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People Behind the Science

Harry Hammond Hess (1906–1969)

arry Hess was the U.S. geologist who 1962 the notion of deep-sea spreading. This
Hplayed the key part in the plate tecton- contended that convection within Earth
ics revolution of the 1960s. was continually creating new ocean floor at
Born in New York, Hess first studied elec- mid-ocean ridges. Material, Hess claimed,
trical engineering and then trained in geol- was incessantly rising from Earth’s mantle
ogy at Yale and later Princeton universities. to create the mid-ocean ridges, which then
From 1931, he began geophysical researches flowed horizontally to constitute new oce-
into the oceans, accompanying F. A. Vening anic crust. It would follow that the farther
Meinesz on a Caribbean submarine expedi- from the mid-ocean ridge, the older would
tion to measure gravity and take soundings. be the crust—an expectation confirmed by
During World War II, Hess continued his research in 1963 by D. H. Matthews and his
oceanographic investigations, undertaking student F. J. Vine into the magnetic anoma-
extended echo-soundings while captain of the lies of the seafloor. Hess envisioned that
assault transport USS Cape Johnson. During the process of seafloor spreading would
the war years, he made studies of the flat- continue as far as the continental margins,
topped sea mounts that he called guyots (after where the oceanic crust would slide down
Arnold Guyot, an earlier Princeton geolo- beneath the lighter continental crust into a
gist). In the postwar years, he was one of the subduction zone, the entire operation thus
main advocates of the Mohole project, whose constituting a kind of terrestrial conveyor
aim was to drill down through Earth’s crust to belt. Within a few years, the plate tectonics
gain access to the upper mantle. revolution that Hess spearheaded had proved
The vast increase in seabed knowledge entirely successful. Hess’s role in this “revo-
(deriving in part from wartime naval activi- Building on Maurice Ewing’s discovery of the lution in the earth sciences” was largely due
ties) led to the recognition that certain parts global distribution of mid-ocean ridges and to his remarkable breadth as geophysicist,
of the ocean floor were anomalously young. their central rift valleys, Hess enunciated in geologist, and oceanographer.

Source: From the Hutchinson Dictionary of Scientific Biography. © Research Machines plc 2003. All Rights Reserved. Helicon Publishing is a division of Research Machines.

Another proposed mechanism of plate motion is called the slab may also be increased by loss of water and reforming of
slab-pull (Figure 18.20). In this mechanism, the sub ducting plate minerals into denser forms.
is colder and therefore denser than the sur rounding hot mantle, What is generally accepted about the plate tectonic the-
so it pulls the surface part of the plate downward. Density of ory is the understanding that the solid materials of Earth

Trench
Ridge

Lithosphere
80 to 100 km
Asthenosphere

?
Plate slides downhill
on this surface

FIGURE 18.19 Ridge-push. A plate may slide downhill on the sloping boundary between the lithosphere and the asthenosphere at the
base of the plate.

18-15 CHAPTER 18 Plate Tectonics 469