1906 - 1969
Harry Hess became a professor of geology at Princeton in 1932. His specialty was the study of arced chains of islands with active volcanoes. When World War II started, he was part of the Naval Reserve. He commanded a submarine base during World War II, and remaining in the Reserves afterwards, rose to the rank of rear admiral.
Hess's first duty during the war was in New York City, estimating enemy positions in the North Atlantic. He then saw active duty, commanding an attack transport ship. He made four major combat landings, incuding Iwo Jima. His ship was equipped for sounding the ocean floor, which was useful for Hess both as ship's commander and as a geologist. In 1945, Hess measured the oceans to the deepest points to date, about seven miles deep. He discovered hundreds of flat-topped mountains on the Pacific floor. He named them guyots (after the first geology professor at Princeton), and found them very puzzling: They looked like their tops were eroded, but they were 2 kilometers under water.
After the war he continued researching guyots and midocean ridges, as well as various mineral studies. With the discovery in 1953 of the Great Global Rift, a volcanic valley running along the midocean ridges, Hess looked back at data he had collected during the war. In 1960 (and with further elaboration in 1962), he hypothesized that the sea-floor was spreading from vents in the Rift, where hot magma oozed up. As the magma cooled it forced the existing sea-floor away from the Rift on either side. This theory accounted for and united several separate puzzles in marine geology: the youth of the ocean floor, the presence of island arcs, the deep sea trenches, and the origin of the midocean ridges.
Hess's puzzling guyots fit into the sea-floor spreading picture, as well. It's believed they were once-active volcanoes which rose above the water's surface before being eroded by the atmosphere down to sea level. As the sea-floor moved (carrying the eroded volcanoes along with it) and subsided, the guyots were moved into deeper and deeper water.)
In 1950, Hess was made head of the geology department at Princeton. He was called on for advice during the Cuban Missile Crisis, and from 1962 until his death he chaired the Space Science Board, a NASA advisory group. In the late 1960s he helped plan the first landing of humans on the moon. He died in August, 1969, a month after the successful lunar mission of Apollo 11.