Marie Tharp
Geologist Marie Tharp (born 1920) is known for her maps of ocean floors. These maps are helpful in showing the structure and evolution of the sea floors.
Marie Tharp is a mapmaker who charted the bottom of the ocean at a time when little was known about undersea geology. Her detailed maps showed features that helped other scientists understand the structure and evolution of the sea floor. In particular, Tharp's discovery of the valley that divides the Mid-Atlantic Ridge convinced other geologists that sea floor was being created at these ridges and spreading outward. The confirmation of "seafloor spreading" led to the eventual acceptance of the theory of continental drift, now called plate tectonics.
Tharp was born in Ypsilanti, Michigan, on July 30, 1920. Her father, William Edgar Tharp, was a soil surveyor for the United States Department of Agriculture's Bureau of Chemistry and Soils; he told his daughter to choose a job simply because she liked doing it. Marie's mother, Bertha Louise (Newton) Tharp, taught German and Latin. The family moved frequently because of William Tharp's mapping
assignments across the country. Marie Tharp attended twenty-four different public schools in Iowa, Michigan, Indiana, Alabama (where she almost flunked out of the 5th grade in Selma), Washington, D.C., New York, and Ohio. In 1943 she received her bachelor's degree from Ohio University.
Since most young men were fighting in World War II at the time Tharp graduated, the University of Michigan opened the doors of its geology department to women for the first time. Tharp entered the masters program, which trained students in basic geology and then guaranteed them a job in the petroleum industry. Graduating in 1944, Tharp was hired as a junior geologist with Stanolind Oil & Gas in Tulsa, Oklahoma. Women were not permitted to search for oil in the field, so Tharp found herself organizing the maps and data for the all-male crews. While working for Stanolind, Tharp earned a B.S. in mathematics from the University of Tulsa in 1948.
The year of her second bachelor's degree, Tharp moved to Columbia University, where a group of scientists were about to revolutionize the study of oceanography. Hired as a research assistant by geologist Maurice Ewing, Tharp actually ended up helping graduate students with their data; she never told anyone that she had a graduate degree in geology. One student, Bruce Heezen, asked for help with his ocean profiles so often that after a while Tharp worked with him exclusively. Heezen and Tharp were to work closely together until his death in 1977. In 1950 the geophysical laboratory moved from Columbia University to the Lamont Geological Observatory in Palisades, New York.
Before the early 1950s, scientists knew very little about the structure of the ocean floor. It was much easier and cheaper to study geology on land. But without knowledge of the structure and evolution of the seafloor, scientists could not form a complete idea of how the entire earth worked. In the 1940s, most people believed that the earth was a shrinking globe, cooling and contracting from its initial hot birth. The work of Heezen, Tharp, and other geologists in the next decade—who gathered data on the sea floor using echo sounding equipment—helped replace that idea with the model of plate tectonics, where thin crustal "plates" shift around on the earth's mantle, colliding and grinding into each other to push up mountains and cause earthquakes.
The Mid-Atlantic Ridge, a mountainous bump that runs roughly parallel to and between the coastlines of the Americas and Africa, was one of the first topographical features on the sea floor to be identified. Initial studies were undertaken by those aboard the British ship H.M.S. Challenger, who discovered in the 1870s that the rise in the center of the Atlantic acted as a barrier between different water temperatures; and by those aboard the German ship Meteor who between 1925 and 1927 revealed the Mid-Atlantic Ridge as rugged and mountainous. The Meteor staff also found several "holes" in the center of the Ridge, but did not connect these holes into the continuous rift valley that they were later discovered to be. In the 1930s, the British geologists Seymour Sewell and John Wiseman suspected that a rift valley split the Ridge, but World War II prevented an expedition to confirm this.
By 1950, when Tharp and Heezen moved to Lamont, the time was right for a series of discoveries. In 1952, the pair decided to make a map of the North Atlantic floor that would show how it would look if all the water were drained away. This type of "physiographic" diagram looked very different from the usual method of drawing contour lines for ocean floor of equal depth. Heezen and Tharp chose the physiographic method because it was a more realistic, three-dimensional picture of the ocean floor, and also because contours were classified by the U.S. Navy from 1952 to 1962.
Tharp assembled her first drawing of the North Atlantic ocean floor in 1952, after rearranging Heezen's data into six seafloor profiles that spanned the Atlantic. This initial map showed a deep valley dividing the crest of the Mid-Atlantic Ridge. Tharp pointed out the valley to Heezen. "He groaned and said, 'It cannot be. It looks too much like continental drift, "' Tharp wrote later in Natural History. The valley represented the place where newly-formed rocks came up from inside the earth, splitting apart the mid-ocean ridge. At the time, Heezen, like most scientists, thought that continental drift was impossible.
While Tharp was working on detailing and clarifying the first map, Heezen kept another assistant busy plotting the location of the epicenters of North Atlantic earthquakes. Beno Gutenberg and Charles F. Richter had already pointed out that earthquake epicenters followed the Mid-Atlantic Ridge quite closely. But Heezen's group found that the epicenters actually fell within the suspected rift valley. The association of topography with seismicity convinced Tharp that the valley was indeed real.
It took Heezen eight months to agree. By studying rift valleys in eastern Africa, Heezen convinced himself that the land in Africa was simply a terrestrial analogy to what was going on in the middle of the Atlantic: the earth's crust was splitting apart in a huge tensional crack. Heezen then began to wonder whether the earthquake epicenters that had been recorded in the centers of other oceans might also lie in rift valleys. Perhaps, he thought, all the mid-ocean ridges could be connected into a huge 40, 000 mile system.
Heezen told Maurice Ewing, director of Lamont, of the valley's discovery. For several years, only Lamont scientists knew of its existence. Heezen presented it to the scientific community in several talks during 1956. In 1959, most of the remaining skeptics were convinced by an underwater movie of the valley, made by French oceanographer Jacques Cousteau towing a camera across it. Today scientists understand how the rift valley represents the pulling apart of the seafloor as the new rock spreads outward from the ridge.
Heezen and Tharp printed their first edition of the North Atlantic map for a second time in 1959. By this time they knew that the Mid-Atlantic Ridge was cut by east-west breaks, now called transform faults. Heezen and Tharp had confirmed only one of these breaks, but they didn't know its exact length or direction. So in its place on the map they put a large legend to cover the space. In the following years,
Tharp and Heezen improved their North Atlantic map and expanded their work to cover the globe, including the South Atlantic, Indian, Arctic, Antarctic, and Pacific oceans. In 1977, three weeks before Heezen's death, they published the World Ocean Floor Panorama, based on all available geological and geophysical data, as well as more than five million miles of ocean-floor soundings. In 1978 Tharp and Heezen received the Hubbard Medal of the National Geographic Society.
After about fifteen years of work behind the scenes, Tharp finally went on research cruises herself, including trips to Africa, the Caribbean, Hawaii, Japan, New Zealand, and Australia. She retired from Lamont in 1983. Since then she has run a map distributing business in South Nyack, New York, and occasionally consults for various oceanographers. She also keeps Heezen's scientific papers and has written several articles on his life and work. Tharp enjoys gardening in her spare time.
Further Reading
Oceanus, winter, 1973-74, pp. 44-48. □
