Hess, Harry Hammond (1906–1969)

US geologist who in 1962 proposed the notion of seafloor spreading. This played a key part in the acceptance of plate tectonics as an explanation of how the Earth's crust is formed and moves.

Hess was born in New York and studied at Yale and Princeton, where he eventually became professor. From 1931, he carried out geophysical research into the oceans, continuing during World War II while in the navy. Later he was one of the main advocates of the Mohole project, whose aim was to drill down through the Earth's crust to gain access to the upper mantle.

Building on the recognition that certain parts of the ocean floor were anomalously young, and the discovery of the global distribution of midocean ridges and central rift valleys, Hess suggested that convection within the Earth was continually creating new ocean floor, rising at midocean ridges and then flowing horizontally to form new oceanic crust. It would follow that the further from the midocean ridge, the older would be the crust – an expectation confirmed by research 1963.

Hess envisaged that the process of seafloor spreading would continue as far as the continental margins, where the oceanic crust would slide down beneath the lighter continental crust into a subduction zone, the entire operation thus constituting a kind of terrestrial conveyor belt.