Stanford University, Calif.
Norman H. Sleep was awarded the 2002 Bucher Medal at the AGU Fall Meeting Honors Ceremony, which was held on 8 December 2002, in San Francisco, California. The medal is given for original contributions to the basic knowledge of the Earth’s crust and lithosphere.
Citation
“Norman Sleep’s major contribution to geophysics has been to use simple physical ideas to understand processes at work within the Earth, particularly those affecting the crust and lithosphere. He entered geophysics soon after the discovery of plate tectonics and has made important contributions to most of the recent advances in our understanding of geodynamics. These contributions cover a great range of topics.
“Mid-Ocean Ridges (1969 to 1991). Sleep used the relationships between the depth of the ocean and the age of the underlying plate to infer the temperature dependence, which is now widely used. He was the first to point out that models of crustal generation on ridges that required large magma chambers to be continuously present beneath slowly spreading ridges were not compatible with heat conservation. Sleep’s simple models explained the differences between the axial valley at slow-spreading ridges and the axial high at fast-spreading ones and the differences in magma chamber geometry and crustal structure as a function of spreading rate. Sleep, with Wolery, first used heat flow data to estimate the volume and age distribution of the heat transferred by water flux through oceanic crust and showed how the hydrothermal fluid would affect the composition from seawater and play a key role in global geochemistry. He showed that magma chambers inferred from seismic imaging occur deeper than expected because much of the hydrothermal circulation occurs by low temperature off-axial flow.
“Continental Margins (1971-1980). Sleep was first to quantify passive margin subsidence, showing that the subsidence history of extensional continental margins closely resembled that of oceanic plates. This result was unexpected, since the continental crust that displayed this behavior was often very much older than the onset of the subsidence. Sleep was the first to realize that the stratigraphic sequence on passive continental margins could be modeled by deposition on a thermally subsiding rifted margin. His idea also had considerable implications for hydrocarbon exploration along margins and in continental interior basins.
“Subduction Zones and Island Arcs (1971-1979). Sleep looked at marine geophysical aspects like marginal basin formation and trench morphology and gravity and developed thermal and mechanical models important to understanding the subduction process and its implications for plate motions.
“Magmatism (1974-1991). Sleep first applied equations governing the two-phase flow of melt through a crystalline matrix. Developments from these equations have provided dynamical models for both magma generation and for metamorphism by fluid infiltration.
“Thermal Evolution of the Mantle (1979-1982). Sleep used petrological data to infer a cooling of the mantle by about 300°C over 3 Ga, and showed that K, U, and Th abundances in the mantle had to be less than 60<37> cosmic to be consistent with whole mantle convection and the observed atmospheric 40Ar.“Archaean Tectonics (1982-1992). Sleep, with Windley, inferred that higher mantle temperatures caused the Archaean crust to be appreciably thicker than the Phanerozoic, but that the geologic record of fault zones in cratons indicated a style of tectonics similar to plate tectonics.
“Martian Tectonics (1982-1994). Sleep, with Phillips, generated the most convincing model for the support of Tharsis–the greatest topographic and geoidal high on the terrestrial planets.
“Mantle Plumes (1987- ). Sleep has been among the leaders in investigating what seafloor topography tells about the thermal structure and flow dynamics of the mantle. With Richards and Hager, he used the bathymetry and gravity at the various hotspot swells as constraints on their fluid mechanics and hence on mantle dynamics. They thus estimated the flux and temperature of the upwelling plume material for different hotspots and thus addressed the relationship between plume processes and plate tectonics in mantle evolution. Using an elegant comparison of a plume-affected hotspot track to a nearby abyssal plain, Sleep argued for a common plume with time-varying flux to explain adjacent continental and oceanic features and thus infer the different responses of continental and oceanic lithosphere to hotspots. He used the shape of the Hawaiian swell to draw inferences about the plume flow and to explain why the lithosphere appears not to be significantly thinned beneath Hawaii. Most recently, Sleep has developed models of the influence of variations in lithospheric thickness on lateral flow of plume material to explain off-ridge topography and volcanism.
“Fault Mechanisms (1992- ). Sleep developed a formula for the coefficient of friction, taking into account porosity, fluid pressure, strain rate, and temperature. He applied this to fault models to infer histories of strain patterns, fluid flow, heat generation, and fracture, and explained why there is such a great range of fault failure, from creep at low shear tractions to sudden failure in major earthquakes.
“These diverse, but profound, efforts of Norman Sleep have greatly influenced many Earth scientists. He is keenly appreciated for both his physical insights (especially by geophysicists) and his attention to the observed record of the Earth (especially by geologists). The Bucher Medal ?recognizes original contributions to the basic knowledge of the Earth’s crust.’ When the medal was established in 1966, ?crust’ connoted to many the layer of mechanical strength as well as the layer of lower density composition. In this broader definition, certainly no one has been more original, or more diverse, than Norman Sleep in contributing to basic understanding of how the outermost layers of the solid Earth came to be: tectonically, thermally, and compositionally. Moreover, he has done this in ways that have not only been original, but also have been enlightening and useful to his colleagues.”
—WILLIAM M. KAULA, University of California, Los Angeles
Response
“Thank you, Bill. I was surprised when I opened the letter from AGU. I was even more surprised when I read on and found that the Bucher Medal is for the study of the Earth’s crust. I have not viewed myself as a crustal scientist. I believe that AGU is recognizing the importance of an integrated approach to crustal and global tectonic studies, rather than my actual contributions. I have had the good fortune to collaborate with many others and would have accomplished little without constant interaction. I cannot thank everyone in 3 minutes so I will describe how others have helped shape my view of the crust.
“As an undergraduate at Michigan State in 1967, establishment of a medal for study of the Earth’s crust would have made as much sense to me as the botany society having a medal specially for plants. I knew from taking Bill Hinze’s classes that much was inferred about the Earth’s deeper interior, yet interior processes had not been linked to the crust, the part of the Earth we could actually examine in detail. Robert Dietz had visited Michigan State and talked about seafloor spreading. The topic got me thinking, but it seemed that any resolution would be in the distant future. After discussing graduate schools with Dr. Hinze, I applied to and was accepted by the Massachusetts Institute of Technology.
“In the summer of 1967 I arrived at MIT in the middle of a scientific revolution. I attended the conference at Woods Hole where results establishing seafloor spreading were presented. With the advent of plate tectonics, the continental crust had suddenly become an unimportant passenger on plates. The oceanic crust had nice magnetic anomalies but was otherwise a 6-km- thick nuisance that got in the way of looking at mantle processes, whose physics was poorly understood. Well, if you didn’t like the oceanic crust, it would soon subduct anyway.
“Gene Simmons hired me to work in the laboratory that summer, preparing equipment to measure heat flow in the ocean basins. I was not much good in the lab, but I did become interested in the thermal structure and subsidence of oceanic lithosphere. When Dave Wones and Dick Naylor organized a seminar on the Appalachians, I ended up with the topic of the passive margin out of laziness. The presentation was to be on the last day and flat-lying sediments seemed simpler than folded mountains.
“I applied thermal contraction, which had worked so well at the ridge, in explaining the passive margin. It worked, despite the fact that I was so impressed with the fall line unconformity that I initially came to believe that surficial erosion thinned the crust instead of it stretching during continental breakup.
“Being from Michigan, I compared the subsidence history of that interior basin with thermal contraction curves, opening a can of worms that has persisted to the present day. However, this work was not leading to a thesis, and the military draft was blowing down my neck. I switched topics to work with Nafi Toksoz on subduction and island arcs. A thesis completed on the deep Earth, I arrived as an assistant professor at Northwestern University. I was again in the middle of the continent and determined to relate plate tectonics to what was going on. It had also become evident that the oceanic crust carried goodies into the mantle that might come up at island arcs.
“Pat Hurley had already shown that degassing of the Earth was related to plate tectonics. Robert Garrels and Fred Mackenzie were studying global geochemical cycles. The crust was now a long-term reservoir and recorder on the effects of complex chemical exchanges within the mantle. Tom Wolery was looking for a thesis topic involving the oxygen cycle. Independently, Bob Garrels and I suggested that he look at ridge hydrothermal systems. The days when one could think that altered basalt was merely dumped back into the ocean were rapidly ending.
“To cut to a recent topic, I did not consider mantle plumes to be a good explanation for hotspots when the hypothesis first came out. Instead, I preferred propagating cracks in the lithosphere. By 1984 I was determined to put plumes to rest and showed that mantle lithosphere delaminating from the base of the lithosphere would produce a positive long wavelength geoid anomaly. When I presented this at the AGU meeting, Mark Richards showed that mantle plumes could do the same thing in that session. We agreed to disagree at first. Soon we could explain a host of phenomena, as plumes are conduits of hot material, not candlelike heat sources under the lithosphere. Colleagues have continued to tweak my interest in plumes with geological problems. In this regard, I would also like to thank George Thompson, Tom Parsons, Mark Anders, and Cindy Ebinger.
“Since the Bucher Medal was established, we have found that the Earth’s crust is incredibly complex, and not just because we can examine it in detail. There are rocky and icy crusts on moons and other planets. The crust is a full active player in global tectonic and geochemical processes and the storehouse of the geological record
—NORMAN H. SLEEP, Stanford University, Calif.