Harry Hammond Hess: Spreading the seafloor
Harry Hess (1906-1969) in his Navy uniform as Captain of
the assault transport Cape Johnson during World War II. After the
war, he remained active in the Naval Reserve, reaching the rank of Rear
Admiral. (Photograph courtesy of Department of Geological and Geophysical
Sciences, Princeton University.)
Harry Hammond Hess, a professor of geology at Princeton University,
was very influential in setting the stage for the emerging plate-tectonics
theory in the early 1960s. He believed in many of the observations Wegener
used in defending his theory of continental drift, but he had very different
views about large-scale movements of the Earth.
Even while serving in the U.S. Navy during World War II, Hess was keenly
interested in the geology of the ocean basins. In between taking part in
the fighting in the Marianas, Leyte, Linguayan, and Iwo Jima, Hess -- with
the cooperation of his crew -- was able to conduct echo-sounding surveys
in the Pacific while cruising from one battle to the next. Building on the
work of English geologist Arthur Holmes in the 1930s, Hess' research ultimately
resulted in a ground-breaking hypothesis that later would be called seafloor
spreading. In 1959, he informally presented this hypothesis in a manuscript
that was widely circulated. Hess, like Wegener, ran into resistance because
little ocean-floor data existed for testing his ideas. In 1962, these ideas
were published in a paper titled "History of Ocean Basins," which
was one of the most important contributions in the development of plate
tectonics. In this classic paper, Hess outlined the basics of how seafloor
spreading works: molten rock (magma) oozes up from the Earth's interior
along the mid-oceanic ridges, creating new seafloor that spreads away from
the active ridge crest and, eventually, sinks into the deep oceanic trenches.
Hess' concept of a mobile seafloor explained several very puzzling geologic
questions. If the oceans have existed for at least 4 billion years, as most
geologists believed, why is there so little sediment deposited on the ocean
floor? Hess reasoned that the sediment has been accumulating for about 300
million years at most. This interval is approximately the time needed for
the ocean floor to move from the ridge crest to the trenches, where oceanic
crust descends into the trench and is destroyed. Meanwhile, magma is continually
rising along the mid-oceanic ridges, where the "recycling" process
is completed by the creation of new oceanic crust. This recycling of the
seafloor also explained why the oldest fossils found on the seafloor are
no more than about 180 million years old. In contrast, marine fossils in
rock strata on land -- some of which are found high in the Himalayas, over
8,500 m above sea level -- can be considerably older. Most important, however,
Hess' ideas also resolved a question that plagued Wegener's theory of continental
drift: how do the continents move? Wegener had a vague notion that the continents
must simply "plow" through the ocean floor, which his critics
rightly argued was physically impossible. With seafloor spreading, the continents
did not have to push through the ocean floor but were carried along as the
ocean floor spread from the ridges.
In 1962, Hess was well aware that solid evidence was still lacking to test
his hypothesis and to convince a more receptive but still skeptical scientific
community. But the Vine-Matthews explanation of magnetic striping of the
seafloor a year later and additional oceanic exploration during subsequent
years ultimately provided the arguments to confirm Hess' model of seafloor
spreading. The theory was strengthened further when dating studies showed
that the seafloor becomes older with distance away from the ridge crests.
Finally, improved seismic data confirmed that oceanic crust was indeed sinking
into the trenches, fully proving Hess' hypothesis, which was based largely
on intuitive geologic reasoning. His basic idea of seafloor spreading along
mid-oceanic ridges has well withstood the test of time.
Hess, who served for years as the head of Princeton's Geology Department,
died in 1969. Unlike Wegener, he was able to see his seafloor-spreading
hypothesis largely accepted and confirmed as knowledge of the ocean floor
increased dramatically during his lifetime. Like Wegener, he was keenly
interested in other sciences in addition to geology. In recognition of his
enormous stature worldwide, in 1962 Hess -- best known for his geologic
research -- was appointed by President John F. Kennedy to the prestigious
position of Chairman of the Space Science Board of the National Academy
of Sciences. Thus, in addition to being a major force in the development
of plate tectonics, Hess also played a prominent role in designing the nation's
space program.
URL: https://pubs.usgs.gov/publications/text/HHH.html
Last updated: 05.05.99
Contact: bhanks@usgs.gov
"Developing
the theory"