of serpentinite are less dense than those of olivine, when the reaction proceeds to the right, the resulting serpentinite has a greater volume than the reactive olivine rock. This reaction, consequently, offered a mechanism for the formation and subsquent disappearance of oceanic ridges.

Aware of new data indicating high heat flows in the oceanic crust, Hess applied this reaction to the formation of oceanic ridges. He placed the crucial 500°C isotherm at a depth well below the Moho. Assuming the upward movement of water from the earth’s interior, he proposed transformation of olivine peridotite to serpentine peridotite at the 500°C isotherm, With the addition of more water, continued serpentinization would occur; at the same time the whole mass of serpentinized peridotite would rise, if the 500°C isotherm migrated upward.

Hess suggested two possible causes for the isotherm rise: either convective overturn in the mantle or intrusion of basalt. Once the rising isotherm reached the crust, where there was no more olivine, the deserpentinization below, brought about by the rising isotherm, would lead to a net loss of serpentinized material. Rising of the oceanic surface would occur whenever the serpentinization was greater than the deserpentinization, while subsequent lowering of the surface would result when deserpentinization exceeded serpentinization.

By 1953 Hess realized that his former solution to the development of guyots was untenable. The difficulty with his earlier solution was that it utilized a mechanism for submergence of guyots that was twenty-five times too slow. Because of the discovery of Upper Cretaceous shallow-water fossils atop guyots, Hess realized that guyots could not have begun sinking until the Late Cretaceous. This was five times more recent than he had formerly supposed. Moreover, he decided that his former estimate of the rate of sediment deposition upon the seafloor was five times too rapid in light of the new estimates of 0.7 kilometer of unconsolidated sediment upon the seafloor. Rather than revise his former solution. Hess applied the serpentinization reaction to the problem. He proposed that guyots formed above a well of serpentinized peridotite. Once the mass of serpentinite began to deserpentinize, the resulting decrease in volume of the mass would cause the required downward movement of the guyot.

In 1959 Hess further developed his 1955 hypothesis for the origin and development of midocean ridges. He presented a new model of the oceanic crust and continued to recognize the importance of the fact that seismic profiles of accumulated oceanic sediment consistently yielded lower values than those established through extrapolation of present sedimentary rates. When Hess reproposed his solution to the problem of midocean ridge development, he opted for convection currents rather than rising basalts as the driving force behind the serpentinizationdeserpentinization transform because of two new several Pacific ridges and association of the central rift valley along the Mid-Atlantic Ridge with shallow earthquakes.

Hess presented his new model of the oceanic crust in December 1959. It differed in two major respects from his 1953/1955 model. He replaced the five-kilometer basalt layer with a five-kilometer layer of serpentinized peridotite. The overall reason for this switch was that seismic velocity data indicated either basalt or serpentinized peridotite. But the newly discovered fact that this layer of oceanic crust (layer 3), the bottom layer, has uniform thickness indicated to Hess that it had to have been formed on location—it was just too even to have been formed, say, in the mantle and then transported piecemeal to its present location. Hess then suggested that layer 3 had been formed by the serpentinization of peridotite because that would give the uniform thickness, and he now believed that the mantle was partly composed of peridotite. He supposed that ascending water serpentinized the peridotite down to a level of five kilometers, the depth at which he placed the critical 500°C isotherm, in order to account for the five-kilometer thickness of layer 3.

The other major difference between this model and Hess’s former one was his more detailed account of the upper two layers. He argued that layer 1, the top layer, was unconsolidated sediment, while layer 2, the middle layer, was consolidated sediment or volcanic rocks or both. He remained puzzled over the lack of seafloor sediment and suggested three solutions without opting for any one of them.

The most obvious alternatives are: (1) The oceans are relatively young… (2) The pre-Cretaceous sediments have in some manner been removed; for example, by incorporation into the continents by continental drift. (3) Nondeposition of any sediment over much of the ocean floor was a common attribute of the past. (’The AMSOC Hole…’ p. 343)

All that he was sure of was that the sedimentaryrock sections of layers 1 and 2 were very incomplete, although he was “rooting against”such a prediction. In the year that followed. Hess close an option that was not extremely obvious to him in 1959: that the ocean floors but not the oceans are very young, since they are continually being created and destroyed through seafloor spreading. However, in 1959 Hess had not developed his seafloor-spreading hypothesis.

In December 1960 Hess, in a preprint, proposed his seafloor-spreading hypothesis—the name’ seafloor spreading’ was given to Hess’s hypothesis by Robert Dietz, an American earth scientist who, with Hess’s preprint in hand, published the first article on seafloor spreading in 1961, one year before Hess’s version was published. With this hypothesis Hess became a proponent of continental drift, for he realized that it offered a solution to the number-one problem faced by continental drift: how to move the continents through the seafloor without having them break up.

Hess proposed that the continents do not plow their way through the seafloor, as formerly suggested by Alfred Wegener, the German earth scientist who presented the first detailed account of continental drift during the second decade of the twentieth century, but are carried passively atop the spreading seafloor. (Arthur Holmes, one of the leading British earth scientists of the twentieth century, proposed a hypothesis of ocean basin formation that was a forerunner of Hess’s seafloor spreading in the 1930’s.)

The central aspect of Hess’s hypothesis was its new solution to the problem of the origin and development of midocean ridges. He realized that his 1955/1959 solution was inadequate and that if he were to propose a slightly different solution incorporating many of the same elements, he could avoid difficulties with his former view and even solve the problem of how layer 3 of oceanic crust forms. He also saw that his new hypothesis solved the problem of guyot formation and explained why no sediments on the ocean floor are older than Cretaceous. Hess claimed that young midocean ridges are located on upward-moving convection currents and are the sites for generation of new seafloor. That is, they are where layer 3 of the oceanic crust, composed of serpentinized peridotite, is created—the place where the peridotite is serpentinized.

Hess explained the five-kilometer thickness of layer 3 by positioning the critical 500°C isotherm five kilometers below the surface, arguing that the rising convection currents would elevate the isotherm to such a level. Once the serpentinized peridotite is created, it is forced outward from ridge axes by the movement of parting convection currents. Eventually the convection cell subsides, and the ephemeral ridge disappears. Meanwhile, the outwardmoving new seafloor ultimately sinks into the mantle on the backs of descending convection currents. This creates ocean trenches. Continents, pushed along by the convection currents, cease moving when convection stops; and their leading edges become deformed as they impinge upon the downwardmoving limbs of convecting material. The continents do not sink into the mantle because of their relatively low density, and sometimes seafloor sediments are metamorphosed and added onto continents at ocean trenches. In addition, since the spreading seafloor moves away from both sides of a ridge axis at equal rates, ridges have a median position between drifting continents. Because of the continual destruction of its material, the seafloor is always young, and sediments are never older than Cretaceous. Guyots form on ridges, where they are truncated by wave erosion. Once guyots move off ridges, they drown themselves well below sea level.

Hess correctly categorized his 1960 preprint as an’ essay in geopoetry, ’ By 1966, however, it began to gain acceptance with the confirmation of the VineMatthews-Morley hypothesis. This hypothesis, independently proposed in 1963 by Fred J. Vine and Drummond H. Matthews at Cambridge University and by Lawrence W. Morley, a geophysicist working for the Geological Survey of Canada, was a direct corollary of Hess’s idea of seafloor spreading and the notion that the geomagnetic field undergoes reversals in polarity. They reasoned that if seafloor spreading occurs and the geomagnetic field reverses its polarity, the seafloor should be made up of strips of alternately normal and reversely magnetized material running parallel to midocean ridges.

Hess learned of the Vine an Matthews version of the Vine-Matthews-Morley hypothesis in late 1963 or early 1964—the article was published in Nature in September 1963, and Vine wrote to Hess about the idea in 1964. Hess was quite excited about it. He eventually worked with Vine and Matthews at Cambridge in 1965, and he arranged for Vine to come to Princeton during the 1965–1966 academic year.

Although many aspects of Hess’s idea of seafloor spreading have become somewhat problematic, his basic idea that the seafloor is created at ridges and sinks into trenches has become accepted background knowledge in the earth sciences.


1. Original Works. Hess published more than 10 monographs, articles and discussion. The most complete listing of his publications is in Harold L. James, ’ Harry Hammond Hess, ’ in Biographical Memoirs. National Academy of Sciences, 43 (1973), 109–128. His two most important works in mineralogy are’ Pyroxenes of Common Mafic Magmas, ’ in American Mineralogist, 26 (1941), 515–535 and 573–594, and Stillwater Igneous Complex, Montana: A Quantitative Mineralogical Study. Geological Society of America Memoir no. 80 (New York. 1960).

Some key articles, particularly relevant to his work on serpentinization, peridotites, the structure of the seafloor, and his idea of seafloor spreading are’ Interpretation of Gravity-Anomalies and Sounding-Profiles Obtained in the West Indies by the International Expedition to the West Indies in 1932, ’ in Transaction of the American Geophysical Union, 13th Annual Meeting (1932), 26–33;’ Island Arcs, Gravity Anomalies, and Serpentine Intrusions: A Contribution to the Ophiolite Problem, ’ in 17th International Geological Congress, Report, II (Moscow, 1937), 263–283;’ Geological Interpretation of Data Collected on Cruise of USS Barracuda in the West Indies— Preliminary Report, ’ in Transactions of the American Geophysical Union, 18th Annual Meeting (1937), 69–77;’ A Primary Peridotite Magma, ’ in American Journal of Science, 235 (1938), 321–344; and’ Gravty Anomalies and Island Arc Structure with Particular Reference to the West Indies, ’ in Proceedings of the American Philosophical Society, 79 (1938), 71–96.

Later works are’ Drowned Ancient Islands of the Pacific Basin, ’ in American Journal of Science, 244 (1946), 772– 791;’ Comment on Mountain Building, ’ in’ Colloquium on Plastic Flow and Deformation within the Earth, ’ Transactions of the American Geophysical Union, 32 (1951), 528–531;’ Geological Hypotheses and the Earth’s Crust under the Oceans, ’ in Royal Society of London Proceedings, A222 (1954), 341–348;’ Serpentines, Orogeny, and Epeirogeny, ’ in A. W. Poldervaart, ed. Crust of the Earth, Geological Society of America, Special Paper 62 (New York, 1955), 391–407;’ The Oceanic Crust, ’ in Journal of Marine Research, 14 (1955), 423–439;’ Nature of the Great Oceanic Ridges, ’ in Preprints of the First International Ocean Congress (Washington, D.C., 1959), 33–34;’ The AMSOC Hole to the Earth’s Mantle, ’ in Transactions of the American Geophysical Union, 40 (1959), 340–345;’ History of Ocean Basins, ’ in A.E.J. Engel. Harold L. James, and B. F. Leonard, eds., Petrologic Studies: A Volume in Honor of A. F. Buddington (New York, 1962), 599–620; and, with Fred Vine, ’ Seafloor Spreading, ’ in The Sea, vol. 4. part 2. Arthur E. Maxwell. A. C. Bullard, E. Goldberg, and J. L. Worzel, eds. (New York, 1970).

Annette Hess collected many of her husband’s privat4 papers and gave them to Princeton. They are presently in the Firestone Library at Princeton University.

II. Secondary Literature. Besides the memoir by James, See A.F. Buddington, Geological Society of America Memorials, 1 (1973), 18–26; and William W. Rubey, ’ Harry Hammond Hess (1906–1969).’ in Year Book of the American Philosophical Society (1970), 126– 129. Two articles on the development and presentation of Hess’s idea of seafloor spreading and its eventual confirmation are Henry Frankel, ’ Hess’s Development of His Seafloor Spreading Hypothesis, ’ in Thomas Nickles, ed., Scientific Discovery: Case Studies (Boston, 1980), 345–366; and’ The Development, Reception and Acceptance of the Vine-Matthews-Morley Hypothesis, ’ in Historical Studies in the physical Sciences, 13 (1982), 1–39.

Henry Frankel

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Mohorovicic Discontinuity (Moho)

Mohorovicic discontinuity (Moho)

The Mohorovicic discontinuity, sometimes referred to as "Moho," is the boundary where Earth's crust meets Earth's upper mantle (approximately 31 mi/50 km below the surface), and where seismic waves travel at a different and more rapid rate than the crust or mantle. The Moho is named after Andrija Mohorovicic (18571936), a Croatian meteorologist and seismologist who was fascinated with the faults and movements in the earth's infrastructure that result in earthquakes. The discovery of the Moho was most important because it helped scientists discover a second layer, or mantle, inside the earth. It also helped scientists to determine more accurately where this second layer was located in relation to Earth's surface, or crust.

Since the early 1900s, scientists were almost certain that Earth, like an onion, was made up of many layers, but they did not know exactly where the layers started and ended. In 1906, Mohorovicic studied Yugoslavian earthquake records, which revealed the existence of two different sets of earth shock waves from one earthquake. Because the second set of waves exactly mirrored the first set, Mohorovicic discovered that the additional set was actually the first bouncing back from a resistant surface, or a layer of different material inside the earth. This resistant surface, or discontinuity, allowed Mohorovicic to postulate the existence of a second stratum of material under the crust. He did this by gauging the time between the waves, which helped him determine how far this layer resided from the earth's surface.

Mohorovicic also noticed from these experiments that the waves, or tremors, traveled at different speeds depending on the thickness of the material inside Earth. This information helped scientists discover the different types of rocks in areas where drilling was impossible. For example, the lowest level of the crust is composed of basaltic rock , the material that rests next to the mantle. After the Moho was discovered, scientists were able to further plot seismic wave movements on sensitive shock recording devices called seismographs. From this information, we know that the outer crust of Earth is 2025 mi (3240 km) thick except under many places in the ocean, where it is only 3 mi (4.8 km) thick. The mantle is only the second interior layer. Deeper within the earth lies the most interior layer, Earth's core. We know from mountains and valleys that Earth's surface has changed and shifted with the ages. Similar to Earth's uneven crust, Earth's mantle is thought to be comparably uneven, mostly caused by enormous pressures inside Earth forcing the weaker areas of the rocky sub-layers out of alignment. When the weaker sub-layers, or plates, give way to pressure or stronger plates, earthquakes result. Ever since the existence of the mantle became certain, scientists sought to probe into the physical nature of the earth's inner layer. And because the Moho is located so much closer to the surface beneath the ocean, there were plans in the late 1950s to drill into the Moho from floating platforms out at sea. After a number of test drillings, and a drop in funding, the projectProject Moholewas abandoned in the mid-1960s.

See also Crust; Earth (planet); Earth, interior structure

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Mohole, Project

Project Mohole, program proposed in 1957 to drill a hole down to the boundary between the crust and the mantle, known as the Mohorovičić discontinuity at about 4 to 43 mi (7 to 70 km) below the earth's surface. Initiated by the American Miscellaneous Society, a loose organization of scientists, the main purposes of the project were to determine the nature of this boundary and to attempt to fill gaps in the geologic record from samples of the rocks encountered. The technology of such a project, however, was beyond the state of drilling technology at that time. Groups such as the National Science Foundation and the National Academy of Science eventually backed phase 1, in which five holes were drilled off the coast of Mexico, the most successful entering 601 ft (183 m) into the ocean floor under 2.2 mi (3.5 km) of water. The project was abandoned by 1966, as funding to support the ever-increasing costs of the project failed to gain congressional approval. Nevertheless, ship positioning and design, along with deepwater drilling technology developed for Project Mohole, were employed in the Deep Sea Drilling Project and future drilling projects.

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Mohorovičić discontinuity

Mohorovičić discontinuity (Moho) The seismic discontinuity that marks the boundary between the Earth's crust and mantle. The mantle has P-wave velocities of about 8.1 km/s, higher than that of the lower-density crustal rocks. The Moho was originally recognized at a depth of some 20 km in Europe by A. Mohorovičić on the basis of the arrival of refracted P-waves before direct P-waves. Previously considered to be very sharp, the discontinuity is now known to be more complex.

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