Six-minute summary: Marie Tharp

Hello readers! It’s time for the final six-minute summary of 2025. We’ll be exploring the pioneering work of Marie Tharp, an American geologist and cartographer who created the first maps of the sea floor. She did this in the 1950s, without any computer assistance, by interpreting thousands of depth measurements collected by research ships. Her work not only revealed the landscape of the sea floor for the first time, but provided key evidence for the theory of plate tectonics, which revolutionised Earth science and changed the way we all view our planet.

A geologist by happenstance

Marie Tharp never intended to be a geologist. She was born in Michigan in 1920, and her education was disrupted by her father being sent all around the USA in his job as a soil surveyor, leading to Marie attending nearly two dozen schools in total. In 1939, she enrolled in Ohio University, where she joined many different courses, and graduated with bachelor’s degrees in English and Music, and with four minors in other subjects.

According to Tharp herself, it was the outbreak of WWII that catalysed her career. With so many men joining the armed forces, universities were losing students, so they started opening their courses to women in order to fill places. In 1943, Tharp and 10 other young women enrolled in a geology program at the University of Michigan, in response to adverts which promised them careers in the petroleum industry. It was the first time women had been admitted to the course.

Tharp graduated with a master’s degree in 1944, and this led to a job with an oil company in Tulsa, Oklahoma. Here, she ended up compiling and mapping the data collected by her male colleagues, as women weren’t allowed to do fieldwork. In search of something interesting to do in her spare time, she enrolled at the University of Tulsa and obtained a BSc in mathematics. She then secured a job at Columbia University, in the recently established Lamont Geological Observatory. This was where she would conduct the bulk of her pioneering work.

The Lamont Geological Observatory had been founded in 1949 by Maurice Ewing, who remained the director of research there. Ewing was recruiting adventurous technicians for Atlantic expeditions, to collect depth measurements with the continuous echo sounder that he had developed for the US Navy. He was also recruiting data analysts to decipher the tens of thousands of rows of latitudes, longitudes and depths sent back from the expeditions. Marie Tharp became one such data analyst, and was assigned to work for a researcher called Bruce Heezen – with whom she would work for most of her career.

The need for seafloor maps

Before 1950, there had only been limited attempts to map the seafloor. The earliest depth measurements in the Atlantic Ocean were conducted in the 19th century using lead weights, and in 1854, a plateau of higher ground had been identified between Europe and America, based on 200 measurements from Matthew Fontaine Maury. The east-west divide was further supported by temperature measurements from Charles Wyville Thomson on the HMS Challenger, which suggested that deep currents didn’t flow past the plateau. Still, it was infeasible to map the vastness of the ocean using only spot measurements with lead weights. Only by the 1950s had echo-sounding technology progressed far enough to make seafloor mapping possible – and by then, of course, the need for maps was keener than ever, due to the development of submarines in the Cold War.

Mapping the Atlantic

Ewing used the ship Atlantis to collect sounding profiles across the Atlantic between 1946 and 1952, and Marie Tharp collated these data to construct her maps. She also borrowed older datasets, such as a profile from the Stewart collected in 1921. Then, in 1951, Ewing acquired the Vema, on which he installed a precision depth recorder for more accurate readings. This allowed Tharp to differentiate between the smoother and rough-textured areas. The Vema was also the first research vessel to have a satellite navigation system, so the depth measurements could be positioned more accurately. Heezen was the chief scientist on the Vema, but Tharp, being a woman, was not permitted to join the voyages.

The data arrived at Lamont in tables of positions and depth measurements, which were then plotted onto charts. According to Tharp, “the ships’ tracks looked like a spider’s web, with rays radiating from Bermuda, where most of the research vessels took on supplies.” These tracks would zigzag whenever the ships fled from storms. Datapoints were scattered, if not entirely random, and Tharp had to construct depth contours between them by hand.

However, Tharp’s maps could not be published initially, as the US government didn’t want Soviet submarines to use them for navigation. So, to share their discoveries with the world, Tharp and Heezen worked with a painter named Heinrich Berann to create “physiographic diagrams” – drawing the seafloor like a landscape, as it would appear from the window of a submarine. It showed the flat abyssal plains and steep cliffs, and highlighted the huge mountain ridge down the centre of the Atlantic Ocean.

The first “map” of the Atlantic Ocean was published in 1957. Later, in 1977, National Geographic published a world map of the oceans, which had been painted with gargantuan dimensions of 6 by 3.5 feet. You can still find reproductions of this map on the walls of Earth Science departments around the world.

Finding the rift valley

Tharp’s greatest discovery was the rift valley in the centre of the Atlantic Ridge. She had created six east-west profiles across the North Atlantic in 1952, and noticed a v-shaped notch in the centre of each one. She knew that this wasn’t a coincidence, and suspected that this rift was a result of the seafloor pulling apart, shifting the continents of Europe and America away from each other. The theory of continental drift had been widely dismissed by the Earth Science community, having been proposed by Alfred Wegener in the 1910s. When Tharp presented her interpretation to Heezen, he dismissed it as “girl talk” (an infamous quote that overshadows the rest of his work).

However, Heezen had recently hired another researcher to plot the location of earthquake epicentres, and it soon became clear that many of these earthquakes aligned perfectly with the rift valley that Tharp had identified. At this point, in 1953, six months on from Tharp’s initial observation, Heezen started taking the rift valley seriously.

Around three years later, in 1956, Ewing and Heezen presented the discovery of the axial rift valley at a major conference in Toronto. Tharp was not listed as a contributor to the project, despite it being her discovery. And Ewing remained a firm disbeliever in continental drift, in spite of the evidence. To others, however, the evidence was irrefutable. When Heezen went to share maps of the rift valley at Princeton in 1957, they caught the attention of Harry Hess, who later went on to demonstrate seafloor spreading, thereby proving that Europe and America were drifting apart, and that the surface of the Earth was divided into shifting plates.

The existence of the rift valley was finally established in 1959, when it was photographed by Jacques Cousteau. He had actually intended to prove Heezen wrong – but had ended up proving him right. The photos were shown to huge crowds at the first International Oceanographic Congress in 1959, where Tharp was present. Later, she recalled seeing the “great black cliffs of the rift valley” and an image depicting “pillows of lava newly extruded from the centre of the Earth”. However, she still hadn’t received any appropriate recognition for her work.

Another rift emerges… Between Heezen and Ewing

In the early 1960s, Ewing and Heezen fell out. Ewing banned Heezen from all Lamont ships and data, and fired Marie Tharp (he couldn’t fire Heezen, who had a tenured faculty position). Tharp continued working on mapping projects for Heezen, being paid through research grants from the navy, but she had to do so from home. When Ewing moved to the University of Texas, Heezen could use Lamont data again – and Tharp used this to finish her maps.

Overshadowed and uncredited

Tharp was not credited in any of the major papers that Heezen and others published between 1959 and 1963. The same was true for many women in science at the time, who were treated merely as computing machines rather than independent thinkers. Tharp, however, made interpretations beyond anything an algorithm would have managed. The Lamont datasets were limited in their coverage, so she had to fill in a lot of gaps with intuition, informed by her knowledge of geology – and her maps are incredibly accurate when compared with modern methods, making her work all the more impressive.

Although Tharp’s contributions were crucial for advancing our understanding of our planet, she never worked in isolation. She was part of a team of data analysts and cartographers, using data collected by teams on research vessels. Heezen facilitated much of this work, even managing to collaborate with Russian scientists at the height of the Cold War, acquiring data from the Soviet ships Ob and Vityaz, which had surveyed the Indian Ocean. He also got data from Japanese ships between Capetown and Australia, data from the UK, Australia and South Africa, and several other US oceanographic institutions – all of which was fed into the maps that Tharp created.

These days, a woman like Marie Tharp would likely progress beyond being a mere data analyst – but in the 1950s and 60s, such career progression was impossible. Tharp was stuck with Heezen as her superior, and according to other scientists who worked with them, the pair often had heated arguments. Apparently, Tharp once tore up huge sheets of maps after Heezen followed his “intuition” and caused her to waste weeks of work. Another scientist remembers her throwing erasers and ink bottles at Heezen when he denied that the rift valley existed. Still, the pair continued to work together until Heezen’s unexpected death from a heart attack in 1977, on board a research vessel off the coast of Iceland.

Recognition at last

Marie Tharp is now recognised as one of the greatest cartographers in history, and thankfully, she lived long enough to receive this recognition herself. The Library of Congress displayed her maps in 1997, and in 2004, the “Marie Tharp Fellowship” was set up to support women scientists at Lamont University. She died in 2006, at the age of 86.

According to her own memoirs:

“I worked in the background for most of my career as a scientist, but I have absolutely no resentments. I thought I was lucky to have a job that was so interesting. Establishing the rift valley and the mid-ocean ridge that went all the way around the world for 40,000 miles – that was something important. You could only do that once. You can’t find anything bigger than that, at least on this planet.”

In summary…

Marie Tharp was a pioneering geologist and cartographer who did not receive the recognition she deserved until very late in her life. She was the first person to identify the rift valley in the centre of the Atlantic Ocean, and her maps provided crucial evidence for establishing the theory of plate tectonics, which forever changed the way we perceive our planet.

Happy reading, and have a lovely week!

Related posts:

Six-minute summary: Alfred Wegener (https://cwclaytonauthor.co.uk/2023/03/13/six-minute-summary-alfred-wegener/)

Six-minute summary: Inge Lehmann (https://cwclaytonauthor.co.uk/2023/11/12/six-minute-summary-inge-lehmann/)

The first maps of French volcanoes (https://cwclaytonauthor.co.uk/2025/06/08/an-18th-century-map-that-changed-science/)

Lessons from Victorian geologists (https://cwclaytonauthor.co.uk/2025/02/02/learning-from-old-science-books-part-one/)