Harry Hammond Hess: Seafloor Spreading Theory and Plate Tectonics Pioneer

Harry Hammond Hess is best known for formulating a groundbreaking theory on the origin and evolution of ocean basins. Building upon observations from Alfred Wegener’s theory of continental drift (proposed in 1912), Hess visualized a process occurring deep below the oceanic crust that caused seafloor spreading. In this model, the seafloor is created at mid‑ocean ridges and sinks at deep‑sea trenches back into the Earth’s mantle. This concept provided a mechanism that catapulted the plate tectonics theory into the mainstream of earth sciences.

Early Years. Harry Hammond Hess was born on May 24, 1906, in New York City to Julian and Elizabeth Engel Hess. His father worked at the New York Stock Exchange, and Harry had one brother, Frank. When Harry was five, his parents photographed him in a sailor suit and fittingly titled the portrait “The Little Admiral,” foreshadowing his future naval career. He attended Asbury Park High School in New Jersey, where he specialized in foreign languages.

In 1923 Hess enrolled at Yale University, planning to major in electrical engineering. He changed his major to geology and received a bachelor’s degree in 1927. The Loangwa Concessions, Ltd. mining company hired Hess to perform exploratory geological mapping and search for mineral deposits in Zimbabwe (then Rhodesia), southern Africa. He did not enjoy this work because he had to survey where he was told rather than where he believed valuable deposits might exist. After two years, he returned to the United States for graduate school, but this experience taught him to appreciate the importance of fieldwork in geological research.

As a doctoral candidate at Princeton University, Hess studied mineralogy (the identification, distribution, and properties of minerals), petrology (the origin, composition, and structure of rocks), and the structure of ocean basins. In 1931 he accompanied Dutch geophysicist Felix A. Vening‑Meinesz on a mission to carry out gravity measurements in the West Indies and the Bahamas. Gravity data at different positions over the Earth’s surface provide insight into subsurface rock composition because gravitational fields are stronger over denser, more massive areas. The measurements had to be performed inside a submarine because a pendulum system was used, and surface ships moved too much due to waves and wind.

One interesting observation from this voyage was that gravity over the Caribbean trench was much weaker than expected. A trench is a long, narrow furrow along the edges of the ocean floor; gravity was expected to be weaker as over all valleys, but the extreme weakness indicated that the underlying structure was unusual. Scientists already knew that volcanoes were often located adjacent to trenches and that earthquakes occurred nearby. Hess began pondering the implications of this finding.

Hess obtained a Ph.D. in geology in 1932. His dissertation focused on the serpentinization of a large peridotite intrusive located in the Blue Ridge Mountains of Virginia. A peridotite is an igneous rock containing olivine and pyroxene; “intrusive” means the rock formed from magma without reaching the Earth’s surface. Serpentinization is a chemical process by which olivine and pyroxene transform into the mineral serpentine, changing peridotite into serpentinite. Hess remained interested in mineralogy throughout his career and published two classic papers: “Pyroxenes of Common Mafic Magmas” (1941) and “Stillwater Igneous Complex, Montana” (1960). NASA later named Hess the principal investigator for pyroxene studies of moon rock samples.

After receiving his doctorate, Hess taught at Rutgers University in New Jersey from 1932 to 1933, then worked as a research associate at the Geophysical Laboratory of the Carnegie Institution of Washington, D.C. for one year. He obtained a teaching position in geology at Princeton University in 1934 and remained associated with Princeton until 1966. That same year he married Annette Burns; they eventually had two sons, George and Frank.

From the Atlantic to the Pacific. Hess had joined the U.S. Navy as a lieutenant to facilitate operations on a navy submarine used for gravity studies following his research with Vening‑Meinesz. He was in the naval reserves when Japan attacked Pearl Harbor on December 7, 1941, and he reported for active duty the next morning. Because of his submarine experience, he became an antisubmarine warfare officer responsible for detecting enemy submarine operation patterns in the North Atlantic. Hess advised the U.S. Navy that German submarines might be using cloud cover north of the Gulf Stream (a current running from the Gulf of Mexico up the U.S. Atlantic coast) to escape detection when surfacing. This suggestion led to clearing out submarines in the North Atlantic within two years.

Hess arranged a transfer to the decoy vessel USS Big Horn to test the submarine detection program’s effectiveness and remained on sea duty for the rest of the war. As commanding officer of the transport vessel USS Cape Johnson, Hess carefully chose his travel routes to Pacific landings on the Marianas, Philippines, and Iwo Jima, continuously performing scientific surveying and profiling of the ocean floor across the North Pacific Ocean.

Hess took advantage of his navy service by patterning travel routes to facilitate studies on ocean floor geology. He installed a deep‑sea echo sounder on his transport ship and used it continuously. This equipment measured seafloor depth by sending a sound signal downward and timing its return after bouncing off the ocean floor. Using these data, Hess constructed bathymetric maps showing the contours of the ocean floor across a large area of the Pacific. While collecting bathymetric data, he discovered flat‑topped underwater volcanoes that he named guyots after Swiss geologist Arnold Guyot, who had founded Princeton’s geology department in 1854. Hess remained in the naval reserves until his death, attaining the rank of rear admiral in 1961.

Baffling Marine Geology Discoveries. In July 1950, a group of scientists studying the Pacific seafloor made surprising discoveries that influenced Hess’s developing ideas about the origin and evolution of ocean basins. Scientists had believed that the oceanic crust was mostly flat and extremely thick due to billions of years of sediment accumulation from continental erosion. Using explosives and seismic waves, the crew determined the oceanic crust’s thickness to be only about four miles (7 km)—much thinner than expected, since continental crust was known to be about five times thicker.

Geologists thought the oceans had existed for 4 billion years, so why was there so little accumulated sediment? The crew took samples from guyots and was surprised to find coral rather than rocky sand. Coral is usually found in shallow areas, yet some guyots lie two miles (3.2 km) underwater. The coral proved to be approximately 130 million years old—hundreds of millions of years younger than expected. Fossil evidence further confirmed the relatively young age of the ocean floor.

A few years later, American oceanographer Maurice Ewing observed that oceanic ridges (underground mountain ranges) have rifts (valleys) running through their centers. The rift seam appeared to result from splitting apart, which was interesting because terrestrial mountain ranges were believed to arise from compression—chunks of land being forced together. The presence of lava and absence of sediment along the ridges also baffled scientists.

Proposal of Seafloor Spreading. Hess contemplated these unexpected discoveries in relation to Alfred Wegener’s theory of continental drift (1912). After noticing that the east coast of South America and the west coast of Africa fit together like jigsaw puzzle pieces and collecting additional fossil evidence, Wegener concluded that the continents had once been connected but split and drifted thousands of miles apart. Wegener offered no explanation for the mechanism driving continental drift, but Hess modified Wegener’s theory and provided a plausible driving force. Wegener thought that drifting continents somehow plowed through the ocean floor, whereas Hess believed they rode along passively as the ocean floor was carried away from ridges.

In 1960 Hess first proposed his theory of seafloor spreading. He explained the mid‑oceanic ridge splitting, the presence of lava surrounding ridges, and the thinness of oceanic crust. The model suggested that oceanic crust was splitting at a seam—the rift in the center of the mid‑oceanic ridge. As it split, melted magma rose through these weak areas and erupted as lava from the spreading ridges, forming new crust composed mostly of basalt as it cooled. The newly formed crust was then carried away by the mantle spreading laterally beneath the crust, away from the ridge, eventually diving back beneath the surface at trenches located along compressional faults. At the trenches, slabs of crust would be forced under other slabs, back into the Earth’s mantle.

Hess proposed convection (heat transfer by fluid motion) as the driving force behind seafloor spreading. The mantle, located just beneath the crust, is solid rock formed mostly of iron and magnesium minerals, but just below the surface it may melt into magma. Although solid, the mantle can flow slowly like a fluid, forming convection cells. Hotter, less dense rock rises; as it cools, it sinks. When it sinks, it pulls the overlying crust down into the mantle. Crust that disappeared into trenches was constantly replenished by newly erupted lava at the ridge. Hess called this paper “an essay in geopoetry” and issued preprints in 1960. The official paper, “History of Ocean Basins,” was published in 1962 by the Geological Society of America in the symposium volume Petrologic Studies: A Volume in Honor of A. F. Buddington (Buddington had been Hess’s petrology professor at Princeton and a close friend).

Hess’s paper was well received, and additional paleomagnetic evidence supporting seafloor spreading soon emerged. Paleomagnetism reconstructs Earth’s ancient magnetic field and continental positions from magnetization in ancient rocks. Because Earth acts like a giant spherical magnet, iron‑rich rocks become magnetized in alignment with the Earth’s poles. When magnetic rocks solidify, they permanently record the direction of Earth’s magnetic field at that time. Every few million years, Earth’s magnetic poles reverse; thus, rock ages and magnetization directions provide a history of magnetic pole directions.

In 1963, two young British geologists—Fred J. Vine and Drummond H. Matthews—and Lawrence Morley of the Canadian Geological Survey independently described magnetic anomalies that further supported seafloor spreading. Magnetic stripes parallel to the mid‑oceanic ridge extended laterally from it, with reversals in magnetic direction every several hundred kilometers. Vine thought that when magma erupted from the rift in the ridge cooled, it magnetized in the direction of the current magnetic field and was then carried away laterally from the ridge. Hess wholeheartedly accepted this hypothesis. Further studies in 1966 showed that the magnetic stripes were indeed parallel to the ridges and bilaterally symmetrical in both magnetics and age. This evidence, together with findings that seafloor becomes older farther from ridges, confirmed lateral crustal movement and further established Hess’s theory of seafloor spreading, as did later fossil evidence and underwater core samples.

Admired and Honored. Beginning in 1962, Hess chaired the Space Science Advisory Board of the National Academy of Sciences, which advised NASA. In 1966, he was at Woods Hole, Massachusetts, chairing a meeting on scientific objectives of lunar exploration when he began having chest pains. He died of a heart attack on August 25, 1969, and was buried at Arlington National Cemetery.

Hess was elected to several academic societies, including the National Academy of Sciences (1952), the American Philosophical Society (1960), and the American Academy of Arts and Sciences (1968). He served as president of the Geodesy Section (1951–53) and the Tectonophysics Section (1956–58) of the American Geophysical Union, the Mineralogical Society of America (1955), and the Geological Society of America (1963). He was also appointed chairman of the Committee for Disposal of Radioactive Wastes, chairman of the Earth Sciences Division of the National Research Council, and chairman of the Space Science Advisory Board of the National Academy of Sciences. Along with oceanographer Walter Munk, he was a principal player in the Mohole Project, which aimed to drill beneath the oceanic crust into the mantle.

The Geological Society of America awarded Hess the Penrose Medal for distinguished achievement in the geological sciences in 1966, and NASA awarded him a Distinguished Public Service Award posthumously. Because of his outstanding achievements, the American Geophysical Union created the Harry H. Hess Medal for outstanding research on the constitution and evolution of Earth and sister planets.

Hess’s friends described his personality as puckish and courageous. Part of his greatness as a scientist was his willingness to entertain new ideas, even if they conflicted with his own previous conclusions. After all, wrong ideas sometimes usher in new eras of scientific accomplishment. By suggesting that ocean basins are continuously recycled, Hess explained why seafloor spreading does not cause Earth to grow, why the sediment layer on the ocean floor is thinner than expected, and why oceanic rocks are younger than continental rocks. Hess’s model of seafloor spreading has become foundational knowledge in the geological sciences and evolved into the theory of plate tectonics. Many questions regarding forces deep within Earth are still actively investigated today.

FURTHER READING: Carruthers, Margaret W., and Susan Clinton. Pioneers of Geology: Discovering Earth’s Secrets. New York: Franklin Watts, 2001.
Hess, Harry H. “Comments on the Pacific Basin.” Geological Survey of Canada. Special Paper (1966): 311-316.

“The Oceanic Crust.” Journal of Marine Research 14 (1955): 423-439.
“Geological Hypotheses and the Earth’s Crust Under the Oceans.” Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences 222 (1954): 341-348.