Earth’s crust is important to us because we happen to live on it. It also contains more than half of Earth’s internal heat production, as well as most of its potassium and phosphorus. Its formation has left the residual mantle in a dramatically different state. Thus, knowing its composition is critical to any understanding of how Earth’s interior works. Roberta Rudnick is the world’s leading authority on the composition of the continental crust and lithosphere. Toward this end, she integrated geophysical properties of the crust with a comprehensive array of geochemical data to elucidate the role of the lower crust, which is inaccessible to direct observation. Today, if one is looking for the best estimate of the average continental crust or of its upper or lower portions, the go-to papers are those of Roberta Rudnick and her coworkers David Fountain and Shan Gao. The continental crust is welded to a much more massive subcontinental lithosphere. To elucidate the origin and evolution of this lithosphere, Roberta has conducted definitive studies on lithospheric peridotite and eclogite xenoliths, concentrating on trace element and isotope systems.
During the past several years, she has also become a leader in using one of the new, often called unconventional tracers, namely, lithium isotopes, to study near-surface continental processes such as weathering and intracrustal fluid flow, as well as recycling of near-surface continental material into the mantle. With her graduate student Fang-Zhen Teng, she demonstrated unequivocally, through detailed field and analytical work on magmatic aureoles, that reactive transport causes kinetic isotope fractionation. This work has opened up a new area of research on using kinetic isotope fractionation to constrain the timescales of diffusive and advective geochemical processes.
Most recently, together with her postdoc Richard Gaschnig and student Ming Tang, she has tackled the thorny problems posed by the long-term chemical evolution of the crust. Ancient crust is sparsely exposed and affected by weathering alteration and is thus subject to serious sampling biases. Her group dealt with both problems by analyzing ancient glacial tills, rather than sampling water- or wind-transported sediments, and developing weathering-resistant chemical proxies to show that ancient continents were richer in Fe and Mg and contained less granitic material than today’s crust.
The Harry H. Hess Medal is intended to honor “outstanding achievements in research on the constitution and evolution of the Earth and other planets.” Roberta’s research scope and accomplishments fit that description perfectly.
—Albrecht W. Hofmann, Max Planck Institute for Chemistry, Mainz, Germany; also at Columbia University, New York
I am deeply moved and humbled by this tremendous honor. Science is not an individual pursuit; it is a collaborative endeavor, whether it be from “standing on the shoulders of giants” per Sir Isaac or through joint ventures with students and fellow scientists. That is why it is just a little embarrassing to stand here as an individual to be honored for accomplishments that reflect the work of so many. For over 3 decades, Bill McDonough and I collaborated on a wide variety of projects from the crust to the mantle and had many debates (yes, zirconium can and does fractionate from hafnium—I think I won a bottle of red on that one). The late Gao Shan, China University of Geosciences, Wuhan, who died far too early, was one of the most creative and inspiring scientists I have worked with. Together we explored the unusual happenings of the North China Craton, which introduced me to a fascinating part of the world and opened the door to many other collaborations. I feel that Shan’s enormous contributions were never adequately celebrated, and I hope that I can share this award with him posthumously.
I have been fortunate to have worked with incredible students from whom I learned more than I taught. There is nothing more satisfying than to see your students go on to achieve at the highest levels: Cin-Ty Lee, Fang-Zhen Teng, Jingao Liu, Xiaoming Liu (no relation), and Ming Tang, among others, keep me in awe of their intelligence, creativity, and overall kind spirits. They have become part of my extended family. I, in turn, benefited from mentoring from quite a few folks: Ross Taylor and Scott McLennan at Australian National University introduced me to the fascinating debate about continental crustal evolution and the use of sedimentary rocks to read the record of ancient Earth. Steve Goldstein, Al Hofmann, and the late Ted Ringwood all served as important mentors post-Ph.D. I’ve had the good fortune to work in collaborative and supportive departments at the University of Maryland and now at the University of California, Santa Barbara.
Finally, my family has supported me every step of the way: my brother Mike and sister Linda are here tonight with their families; my 95-year-old mother (Janet Rudnick) could not make it but continues to be an inspiration. My son, Patrick McDonough, is one of the finest humans I know. I thank you all, and those whom time does not permit me to mention by name, from the bottom of my heart.
—Roberta Rudnick, University of California, Santa Barbara