Scientists once rejected continental drift, then a woman in 1977 drew the ocean floor and changed Earth science forever

Marie Tharp ocean floor map: For much of the mid-20th century, many American scientists rejected continental drift, the controversial idea that Earth's continents were not fixed in place. Marie Tharp's seafloor maps, culminating in a world map completed in 1977, helped tilt scientific opinion toward acceptance of continental drift and clear a path for the emerging theory of plate tectonics, as per a report.

Created by geologist and cartographer Marie Tharp, geologist Bruce Heezen and Austrian painter Heinrich Berann, the map gave scientists and the public their first glimpse of what the world's ocean floor looks like. It marked the culmination of more than three decades of Tharp's work at Columbia University.

A career shaped by opportunity and timing

Born in Michigan in 1920, Tharp grew up accompanying her father, who surveyed land and made maps for the US Department of Agriculture's Bureau of Soils, as per a Science News report. Because his work frequently moved the family, she attended nearly two dozen schools before graduating from high school.


During World War II, the shortage of male students prompted the University of Michigan's geology department to open its doors to women. Having already taken geology classes, Tharp seized the opportunity and earned a master's degree in geology in 1943 after completing a summer field course in geologic mapping and working part time as a draftsperson for the US Geological Survey, according to the report.

After taking a job with an oil company and later earning a second master's degree in mathematics from the University of Tulsa, she moved to New York City in 1948. Although her qualifications earned her an interview at Columbia University, the only position available to a woman was as a draftsperson assisting male graduate students in geology.

The following year, Tharp became one of the first women employed by Columbia's newly founded Lamont Geological Observatory, where she began working closely with geologist Bruce Heezen.

Mapping an unseen world

While Heezen and other scientists collected ocean data at sea, Tharp analyzed, plotted and mapped the information on land. Women were often not allowed aboard research ships, leaving them to analyze and interpret the data collected by others.

Using echo soundings gathered by research vessels, Tharp created detailed profiles of the Atlantic seafloor. As she examined the data, she noticed a notch near the crest of the Mid-Atlantic Ridge in profile after profile. She believed the notches represented a continuous rift valley that might be where molten material rose from below, forming new crust and pushing the ocean floor apart.

At the time, Heezen, like many American scientists, considered continental drift "almost a form of scientific heresy," as per the Science News report. It took Tharp more than a year to convince him the rift valley was real, and several more years for the pair to complete their first map of the North Atlantic in 1957.

Maps that helped change scientific thinking

To publish their work despite U.S. Navy restrictions on detailed topographic maps, Tharp and Heezen used physiographic diagrams, creating three-dimensional sketches of the ocean floor based on geological interpretation.

As they continued mapping the Atlantic and later other oceans, evidence accumulated for a continuous mid-ocean ridge with a central rift valley stretching about 60,000 kilometers around the globe. Their physiographic maps gave scientists a visual comparison with familiar continental landforms, helping convince many that the submarine rift valley marked where the continents on either side of the Atlantic had pulled away from one another. The maps helped strengthen support for continental drift and clear a path for the emerging theory of plate tectonics.

National Geographic later invited Tharp, Heezen and Austrian painter Heinrich Berann to create panoramic illustrations of the ocean floor, which appeared as poster-sized supplements in the magazine between 1967 and 1971.

In 1973, the trio received funding from the US Navy to produce a complete map of the world's ocean floors. After four years of work, they completed the iconic map in 1977, just weeks before Heezen died of a heart attack while in a submarine exploring the mid-ocean ridge near Iceland.

Although Tharp's contributions were not fully recognized until later in her life, she eventually received honors from several institutions, including the Library of Congress, which named her one of the four greatest cartographers of the 20th century.

Looking back on her work, Tharp said, "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 can’t find anything bigger than that, at least on this planet," as quoted by Science News.