Great Geologists | 111
After the war, increasing amounts of data on the ocean crust, ocean floor topography and sedimentation were gathered by
the international scientific community at large. Hess saw his role as the synthesiser of these data, providing explanations
of what they implied, and his ideas began to evolve very rapidly. A strong believer that science moves forward in an
iterative process, he was not afraid to advance a hypothesis and then overturn it as new data emerged. This constant
development and refining of hypotheses were important steps towards the concept of sea-floor spreading, which he first
put forth in 1960. One might say that Hess carried out his thinking in public.
Building on ideas first introduced by Arthur Holmes and others, Hess believed that mantle convection might provide an
explanation for deformation of oceanic crust. By 1953, he had turned his attention to the mid-ocean ridge known to be
in the Atlantic. Having previously considered this to be an old, folded mountain belt, he now related its origin to upward
convection in the mantle driving intrusion and uplift. When this convection ceased, the ridge subsided. However, he
was soon forced to reverse the notion once he realized that rising convection causes deserpentinisation (serpentine is
transformed into olivine at temperatures above 500°C and water is released) and, hence, sinking of the ocean floor. This
concept also provided him with a mechanism to sink guyots deep below sea level. (The fact that some guyots had Late
Cretaceous fossils at their crests challenged Hess’s original idea of them being Precambrian islands.)
The late 1950s saw Hess constantly updating his models for the origin of oceanic features, such as mid-ocean ridges,
guyots and island arcs. The tectogene hypothesis became increasingly challenged. As the debate for and against
continental drift grew, Hess was initially a “fixist,” arguing in 1955 that Early Palaeozoic folded mountain belts in North
America and north-western Europe might continue beneath the Atlantic and be hidden by younger sediments. However,
by 1959, he was happy to switch to “mobilism” as the first palaeomagnetic studies demonstrating polar wander paths
were published. Interaction with the Australian geologist, Warren Carey, who favoured an expanding Earth, but in doing so,
promoted mobilist ideas, may also have been an influence.
By 1960, Hess was ready to assess all the oceanic features, which he had spent much of his career discussing, in terms
of mobilsm. The result was a manuscript entitled Evolution of the Ocean Basins, a report to the Office of Naval Research
widely circulated in 1960, but not formally published until 1962 as History of the Ocean Basins in a Geological Society of
America publication. Hess envisaged that oceans grew from their centers, with molten material (basalt) rising up from
the Earth’s mantle along the mid-ocean ridges, driven by mantle convection. The presence of a rift valley at the center
of ocean ridges, as detected from bathymetric surveys by Mary Tharp and Bruce Heezen, was a crucial observation that
helped Hess to develop his ideas.
The extrusion of basaltic lava created new sea floor, which spread away from the ridge in both directions. The ocean ridge
was thermally expanded and, consequently, higher than the ocean floor further away. As spreading continued, the older
ocean floor cooled and subsided to the level of the abyssal plain, which is approximately 4 km deep. Ocean trenches were
areas where ocean floor was destroyed and recycled — a point that Hess expanded upon in a joint paper with Robert
Fischer, of the Scripps Institute of Oceanography, in 1963. His long-standing conundrum of the origin of guyots could also
be explained by this theory; i.e., they evolved from wave-eroded volcanic peaks that formed at ridge crests and then, as
spreading moved them away from the ridge, they subsided with the cooling oceanic crust they were resting upon.
Hess did not initially call this theory “sea-floor spreading” — that term was introduced by Robert Dietz in a 1961 Nature
paper, which contained many similar ideas to those that had been circulated by Hess in 1960. Once introduced, the term
became widely accepted.
Hess was well aware that his ideas were both provisional and controversial. His History of the Ocean Basins paper
contained the line, “I shall consider this paper an essay in geopoetry;” thereby, emphasising its speculative nature. If it
were poetry, it scanned well. The paper explained many observations in an integrated way and offered the solution that
proponents of continental drift had long sought — a mechanism for continental movement along the conveyor belt of sea-
floor spreading.