Simkin was a 1955 civil engineering graduate of Swarthmore, and received a Ph.D. in geology from Princeton in 1966. He began as a civil engineer but said he became interested in geology because he loved climbing mountains.
He started at the Smithsonian in 1967, and from 1972 to 2003 he was a curator of petrology (study of rocks) and volcanology (study of volcanic phenomena) at its Natural History Museum. He edited and compiled several books on volcanoes and was the lead author of This Dynamic Planet, a world map of volcanoes, earthquakes, and plate tectonics.
Simkin received awards for outstanding contributions to volcanology and for lifetime contributions to science. He was a former president of the Geological Society of Washington.
He is survived by Sharon, his wife of 43 years; two children; and two grandchildren.
Graduate memorials are prepared by the APGA.