Marie Tharp – Centenary Year – 2020

The summer of 2020 marks the centenary year of the birth of one of the great cartographers of the 20th century, Marie Tharp, who was born on 30th July 1920.

Her story is little known outside the rarefied sphere of the Earth Sciences and it is worth repeating and celebrating here. It is one of a struggle against adversity to make a ground breaking discovery in oceanography which contributed to the growing evidence for the theory that would later become known as plate tectonics.

Five years before her birth, the German meteorologist Alfred Wegener had published a book “The Origin of Continents and Oceans”. Wegener, like others before him, had noted the almost jigsaw-like fit of the east coast of South America with that of the west coast of Africa, but he also drew on a number of other lines of evidence including geology and palaeontology to propose a theory in 1912 which became referred to as continental drift and which he expounded in his book.

The modern day continents had once been locked together in one large landmass, named Pangaea (from the Greek for “all Earth”), but had subsequently drifted apart. The theory was not widely accepted at the time mainly due to a lack of evidence for a convincing mechanism.

Marie’s father was a soil surveyor for the US Department of Agriculture. As a child she had helped him with his work which gave her an early insight into map making. She graduated from Ohio University in 1943 with degrees in English and Music, two years after the US had entered World War II. The conflict opened up opportunities for women in areas that had previously been the preserve of men, such as the oil industry. Marie completed a master’s degree in Petroleum Geology and took a job as a geologist with an oil company in Oklahoma but, as a woman in those times, she was not allowed to work in the field and was confined to office work collating data and maps. Dissatisfied, she moved to New York in 1948 and took a job as a research assistant at Columbia University under Maurice “Doc” Ewing.

He was a pioneering research geophysicist and in the following year became the founder director of the university’s Lamont Geological Observatory (later the Lamont-Doherty Earth Observatory – LDEO).

In the years following the publication of Wegener’s hypothesis, a number of scientists had been working on a theory of plate motions. In 1920 the British geologist Arthur Holmes had proposed that plate boundaries might exist under the oceans and, later in the same decade, that the driving force for plate motions could be large scale convection currents in the Earth’s upper mantle. During the 1930s, 40s and 50s this theory gained some traction mainly amongst scientists in Europe.

Over in the US, Maurice Ewing assembled a research team at Lamont including Frank Press and Jack Oliver (seismologists), Joe Worzel (gravity & acoustics), Jack Nafe (oceanographer), and Bruce Heezen and Marie Tharp (geologists and cartographers). At the start of the 1950s knowledge of the ocean floors was scant to say the least. There was known to be a mid-Atlantic ridge, but apart from that, it was generally assumed that the ocean floors were largely flat and featureless. Interestingly the existence of the ridge was discovered during an oceanographic survey expedition by HMS Challenger (1872 to 76) which measured depths with a weighted line marked in 25 fathom intervals (150ft, 46m).

In 1947 Maurice Ewing and his team had acquired data with the Woods Hole Oceanographic Institution (WHOI) research vessel the Atlantis and had shown the basaltic (rather than continental granitic) nature of the basement rocks and that the oceanic crust was much thinner than continental crust.

At Lamont, Heezen and Tharp were teamed together, he as researcher and she as cartographer. He acquired data on the research vessel the Vema and Marie, again due to her gender, was not at that time allowed to go offshore on a vessel as field crew. Fortunately, society has advanced somewhat since those days.

One of Marie’s earliest achievements was in 1952 when she processed thousands of echo sounder recordings in the North Atlantic to compile six transatlantic west-east bathymetric profiles at different latitudes. The most northerly profile started at around 40 degrees north (Massachusetts) and the most southerly at around 8 degrees south (Recife, Brazil).

She identified a V-shaped notch running through the axis of the ridge which she believed to be a rift valley, where new material welled up as the rift pulled apart under tension. Heezen had been an advocate of Carey’s “expanding Earth” theory and was originally sceptical of the rift valley proposal which he famously dismissed as “girl talk”. However, after the epicentres of oceanic earthquakes were plotted and found to lie along the rift, the evidence was undeniable.

A pioneering paper by the British physicist Keith Runcorn in 1956 and successive work by his research group at Newcastle University on magnetic polar wandering measurements in Europe and the US lent significant support to the theory of the movement of the continents. Runcorn's research in palaeomagnestism gave fresh impetus to the study of plate motions.

Heezen and Tharp published the first physiographic map of the North Atlantic basin in 1957.

This used the artistic techniques of hachuring and stippling to represent the topographic features of the seabed. This was the old school method of terrain modelling. It was a painstaking process, drawn by hand, and resulted in a map that is also a work of art. The map showed that the mid-Atlantic rise with its axial rift was offset in sections along its length by what would later be understood to be transform faults (Wilson). Tharp was subsequently able to demonstrate that this pattern extended into the South Atlantic and, indeed, similar structures existed in the Indian Ocean, Persian Gulf, Gulf of Aden and Red Sea forming a continuous global rift zone of some 40,000 km in length.

The development of the modern day theory of plate tectonics accelerated in the 1960s with the pioneering work on seafloor spreading by Hess, Dietz, Vine, Matthews and Morley and indeed many others and that deserves to be treated in more detail in another article.

Following the publication of the first physiographic map, Tharp and Heezen went on to map the other ocean basins and later collaborated with the Austrian alpine artist Heinrich Berann.

Berann painted the World Ocean Floor which was sponsored by National Geographic and published in 1977, collating the past 25 years of Marie’s work in one global map.

Moving on to the digital age, the GEBCO organisation (General Bathymetric Chart of the Oceans) was established to provide publicly available bathymetry datasets of the world’s oceans. The Seabed 2030 initiative is a collaborative project between GEBCO and the Nippon Foundation with the aim to facilitate the complete mapping of the global oceans by the year 2030. Currently, in 2020, around one fifth of the ocean floor has been mapped to modern standards and significant momentum now exists with many international partners contributing to data collection and technical innovation.

One such example has been the recent return to Plymouth of the SEA-KIT uncrewed surface vessel (USV) Maxlimer on 14th August 2020 after 22 days of successful remote survey operations on Europe’s continental margin. The future mapping of the ocean floors undoubtedly belongs to USVs and digital mapping software, but let us not forget the ground-breaking work of one of the great oceanographic cartographers of the last century, Marie Tharp.