(1951- ) American Petrologist, Geochemist
The most common and well-known divergent boundary is a mid-ocean ridge. Indeed, in the life of a margin where plates pull apart, they mark it for probably 99 percent of the time. They produce only basalt volcanism. However, most of these boundaries begin on continental crust. These early stages of rifting are very complex in many ways, including volcanism. Not only is there basalt volcanism, as in the later stages, but there also may be rhyolite or silicic volcanism as well. Gail Mahood is one of the foremost experts on this rare and complex volcanic activity. Rhyo-lites are blends of magmas resulting from partial crystallization of the basalts and from partial melting of continental crust through which the basalt travels. Because the composition of the basalt and lower crust vary, the composition of the rhyolite is also highly variable. This variability occurs on the major element scale, but even more so on the minor and trace element scale. Because rhyolites are the last bits of liquid in a crystallizing basalt or the first bits of melt from heated crust, they contain only the lowest temperature minerals, as well as all of the elements that do not fit into standard minerals known as incompatible elements. Mahood uses these elements and isotopes, both stable and radioactive, to unravel the processes of formation of rhyolitic magmas. These elements can not only help to determine the source of the magma but also the igneous processes that occur during the ascent and eruption of these rocks. One of her more notable papers is “Synextensional Magmatism in the Basin and Range Province; A Study from the Eastern Great Basin.”
Gail Mahood is also interested in the mechanics of the volcanic eruptions. Rhyolite tends to be very sticky and viscous. Coupled with a potentially and commonly high water content that explodes to steam as the eruption occurs, these volcanoes can be very dangerous. They produce enormous amounts of ash in highly explosive eruptions with high eruption columns that result in widespread ash deposits. The biggest volcanic eruptions in North America in recent geologic history were from rhyolite volcanoes. Mahood studies several of these famous deposits like the Bishop Tuff but also several others from the southwestern United States, especially in California and Colorado. Her paper, “Correlation of Ash Flow Tuffs,” is seminal reading. She has also done extensive research on rhyolite volcanoes from northwestern Mexico and in Alaska and Italy. Additionally Mahood has investigated the ancient magma chambers that fed the rhyolite volcanoes in the Sierra Nevada, California, and the Andes of Chile.
Mahood has applied her geologic research to Mesoamerican archaeology, where she also has a strong interest. Obsidian (volcanic glass) is commonly produced in these rhyolite volcanoes. Ma-hood provides constraints on migration and trading routes by determining the source of obsidian, which was used extensively in tools and weapons by the native peoples of the southwestern United States, Mexico, and Central America.
Gail Mahood was born on June 27, 1951, in Oakland, California. She enrolled at the University of California at Berkeley in 1969, but left college after one tumultuous year. She enrolled at the College of Marin in Kentfield, California, in 1971, but returned to the University of California at Berkeley the next year. She earned a bachelor of arts degree in geology in 1974, and remained for graduate studies. She earned a Ph.D in 1980 as an advisee of ian s. carmichael. During her graduate career she was a National Science Foundation Fellow as well as a geologist for the Proyecto Ar-queologico Copan in Honduras. In 1979, Mahood joined the faculty at Stanford University, California, where she remains today. She served as chair of the department from 1996 to 1999. She was a visiting professor at Pennsylvania State University at State College in 1983 and at the University of Michigan at Ann Arbor in 1989 under a National Science Foundation program. Gail Mahood is married to Wes Hildreth, a well-known volcanolo-gist/petrologist with the U.S. Geological Survey.
Gail Mahood is amid a productive career. She is an author of some 45 articles in international journals and professional volumes. Many of these are seminal papers on igneous processes, especially volcanic, and appear in high-profile journals like Science and Nature. Gail Mahood has performed outstanding service to the profession. In addition to serving on numerous committees and panels, she served as councilor for the Geological Society of America in 1996 to 1999. She also served on several committees for the American Geophysical
Union and panels for the National Science Foundation, National Research Council, and she even testified before the U.S. Congress on the role of the U.S. Geological Survey. Mahood served in numerous editorial roles as well. She was the founding editor of Proceedings in Volcanology at the International Association of Volcanology and Chemistry of the Earth’s Interior, the executive editor and a member of the editorial board for the Bulletin of Volcanology, and an associate editor for the Geological Society of America Bulletin. She additionally served as an external reviewer for the geology department at Dalhousie University in Halifax, Canada.
