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Science and Scientists
Seafloor Spreading

The Science
Harry Hammond Hess's idea of seafloor spreading as the reason for continental drift had the same impact on geology that Charles Darwin's evolution theory had on biology.

The Scientists
Harry Hammond Hess (1906-1969), an American geologist
Alfred Lothar Wegener (1880-1930), a German scientist-explorer
Robert S. Dietz (1914-1995), an American geologist
Matthew F. Maury (1822-1891), a U.S. Navy officer and oceanographer

A Geophysical "Fairy Tale"
The Princeton University professor Harry Hammond Hess is noted for his scientific contributions to the field of geology, specifically for his groundbreaking History of the Ocean Basins (1962), in which he proposed seafloor spreading as the long-sought-after mechanism for Alfred Lothar Wegener's theory of continetal drift, which he had proposed fifty years before. The elements of seafloor spreading, the splitting of the original Pangaea supercontinent into several continental-size plates, and movement of those plates to their present positions are collectively known as "plate tectonics."

Yet the hostility of the geologic community toward previous seafloor spreading hypotheses kept Hess from publishing his theory. Robert S. Dietz, working for the Navy, published virtually identical ideas and coined the phrase "seafloor spreading" in a 1961 article, "Continent and Ocean Basin Evolution by Spreading of the Seafloor," published in Nature.

World War II (1939-1945) delayed research into the question, and from the 1930's to the mid-1950's, continental drift remained a theory held with great passion by a minority of geologists. As late as 1966, the University of Hamburg physicist Pascual Jordan described the theory as the geophysicists' "favorite fairy tale."

The Mid-Atlantic Ridge
Postwar advances in technology, in methodology, and in the new science of paleomagnetism (the study of the direction and intensity of the Earth's magnetic field through geologic time) lent support to Wegener's theory. The U.S. Navy directed its interest to the ocean floor, and other seagoing nations also initiated active research programs that led to the International Geophysical Year (July, 1957, to December, 1958), the first multinational research effort. This effort focused on almost every area of geologic research and led scientists to realize that the Earth, particularly the ocean, was very different from what they had previously imagined. One of the most curious features was the Mid-Atlantic Ridge, which spans the Atlantic Ocean from north to south. Understanding the feature led to an understanding of plate tectonics.

Matthew F. Maury, director of the U.S. Navy's Department of Charts and Instruments, had first recognized the Mid-Atlantic Ridge in 1850 while measuring ocean depths 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 the HMS Challenger expedition (1872-1876) supplemented Maury's map, but the details of the Mid-Atlantic Ridge remained vague. 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.

The idea of a seismically active ridge received further support in 1954. That year 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 C. Heezen continued the German technique of echo sounding 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 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, and is mostly devoid of sediment cover. The question of whether the ridge system was covered with sediment--and the amount and age of that sediment--was important: It would reveal clues to the age of the ridge itself. Ivan Tolstoy and Ewing first characterized the sediment cover in 1949, describing a main ridge of thin sediment and flanks of thick sediment. The age of the sediments increased as one moved from the ridge toward the continents, the oldest age being only about sixty-five million years old.

Central to the interpretation of this underwater mountain range was the early 1950's paleomagnetic research 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, the 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 other oceanographers revealed a symmetrical, zebralike pattern about the midoceanic ridge in 1957.

A Bold New Synthesis
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 theory 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 locations of upwelling "mantle convection cells": that is, areas of the Earth's mantle that progressively moved the seafloor outward from the ridge and eventually under the continents. The mantle is part of the molten core of the Earth. Hess suggested that different parts of it (cells) may spin like wheels, driven by changes in temperature (convection) between the lower and upper parts of the mantle.

Impact
This driving mechanism brought together the divergent data of post-World War II research into one coherent theory. It explained the rift valley in the middle of the ridge, the correlation of the ridge with earthquake epicenters, the continuation of the ridge throughout the oceans, the thin sediment in the middle of the ridge and its thickening toward the edges, and the symmetrical paleomagnetic zebra patterns. In addition, the energy of the mantle convection currents was sufficient to drive the continents.

In 1966, in recognition of his scientific breakthrough, Hess received the Geological Society of America's Penrose Medal, the geologist's equivalent of the Nobel Prize. By 1967, seafloor spreading was the dominant theory, and virtually all earth scientists began to reinterpret their data in the light of the new theory.

See Also
Continental Drift; Plate Tectonics.

Further Reading
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.

James, H. L. "Harry Hammond Hess." National Academy of Sciences Biographical Memoirs 43 (1973): 108-128.

Le Grand, Homer E. Drifting Continents and Shifting Theories. New York: Cambridge University Press, 1988.

Scientific American. Continents Adrift. San Francisco: W. H. Freeman, 1973.

Sullivan, Walter. Continents in Motion: The New Earth Debate. New York: McGraw-Hill, 1974.

Young, Patrick. Drifting Continents, Shifting Seas. New York: Impact Books, 1976.

Richard C. Jones and Anthony N. Stranges