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American oceanographic cartographer (b. July 30, 1920, Ypsilanti, Mich.-d. Aug. 23, 2006, Nyack, N.Y.), pioneered ocean-floor mapping, which provided crucial support for the acceptance of seafloor spreading and continental drift. Tharp obtained a master's degree (1944) in geology from the University of Michigan. After working as a field geologist in Oklahoma, she was hired in the late 1940s to be a technical assistant at Columbia University's geology department, which soon became the Lamont Geological Observatory (and later the Lamont-Doherty Earth Observatory) in Palisades, N.Y. Tharp began a longtime collaboration there with Bruce Heezen. Using sonar ocean-depth measurements that he obtained, Tharp created three-dimensional relief maps of the ocean floor that revealed global-scale undersea ridges. From Tharp's maps of the mid-Atlantic ridge in the 1950s, Tharp and Heezen first discerned the undersea rift valleys from which seafloor spreading takes place. The work of Tharp and Heezen culminated in the "World Ocean Floor," a detailed map published shortly after Heezen's death in 1977.