Timothy L. Grove was awarded the 2018 Harry H. Hess Medal at the AGU Fall Meeting Honors Ceremony held on 12 December 2018 in Washington, D. C. The medal is for “outstanding achievements in research on the constitution and evolution of the Earth and other planets.”  

The formation of igneous rocks is the fundamental process of planetary differentiation. Tim Grove is one of the world’s experts in igneous petrogenesis, having used a unique blend of fieldwork and laboratory experiments to make several key conceptual breakthroughs about the Earth and extraterrestrial bodies.

Mid-ocean ridge basalt (MORB), formed at divergent plate boundaries, is the most voluminous volcanic rock type on Earth. Tim experimentally mapped out the compositional space of MORB melts and developed a quantitative model that predicts the composition of magmas produced by mantle melting as a function of mantle composition, pressure, and temperature. This work showed that MORB compositions can be explained by low-degree melting over variable pressures and temperatures. This is perhaps the single most important body of experimental work in defining the nature of primary magmas of mid-ocean ridge magmatism.

Subduction zones constitute another major tectonic setting on Earth where magmas form. It has long been recognized that the compositions of most igneous rocks formed at convergent plate boundaries define a “calc-alkaline” trend, characterized by increasing alkali content without iron enrichment. Guided by fieldwork and phase equilibrium experiments, Tim showed that the calc-alkaline trend can be explained by the presence of magmatic water. This work serves as a foundation of our modern understanding of the role of water in the evolution of subduction zone magmas. Tim extended his knowledge of modern subduction zone processes to the Archean, showing that komatiite magmas were produced by hydrous melting in a subduction environment.

Tim has also made major contributions to our understanding of petrogenesis on the Moon and asteroids. He demonstrated that Apollo mare basalts were simple extrusions without extensive episodes of ponding on the lunar surface. His studies of lunar mafic glasses showed that the lunar mantle is compositionally heterogeneous, consistent with it being a frozen relic of the ancient magma ocean. He also showed that different meteorite classes can be related to one another by igneous differentiation processes and that these rocks sample different zones of differentiated planetesimals.

On top of his outstanding research achievements, Tim has an exceptional record of service, including as AGU president when he oversaw significant changes in AGU’s governance structure. Tim’s legacy continues beyond his own research through his education of a large number of extraordinary students who are successful and influential scientists in their own right.

—Benjamin P. Weiss and Oliver E. Jagoutz, Massachusetts Institute of Technology, Cambridge

I feel very honored to be receiving the Hess Medal. The citation is almost exclusively about scientific accomplishments, and I would like to take some time to talk about how all that science happened. One of the most rewarding things for me has been working with many graduate students, postdocs, and collaborators as together we have moved science forward. I have always thought that mentoring and training the next generation of scientists is one of my primary responsibilities. I am very proud of you all and feel that you are also an important part of my work and that we all share this award. And there is a Harry Hess connection that relates to mentoring. Harry Hess was one of my first scientific heroes. In 1970 at University of Colorado Boulder, we went on a field trip to the Stillwater layered igneous complex in Montana. I asked my professor/mentor, Bill Braddock (Princeton, Ph.D., 1959), if there was something I could read before the trip. He replied, Hess’s 1960 memoir. So I did, and I saw the remarkable breadth of approach that Hess had taken. Hess tied together excellent field observations with careful characterization of mineralogy and petrology to tell an amazing story of the crystallization processes that led to the development of this layered series. In it, Hess discussed phase equilibria of the pyroxene minerals, compositional evolution of the crystallizing melts, and the fluid dynamics of magma chamber processes. In 1971 I went to graduate school at Harvard, where we studied lunar samples newly returned by the Apollo missions that Harry Hess helped to plan. Then it was off to a postdoc at Stony Brook, where Don Lindsley (Princeton, AB, 1956, who worked for Harry) taught me everything he knew about experimental petrology. Next was a job as a professor at the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution, where we studied the processes that led to the formation of the rocks on the ocean floor, an area where Hess was a pioneer. Hess inspired new thinking in many areas of the Earth and planetary sciences, and he mentored outstanding geologists like Braddock and Lindsley, who, in turn, mentored me. In closing, it has also been deeply satisfying to help AGU move into a new era of governance and engagement with society. Also, I have had incredible support from my wife and sons, Matthew and Michael, while I have embarked on my scientific adventures. Thanks to you all. —Timothy L. Grove, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge