Great Geologists | 103
Marie Tharp working on a map of the Atlantic Ocean floor in the
early 1950’s. Note the profiles that were to reveal the rift within the
mid-ocean ridge.
Ocean-floor contour maps were classified as confidential until
1961, so Tharp and Heezen used “physiographic mapping,”
developed by the Columbia University geomorphologist
Armin Lobeck, to portray a somewhat three-dimensional
representation of ocean-floor topography. Making such maps
required a combination of geological and geomorphological
knowledge, plus mathematical skills to convert the raw
bathymetric data into profiles, maps and diagrams. Tharp’s
diverse education, therefore, stood her in good stead. The
process of creating profiles and then stitching these together
to create the three-dimensional physiographic diagrams was
a painstaking process and involved some degree of informed
speculation on the geomorphology between data points.
Nonetheless, the results were striking and once the first map
was published in 1957 (an article on assisting in the placement
of submarine cables), there was considerable interest in their
production.
By the mid-1960s, the National Geographic Society had
interest in the maps reaching a wider audience. This resulted
in a classic series of maps being published between 1967 and
1971 in National Geographic Magazine that documented the
ocean-floor of the Indian, Atlantic, Pacific and Arctic oceans.
The Society hired an Austrian artist, Heinrich Berann, to colour
the black and white originals produced by Tharp and Heezen,
producing extremely striking images. Their last project,
Marie Tharp and Bruce Heezen with an early version of
their Atlantic Ocean floor map.
completed in 1977, was the much-celebrated map of the entire
global ocean-floor, funded by the US Office of Naval Research.
Ever since the Challenger expedition of 1872, it had been
known that the floor of the central Atlantic Ocean possessed
an undersea rise; but until the detailed wartime bathymetry
became available, its size and character were unknown. Soon,
submerged mountain chains were also known from the Indian
and Pacific oceans as the true nature of the ocean floor finally
became apparent.
By 1960, the notion of seafloor spreading was beginning
to take hold in the geology/oceanography community. The
Princeton professor and former naval officer Harry Hess
speculated, as had Arthur Holmes many years before, that
convective overturn within the mantle was driving migration
of the crust. Central to this argument were the discoveries
regarding the nature of mid-ocean ridges. It was Marie Tharp,
who in 1952 had demonstrated the existence of medial graben
along ridge crests, and then with an initially skeptical Bruce
Heezen, had suggested that the crust was extending at right
angles to the trend of the ridges. This was consistent with the
notion that the ridges were the sites of upwelling convective
currents and the ocean crust migrated laterally away from
the ridges at rates approximating 1cm/year. Maurice Hill and
John Swallow of the University of Cambridge independently