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Hess, Harry Hammond

crust ocean sea mid

(1906–69) US geologist and geophysicist: proposed sea-floor spreading hypothesis.

Hess spent most of his academic life at Princeton University, moving there in 1934. During the Second World War he distinguished himself in the US Navy by conducting echo-sounding work in the Pacific, during which he discovered a large number of strikingly flat-topped seamounts, which he interpreted as sunken islands, naming them guyots (after Arnold Guyot (1807–84), an earlier Princeton geologist).

Following the war there was a great increase in knowledge about the sea bed, and it became apparent that parts of the ocean floor were anomalously young. In 1962, following the discovery of the global extent of the mid-ocean ridges and their central rift valleys by , Hess proposed his sea-floor spreading hypothesis to account for these facts. He suggested that material was continuously rising from the Earth’s mantle to create the mid-ocean ridges, which then spread out horizontally to form new oceanic crust; the further from the mid-ocean ridge, therefore, the older the crust would be. He envisaged that this process would continue as far as the continental margin, where the oceanic crust would sink beneath the lighter continental crust into a subduction zone, the whole process thus forming a kind of giant conveyor belt . Palaeomagnetic and oceanographic work, notably by , confirmed the hypothesis. Later, as chairman of the Space Science Board of the National Academy of Sciences, Hess also had an influential effect on the American space programme.

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