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In the 1940s, most of the world map was a flat blue space. Scientists expected the deep ocean floor to be a smooth plain of mud. Marie Tharp's maps showed mountains, canyons and a crack running down the middle of the Atlantic, and that crack became key evidence for the moving plates you now study in geography. This post tells how she found it, what she did when she was told she was wrong, and the working habits a student can borrow.
Who was Marie Tharp?
In her own account, "Connect the Dots", republished by Columbia's Climate School, Tharp says her father was a soil surveyor who made county soil maps, so the family moved constantly and she attended nearly two dozen schools. She studied English and music at Ohio University. Then, she wrote, "I never would have gotten the chance to study geology if it hadn't been for Pearl Harbor." With men away at war, the University of Michigan opened geology to women; she earned a master's degree, worked for an oil company in Tulsa and took a maths degree at night.
In 1948 she went to Columbia, where the geophysicist Maurice Ewing asked her, "Can you draft?" He hired her. She was born on 30 July 1920, and Columbia notes that she died in 2006.
How do you map a sea floor you cannot see?
The data came from echo sounders. Tharp explains that a ship sent out a sound ping and timed the echo from the bottom, and a stylus burned a continuous depth profile onto a paper strip. Cruises of the research ship Atlantis collected tens of thousands of depth readings across the North Atlantic between 1946 and 1952.
Tharp and her colleague Hester Haring plotted these readings on standard sheets at a scale of 1:1,000,000. A quick way to read that scale: 1 cm on the sheet is 1,000,000 cm on the Earth, which is 10 km. A 30 cm wide sheet therefore covers about 300 km of ocean.
The echo sounder itself is a small physics problem, and the NCERT Class 9 science chapter on sound sets one just like it. Sound travels down, reflects and comes back, so depth = (speed × time) ÷ 2. Using NCERT's figure of 1,500 m/s for sound in seawater and an echo that returns after 4 s, the depth is 1,500 × 4 ÷ 2 = 3,000 m. Forgetting to halve the time is the usual mistake in this kind of question.
What did she notice in the profiles?
By 1952 Tharp had assembled six roughly parallel profiles across the North Atlantic. "The individual mountains didn't match up," she wrote, "but the cleft did." Each profile showed a V-shaped notch in the centre of the ridge. She thought it might be a rift valley running along the whole crest.
Heezen, she recalled, "groaned and said, 'It cannot be. It looks too much like continental drift.'" He initially dismissed her reading as "girl talk." At that time, she wrote, almost everyone in the United States thought continental drift was impossible.
How was the rift confirmed?
The confirmation came from a second, independent data set. Heezen had hired Howard Foster to plot the positions of ocean earthquakes on maps of the same scale as Tharp's. When the two were laid together on a light table, the earthquake locations fell inside the valley. In mid-1953, about eight months after her first six profiles, Heezen accepted the idea.
They then traced the valley into the Gulf of Aden and the rift valleys of East Africa, and concluded that the Mid-Atlantic Ridge was part of a mid-ocean ridge system about 40,000 miles long, roughly 64,000 km, running through all the oceans. The Woods Hole Oceanographic Institution describes her as co-discoverer of the central rift valley and co-creator, with Heezen, of the first world ocean floor map, published in 1977.
The NCERT Class 11 geography chapter on the distribution of oceans and continents explains why this mattered: ocean floor mapping, the young age of rocks near the ridge crest and their matching magnetism led Harry Hess in 1961 to propose sea floor spreading. Our post on Alfred Wegener and continental drift covers the earlier part of that argument.
Was her work recognised?
Not at first. A 2002 Library of Congress profile notes that women were not allowed on the Lamont ship at the time, so Heezen made his early cruises without her. In 1997 the Library's Phillips Society named her one of the 20th century's outstanding cartographers, and in 2001 Columbia gave her its first Lamont-Doherty Heritage Award.
What can a student take from how Tharp worked?
Look for what repeats, not what stands out. The tall peaks did not line up; the small notch did.
Put different evidence on the same scale. Plotting depths and earthquakes at 1:1,000,000 is what let the two maps be compared at all.
Keep a view you can defend with data. Tharp kept looking for the rift in every new data set until the earthquakes settled it.
Expect to revise. She later corrected an error in her Indian Ocean map when new data arrived.
Some ideas, such as an echo bouncing off the sea floor, are easier to understand by watching than by reading. The Fermi Simulation Lab has interactive simulations for ideas like these that are easier to see than read. For the older story of how rocks record time, read our post on James Hutton and deep time.
Try it now: draw three depth profiles across an imaginary ridge, add a notch to each, and see how much clearer the pattern is when you stack them.
