110 | Great Geologists
The Bear Seamount, a guyot from the North Atlantic.
The age of the ocean crust demonstrates sea-floor spreading.
Whilst undertaking his PhD research, Hess had the opportunity to participate in gravity and bathymetric surveys in the
Caribbean Sea aboard the submarine USS S-48. This experience and a subsequent survey aboard the USS Barracuda in
1937 would be the catalysts sparking his interest in marine geology. The S-48 surveys were carried out along with the
Dutch geophysicist, Vening Meinesz, who became a mentor to Hess as they discussed the origin of the oceanic and
gravimetric features they had observed.
Of particular interest was the coincidence of negative gravity anomalies and ocean deeps adjacent to island arcs,
in features Hess and Meinesz described as “tectogenes,” which were ascribed to down-buckling of the crust. No
explanation for the warping of the crust was given, although the concept would subsequently be adapted thirty years later
as part of the process of subduction. Hess speculated that sediment infilling the basin above a tectogene would eventually
be deformed as tectogenesis continued, forming mountain belts, such as the Alps. The ultramafic serpentines of his PhD
research had their place in this theory, too. They were intruded from the mantle in an early stage of tectogenesis and
subsequently uplifted and deformed. Hence, serpentine belts, which Hess knew from the Alps and many other ancient
mountain belts, were evidence for deformed and uplifted tectogenes. These ideas now seem far from plate tectonics but,
nonetheless, were stepping stones towards that understanding.
Hess’s research activities were put on hold in 1941, with the entry of the U.S. into World War II; although, as already
mentioned, the sonar aboard the USS Cape Johnson allowed him to gather valuable bathymetric data from the Pacific
Ocean. After the war, he was able to evaluate the implications of these data. A surprising discovery was the presence
of twenty deeply submerged, reefless, flat-topped seamounts that he named “guyots” after Arnold Guyot, the Swiss
oceanographer, who founded Earth Sciences at Princeton. A further 140 guyots were identified through examination of
bathymetric charts from the U.S. Navy’s Hydrographic Office. They were circular or oval in shape and ranged from two to
sixty miles in diameter. Their origin presented Hess with a puzzle. The flat tops suggested erosion by wave action, but if
this were so, why were reefs not present?
Hess’s first solution was to suggest that these guyots were Precambrian oceanic islands formed before the advent of
reefal organisms. During the time since their formation, the sea level had risen because of constant sedimentation on the
ocean floor, along with isostatic adjustments. His observations and ideas on guyots were published in 1946, in a paper
that firmly established Hess’s already considerable scientific reputation, even though the erroneous theory on the origin of
guyots would subsequently be rejected, not least by Hess, himself.