Hess Identifies the Cause of Continental Drift

Date 1960

Harry Hammond Hess’s identification of seafloor spreading as the mechanism for continental displacement revolutionized geologic paradigms and had the same degree of impact on the Earth sciences that Charles Darwin’s theory of evolution had had on biology.

Also known as Seafloor spreading

Locale Princeton, New Jersey

Key Figures

• Harry Hammond Hess (1906-1969), American geologist
• Alfred Wegener (1880-1930), German scientist and explorer
• Robert S. Dietz (1914-1995), American geologist

Summary of Event

Princeton University professor Harry Hammond Hess was noted for his scientific contributions to the field of geology, specifically his groundbreaking “History of the Ocean Basins” (published in 1962 but widely circulated beginning in 1960), in which he proposed seafloor spreading as the long-sought-after mechanism for Alfred Wegener’s continental drift theory. The elements of seafloor spreading, the rifting of the Pangaea continent into several continental-size plates, and lateral displacement of those plates to their present latitudes remain the dominant paradigm in the earth sciences and are collectively known as plate tectonics. The idea of drifting continents was not original with Hess, but the hostility the geologic community demonstrated toward the idea inhibited Hess initially from publishing his theory.

Wegener’s 1915 theory of drifting continents lay dormant after his death in 1930. Several problems kept it from universal acceptance. First, the jigsaw-puzzle fit was not exact. It did not account very well for Central America, and Ireland and Newfoundland did not correlate geologically. Second, Sir Harold Jeffreys, leading geophysicist of the time, presented evidence in the 1923-1924 meeting of the British Association for the Advancement of Science that Wegener’s forces were too small by three orders of magnitude to drive the drift mechanism.

Other geophysicists questioned Wegener’s objectivity and accused him of generalizing on generalizations. A third problem was the repeated failure of Wegener’s own test to prove his hypothesis. The timing of radio signals across the Atlantic in 1922, 1927, 1936, 1938, and 1948 did not reveal a widening of the Atlantic through progressively longer time intervals. Finally, in the eyes of the geologic community, Wegener was not a geologist but an outsider attempting to restructure their science.

Wegener’s supporters tried to mollify the scientific community. Harvard University geologist Reginald A. Daly, an ardent believer in continential drift, wrote in his 1926 book, Our Mobile Earth: “So obvious are his logical inconsequences and his failure properly to weigh ascertained facts that there is danger of a too speedy rejection of the main idea involved.” Geologist Alexander L. du Toit of South Africa’s Johannesburg University and S. Warren Carey of the University of Tasmania, Hobart, defended Wegener’s theory in their 1937 publications, and the theory rallied again in England with the 1944 publication of Arthur Holmes’s textbook Principles of Physical Geology.

For the most part, resistance to Wegener’s theory continued. World War II delayed research into the question, and from the 1930’s to the mid-1950’s, continental drift remained a paradigm held with great passion by a minority of geologists but deficient of evidence according to the majority. Bailey Willis, of Stanford University, criticized it in his 1944 article, “Continental Drift, Ein Märchen (a Fairy Tale),” published in the American Journal of Science. Even as supporting evidence was accumulating in the mid-1950’s, Crust of the Earth, a 1955 landmark publication of the Geological Society of America, ignored continential drift, although it mentions both Wegener and du Toit in unrelated discussions. As late as 1966, University of Hamburg physicist Pascual Jordan described the theory as the geophysicists’ “favorite fairy tale.”

Postwar advances in technology, methodology, and the new science of paleomagnetism lent support to Wegener’s theory. Paleomagnetism enabled scientists for the first time to reconstruct, from igneous rocks, a fossil imprint of the earth’s earlier magnetic fields. Geologic research in these new areas continued to focus on data gathering, but now particularly in the oceans, where the U.S. Navy had directed its interest to the ocean floor. Other seagoing nations also initiated active research programs that culminated in the International Geophysical Year (IGY, July, 1957, to December, 1958), the first multinational research effort. The result of this effort was that in almost every area of geologic research, scientists found the earth and particularly the ocean very different from what they had imagined originally. One of the most curious features was the Mid-Atlantic Ridge . Understanding it led to an understanding of plate tectonics.

Matthew F. Maury, director of the U.S. Navy’s Department of Charts and Instruments, first recognized the Mid-Atlantic Ridge in 1850 while collecting bathemetric measurements aboard the USS Dolphin. Maury named it the “Dolphin Rise” and published a map of it in his The Physical Geography of the Sea (1855). Data from HMS Challenger expedition (1872 to 1876) supplemented Maury’s map, but the details of the Mid-Atlantic Ridge remained vague until the German meteor expedition of 1925 to 1927 led by Nobel laureate Fritz Haber . The expedition collected much oceanographic data and utilized an echo sounder to map the ocean floor. In 1933, German oceanographers Theodor Stocks and Georg Wust produced the first detailed map of the ridge noting a valley that seemed to be bisecting it. Later, in 1935, geophysicist Nicholas H. Heck found a strong correlation between earthquakes and the Mid-Atlantic Ridge.

Interrupted by World War II, oceanic exploration resumed in the late 1940’s as a predominantly American venture. These studies produced a host of seemingly divergent data. In 1950, Maurice Ewing of the Lamont-Geological Observatory established that no continental crust exists beneath the ocean basins. In 1952, Roger Revelle, director of Scripts Institute of Oceanography at La Jolla, California, and his student Arthur E. Maxwell determined the rate of heat flow from the earth’s interior in the Pacific Ocean. They found it was similar to continental heat flow rates that Edward Bullard of the University of Cambridge first described in 1935, except over the oceanic ridge, where the rate was substantially higher.

The idea of a structurally active ridge received further support in 1954 when Jean P. Rothé, director of the International Bureau of Seismology in Strasbourg, mapped a continuous belt of earthquake epicenters from Iceland through the mid-Atlantic around South Africa, through the Indian Ocean, and on to the African Rifts and the Red Sea. In 1956, Maurice Ewing and Bruce Heezen continued the German technique of echo soundings at Lamont-Geological Observatory and found that the Mid-Atlantic Ridge was more than 64,000 kilometers long and, more important, that it had a rift (tensional) valley along the entire crest. In 1961, Ewing and Mark Landisman discovered that this ridge system extends throughout all the world’s oceans, is seismically and volcanically active, continues to exhibit a tensional valley, and is mostly devoid of sediment cover.

Ivan Tolstoy and Ewing first characterized the geologically important question of sediment cover in 1949, describing a main ridge of thin sediment and flanks of thick sediment. The age of the sediments increased from the ridge toward the continents, the oldest being only Cretaceous in age (65 million years). In 1959, Victor Vacquier discovered that enormously long faults crossed the oceans’ floor in many places. The faults have offsets of hundreds of kilometers but do not extend into the continents.

Central to the interpretation of this ocean mountain range were the early 1950’s paleomagnetic researches of Patrick M. S. Blackett and his student, Keith Runcorn, at the University of Manchester. Their studies of fossil magnetism suggested that in the geologic past, the inclination, declination, and even the polarity of the earth’s magnetic field had been very different from current orientations. The seemingly chaotic data formed a consistent pattern only upon assuming that the continents had moved relative to the magnetic poles and to one another. Magnetic studies of the seafloor by oceanographers R. G. Mason, A. D. Raff, and Vacquier revealed a symmetrical, zebralike pattern about the midoceanic ridge in 1957.

By the end of the 1950’s, it was clear that existing geologic theories clearly had failed to predict or explain these seemingly unrelated phenomena; the new data required a new paradigm. In 1960, Hess circulated among his colleagues his theory of seafloor spreading to illuminate the working of Wegener’s continental displacement hypothesis.

Significance

The period between 1960 and 1965 was one of great uncertainty and multiple directions for geologists. In 1960, Hess synthesized the oceanic data of the 1950’s into a bold new theory. Hess’s paradigm was so novel and radical that he did not attempt to publish it in the usual professional journals, but included it in a 1960 report to the Office of Naval Research. Hess also widely circulated reprints among his colleagues.

In his 1960 report, Hess proposed that the midoceanic ridges were the loci of upwelling mantle convection cells that progressively moved the seafloor laterally and eventually under the continents. (Nearly thirty years earlier, in 1931, Arthur Holmes, of the University of Edinburgh, proposed that convection currents may exist in the mantle.) This driving mechanism brought together the divergent data of post-World War II research into one coherent theory. This creation of new seafloor from the upwelling of the mantle explained the high heat flow above the midoceanic ridge, the tensional rift-valley in the middle of the ridge, the correlation of the ridge with earthquake epicenters, the transform faults associated with the ocean floor, the continuation of the ridge throughout the oceans, the absence of continental-type rocks in the oceans, the thin sediment in the middle of the ridge and its thickening toward the edges, the symmetrical paleomagnetic zebra patterns, and the energy of the convection currents that was sufficient to drive the continents.

Robert S. Dietz, working for the Navy Electronic Laboratory in San Diego, published virtually identical ideas and coined the phrase “seafloor spreading” in a 1961 article “Continent and Ocean Basin Evolution by Spreading of the Sea Floor,” published in Nature. Hess’s reluctance to publish in mainstream scientific journals muddles the question of intellectual priority. He had scheduled his theory for publication in a multivolume set The Sea: Ideas and Observations, but delays kept it from the presses until 1963. Meanwhile, Hess was aware of Dietz’s 1961 article, and in November, 1962, he published his ideas through a more expedient route, a collection of essays dedicated to his former teacher, Petrologic Studies: A Volume in Honor of A. F. Buddington. Dietz acknowledged Hess’s priority of the mobile seafloor theory, and Hess credited Dietz for coining the term “seafloor spreading.”

With the theory of seafloor spreading fully illuminated by both Hess and Dietz, the geologic community began reevaluating data and redesigning field studies to test it. One of the first tests occurred in 1963, following a prediction by Drummond H. Mathews and his student Frederick J. Vine at the University of Cambridge. They hypothesized that the magnetic pattern of polar reversals should be symmetrical along the axis of the oceanic ridge and correlate with its morphology. Their hypothesis proved correct and initiated a revolution in the earth sciences. In 1966, in recognition of his scientific breakthrough, Hess received the Geological Society of America’s Penrose Medal, the geologists’ equivalent of the Nobel Prize. By 1967, seafloor spreading was the dominant paradigm, and virtually all earth science specialists began to reinterpret their data in the light of the new paradigm.

Bibliography

Engel, A. E. J., H. L. James, and B. F. Leonard, eds. Petrologic Studies: A Volume in Honor of A. F. Buddington. New York: Geological Society of America, 1962. This is the primary source for Hess’s theory based on his earlier Report to the Office of Naval Research, Evolution of Ocean Basins, Contract No. 1858(10), NR 081-067. This may be appropriate for the more advanced reader.

James, H. L. “Harry Hammond Hess.” National Academy of Sciences Biographical Memoirs 43 (1973): 108-128. This biographical sketch yields insight into Hess, the man, and the full scope of his career and scientific work. For a wide audience.

Le Grand, Homer E. Drifting Continents and Shifting Theories. New York: Cambridge University Press, 1988. A very comprehensive work illustrating how science works in periods of revolution. Although written for an advanced audience, there is much here for the reader at any level. Comprehensive bibliography.

McCoy, Roger M. Ending in Ice: The Revolutionary Idea and Tragic Expedition of Alfred Wegener. New York: Oxford University Press, 2006. Narrative of Wegener’s original development of the continental drift theory and the final expedition he mounted in pursuit of evidence to support that theory. Bibliographic references and index.

Scientific American. Continents Adrift. San Francisco: W. H. Freeman, 1973. This is an excellent collection of landmark papers (1952 to 1970) from the geologic revolution. Designed for the general reader, with typically excellent illustrations and photographs.

Sullivan, Walter. Continents in Motion: The New Earth Debate. New York: McGraw-Hill, 1974. This book has something for the reader at any level of geologic sophistication. Extremely readable and well written, with comprehensible drawings and photographs. Develops like a detective story rather than a textbook.

Vrielynck, Bruno, and Philippe Bouysse. The Changing Face of the Earth: The Break-Up of Pangaea and Continental Drift over the Past 250 Million Years in Ten Steps. Paris: UNESCO, 2003. Booklet of maps showing the evolution of the Earth’s surface over 250 million years. Invaluable for understanding the effects of continental drift.

Young, Patrick. Drifting Continents, Shifting Seas. New York: Impact Books, 1976. This is an introductory book to the story of plate tectonics. The emphasis is on the people and chronological development. Very readable and well illustrated.