Before the mid-20th century, little was known about the ocean bottom. Many scientists regarded it as a mostly flat, dull expanse whose principal function was to act as a plinth for the masses of water on top of it. There were some mountains down there — that much was known — but no one was certain if they were isolated anomalies or part of a larger geologic system.

Image Marie Tharp of South Nyack, N.Y., in 2001. She charted the ocean floor in the 1950s, 60s and 70s. Credit... Bruce Gilbert

In 1949, Mr. Ewing and his team moved their base of operations to the newly opened Lamont Geological Observatory (now the Lamont-Doherty Earth Observatory), in Palisades, N.Y. There, they embarked on a quest to map the sea floor using sonar soundings. Because a research vessel was considered no fit place for a woman, Ms. Tharp, working with pen, ruler and what turned out to be a dangerous bottle of India ink, plotted the data back in Palisades.

She noticed something striking. Charting the soundings in 1953, she observed what seemed to be a depression in the mid-Atlantic Ridge, a volcanic chain that traverses the ocean from north to south. She wondered whether the depression was evidence of a continuous rift — a crack in the world — down the middle of the ridge. And she wondered in turn whether that rift might be evidence of what scientists now call seafloor spreading, popularly known as continental drift.

She and Mr. Heezen argued about it. She threw erasers and bottles of ink at him. It took him some time to come around.

“I discounted it as girl talk and didn’t believe it for a year,” Mr. Heezen later said in an interview.

Over time, Ms. Tharp’s hunch was proved correct. Her work with Mr. Heezen, along with the work of other scientists, helped establish the existence of a continuous belt of undersea mountains girdling the earth, with a rift down the center of it.

“It was only in the early 60’s that the marine scientists around the world started putting the pieces of the puzzle together and understanding that these huge volcanic mountain chains were actually where seafloor spreading was taking place,” Michael Purdy, the director of the Lamont-Doherty observatory, said in a telephone interview yesterday.