Geologist Marie Tharp mapped the ocean floor and helped solve one of science's biggest controversies

Geologist Marie Tharp sits at a large drafting table holding a pen and working on a map of the Atlantic Ocean floor.
Marie Tharp at her drafting table in Lamont Hall, circa 1961.Lamont-Doherty Earth Observatory and the estate of Marie Tharp
  • Until the 1950s, scientists didn't have a clear understanding of what the seafloor looked like.

  • Geologist Marie Tharp turned years of data into easily digestible maps.

  • She also discovered the Mid-Atlantic rift, which contributed evidence to the plate tectonics theory.

Why do Earth's continents look like they fit together? In the early 20th century, this was one of geology's greatest scientific mysteries and part of the solution lay at the bottom of the ocean.

Back then, no one knew what the ocean floor looked like — until one woman used her many talents to find out.

When she reflected on her life, geologist Marie Tharp recollected being able to fill in the blanks of the ocean floor, which she saw as a fascinating jigsaw puzzle. She pinpointed mountains, volcanoes, and canyons that lurked under oceans, much like they do above sea level.

"It was a once-in-a-lifetime — a once-in-the-history-of-the-world — opportunity for anyone, but especially for a woman in the 1940s," Tharp wrote.

A geologist with Columbia University's Lamont Geological Laboratory (now the Lamont-Doherty Earth Observatory), Tharp took sheets and sheets of depth readings and turned them into incredible maps. Her maps not only helped her see the ocean floor in a whole new way, but they also helped change the way geologists and oceanographers thought about the Earth.

When continental drift was a controversy

When Tharp was still in elementary school, some scientists were tearing apart Alfred Wegener's theory of continental drift.

A German meteorologist and astronomer, Wegener first presented his ideas in 1912. Like others, he'd noticed that the continents looked as if they had once fit together. Even though oceans separated them, their rock layers matched.

"It is just as if we were to refit the torn pieces of a newspaper by matching their edges," Wegener wrote in "The Origin of Continents and Oceans."

If he was correct, moving continents would explain many puzzling mysteries, like why Australian marsupials were more similar to South American animals than those in closer Asian countries.

Wegener's theory gained support, but many US scientists found it implausible. One influential geologist, Bailey Willis, called it a "fairy tale." Instead, Willis stuck with his own theory that there had once been land bridges between the continents that had since sunk.

By the time Tharp enrolled in a two-year geology program at the University of Michigan in 1943, her instructors likely didn't advocate for Wegener's theory.

There wasn't yet "any satisfactory unifying hypothesis that could explain the main features and processes at the Earth's surface," geologist Bettie Higgs wrote of Tharp's education.

Studying the ocean from land

When Tharp arrived at Columbia University for a job with Maurice Ewing at Lamont, she joined a group of about two dozen men and women on the geophysics team in 1948. The other women were mostly bookkeepers, assistants, or human calculators doing mathematical work.

Like many of her male colleagues, Tharp had a graduate degree in geology. But she wasn't allowed on research vessels. Some sailors still believed it was "unlucky" to have a woman aboard unless they were passengers. In 1956, graduate student Roberta Eike lost her fellowship after sneaking aboard a ship in Massachusetts. Tharp didn't sail on a mission until 1968.

Instead, Tharp spent much of her time assisting the men with calculations and diagrams. After about three years, she nearly quit. Whether she was overworked or frustrated at working on others' projects instead of her own isn't clear. But when she came back, she embarked on a partnership that changed the trajectory of her career.

Mapping the seafloor

By the early 1950s, echo sounders and precision depth recorders (PDRs) allowed scientists to continuously record seafloor depths, giving them a more complete picture than they'd ever had before.

The only problem was they produced loads of data. Bruce Heezen, who started at Lamont around the same time as Tharp, handed it off to her in 1952. There were long rolls of paper containing lines that rose and dipped, depending on how long it took the echo sounders' electronic ping to travel from the seafloor to the ship.

Peope stand around a large echo sounding machine with a large piece of paper and depth data
Recordings of the Australian continental slopes from a precision echo sounding recorder on a plotting table in the 1960s.Frank Albert Charles Burke/Fairfax Media via Getty Images

It was Tharp's job to translate 3,000 feet of paper and undulating lines into a map containing all the data from the various ships. To do so, she had to compile snippets from different trips to get a continuous line from coast to coast. And she had to do that up and down the continents, from Nova Scotia, Massachusetts, France, and the Strait of Gibraltar.

The result "looked like a spider's web," Tharp said, with lines resembling telephone wires spanning the Atlantic Ocean.

After weeks of looking at the data and plotting the lines, Tharp had noticed a pattern. She had about half a dozen lines running across the ocean, and many had a v-shaped dip in a similar spot, right on top of an underwater mountain chain, the Mid-Atlantic Rift.

It looked like a rift. But it couldn't be, Heezen told her, because that would be too much like continental drift. He and "almost everyone else at Lamont, and in the United States, thought continental drift was impossible," according to Tharp.

It would take Heezen months to accept what he'd dismissed as Tharp's "girl talk."

The ups and downs at the bottom of the sea

In the meantime, Tharp and Heezen decided to use physiographic diagrams to capture what they were seeing in the data. If the echo sounder recorded a large mountain, Tharp would make it appear tall next to shorter mounds. In spots where ships hadn't recorded depth information, Tharp made educated deductions.

Shading and closely spaced lines gave an illusion of depth, making pointy peaks and flat sea mounts seem to rise off the page. Amateurs and experts alike could easily get a sense of the underwater topography from Tharp's map.

"But we also had an ulterior motive," Tharp wrote. "Detailed contour maps of the ocean floor were classified by the US Navy, so the physiographic diagrams gave us a way to publish our data."

Heezen had graduate student Howard Foster placing dots at the latitudes and longitudes of tens of thousands of earthquake epicenters on another map. When Fosters' map was laid on top of the Tharp's diagram, the quakes snaked along the Mid-Atlantic Ridge where she'd found the rift.

A map of the world with the mid-ocean ridge highlighted in red
The mid-ocean ridge traverses the globe.USGS

Tharp was then able to use other seismic data to extend the ridge into the Indian Ocean and the East African Rift Valley. What Heezen would later call a "monstrous scar" split the globe into pieces and the theory of continental drift now seemed undeniable.

A geological rift 

In 1956, Heezen and Ewing, the head of the Lamont lab, published a paper noting a ridge covering about 40,000 miles of the ocean's floor, without crediting Tharp. Two years later, at the 1959 International Oceanographic Conference, Heezen presented on continental drift and the rift.

By this time, other researchers in several fields were accumulating evidence that aligned with Wegener's theory. Proof of Tharp's rift came from an unexpected source. At the conference, underwater explorer Jacques Cousteau showed a video he'd filmed of the rift valley with an underwater camera.

Jacques Cousteau wears a red hat and steers his boat, Calypso.
French explorer Jacques Cousteau in 1973 on his boat, Calypso.ABC Photo Archives/Disney General Entertainment Content via Getty Images

Originally, Cousteau had planned to prove Tharp and Heezen wrong. Instead, the footage "helped a lot of people believe in our rift valley," Tharp said.

Over the next decade, scientists in an array of fields contributed to the theory of plate tectonics with new explanations of everything from seafloor faults to the formation of the Earth's crust.

Marie Tharp's legacy

Tharp and Heezen continued to collaborate until his death in 1977. Their final project together was the World Ocean Floor Map. They recruited artist Heinrich Berann, who had worked with them on many maps for National Geographic, to paint it.

The map depicts the mid-ocean ridge zigzagging its way around the world. It looks like if you ran your fingers over the paper, you'd be able to feel where it rises and falls.

A colorful world map by Heezen, Tharp, and Berann showing the ocean floor's mountain ranges and rift valleys
The Heezen-Tharp “World Ocean Floor” map painted by Heinrich Berann.Marie Tharp Maps, LLC

After Heezen's death, organizations that had hired him and Tharp to work on projects reassigned them. A few years later, she was asked to retire from Lamont. It's not clear why, and Tharp biographer Hali Felt noted that at 63 she wasn't "the oldest person there by any means."

Tharp continued working on her own projects and started a cartography business. She protected and promoted Heezen's legacy, but she also "demanded fair recognition for work that was hers," according to Higgs.

Her contributions to oceanography and geology gained recognition with several awards, including the National Geographic Society's Hubbard Medal and the Woods Hole Oceanographic Institution's Mary Sears Woman Pioneer in Oceanography Award.

In 1997, the Library of Congress included some of Tharp's maps in an exhibition displaying items from Abraham Lincoln, Thomas Edison, and George Washington.

Tharp died in 2006. Over 70 years since she watched her colleagues head off in ships she wasn't allowed to sail on, the US Navy announced it was renaming a research vessel in her honor, the USNS Marie Tharp.

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