Dietz was a marine geologist, geophysicist and oceanographer who conducted pioneering research along with Harry Hammond Hess concerning seafloor spreading, published as early as 1960–1961. While at the Scripps Institution of Oceanography he observed the nature of the Emperor chain of seamounts that extended from the northwest end of the Hawaiian Island–Midway chain and speculated over lunch with Robert Fisher in 1953 that something must be carrying these old volcanic mountains northward like a conveyor belt. In later work he became interested in meteorite impacts, was the first to recognize the Sudbury Basin as an ancient impact event, and discovered a number of other impact craters.
He championed the use of shatter cones as evidence for ancient impact structures.
The Arizona State University School of Earth and Space Exploration sponsors annual Robert South. Dietz Memorial Public Lectures, which have been given by: 2006 National Center for Science Education Executive Director Eugenie Scott 2007 National Aeronautics and Space Administration Astronaut John M. Grumsfeld 2011 John Grotzinger, Caltech.