Relief map of the Atlantic Ocean showing the Mid-Atlantic Ridge and surrounding continents including North and South America, Europe, and Africa.

▪ Above: Artist Heinrich Berann painted this world map of the ocean floor, created by Marie Tharp and Bruce Heezen — the first realistic depiction of complete ocean-floor bathymetry. Geography and Map Division/Used by permission of the estate of Marie Tharp

The woman who mapped the ocean

The revolutionary sea floor charts of Marie Tharp.

By Meagan Snow and Carissa Pastuch

Marie Tharp was an American geologist and marine cartographer whose groundbreaking visualizations of ocean floors and discovery of the mid-Atlantic rift valley challenged the widely accepted geological views of the time.

Although plate tectonics (the idea that the Earth’s surface is built of moving plates) is accepted today, Tharp’s theory originally met opposition within the scientific community. Over time, her work sparked the plate tectonics revolution, a shift in geological thinking that transformed how scientists understand and map the Earth.

Tharp was introduced to geology and cartography at a young age, often accompanying her father, a U.S. Department of Agriculture soil surveyor, into the field to survey and map soil. During World War II, she earned a master’s degree in geology at the University of Michigan, then moved to Tulsa, Oklahoma, to work for Stanolind Oil and Gas Company while completing a mathematics degree at the University of Tulsa.

Dissatisfied, Tharp moved to New York to work as a cartographic assistant at the Lamont Geological Observatory. Barred from ocean expeditions as a woman, she stayed ashore and used sonar depth measurements taken by male colleagues to manually create revolutionary visualizations of the sea floor.

At the time, it was newly understood that the ocean floor was not just a flat surface. Tharp used depth measurements — known as soundings, which record the time it takes a sound signal to travel from a ship to the ocean floor and back — to visualize this for the first time.

With those soundings, she created six depth profiles of the North Atlantic. Across the profiles, she noticed a V-shaped indentation — a large valley, she surmised, in the center of the ocean floor. Tharp began creating physiographic diagrams that visualized the ocean floor’s transition from smooth plains to spiked undersea mountains — all done without computers.

Geologist and oceanographic cartographer Marie Tharp seated beside a globe illustrating seafloor features, with maps of the ocean floor displayed behind her.

▪ Through her work, Tharp advanced the theory of plate tectonics and changed the way scientists understand and map the Earth. Used by permission of the Lamont-Doherty Earth Observatory and the estate of Marie Tharp

Tharp started with the sounding measurements, creating two-dimensional profiles, then three-dimensional sketches. All these visualizations followed the tracks of ships that had recorded the depth measurements. Next, she took the raw data and, using her knowledge of geology and mapping, connected the profiles, filling gaps to create the first complete physiographic diagram view of the ocean floor.

Meanwhile, Tharp’s colleague Bruce Heezen was involved in a Bell Laboratories research project to find the best locations to install new underwater cables. Together with Howard Foster, Heezen mapped the location of underwater earthquake epicenters in the Atlantic.

As the map emerged, Heezen noticed that underwater earthquake epicenters aligned with the rift valley on Tharp’s diagrams — a discovery that supported theories of continental drift and plate tectonics, then-controversial ideas now widely accepted.

The pattern of earthquakes around mid-oceanic ridges held even as Tharp and Howard moved their mapping out of the Atlantic. These findings were presented at the 1956 American Geophysical Union meeting, and, that same year, Tharp published the North Atlantic physiographic map, drawn in pen and ink.

Over the course of 20 years, Tharp continued her partnership with Heezen, mapping the ocean floor and incorporating new sources of data as they emerged. In 1977, the team commissioned Austrian painter Heinrich Berann — famous for his Alpine panoramas — to hand-paint the world ocean floor map, the first realistic depiction of complete ocean-floor bathymetry. The Library’s Geography and Map Division holds the original manuscript.

In 1995, Tharp donated her and Heezen’s research papers to the Library — maps, journals, research papers, correspondence and geologic and cartographic data that changed the way we understand the ocean’s depths.

—Meagan Snow is a geospatial data visualization librarian and Carissa Pastuch a reference specialist in the Geography and Map Division.

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