Collaborative Research: A Multidisciplinary Approach to Constraining the Geodynamical Landscape of the Deep Mantle

About This Grant

Approximately 3000 km beneath Earth’s surface lies the boundary between the liquid iron core and the rocky mantle above. The nature of this region, the deep mantle and the shape of the boundary, remains mysterious, but it is important in that the heat emanating from this region powers mantle convection, the process that drives plate tectonics. If the properties of this region can be constrained, much progress in understanding the evolution of our mantle and the flow of the liquid outer core can be made. This project involves combining observations of global scale Earth processes—including large scale vibrations of the Earth triggered after an earthquake, the warping of Earth crust due to the unloading of ice sheets and its associated changes in earth’s gravitational field— to constrain the density, viscosity and shape of this boundary. With this knowledge, the team will make interpretations that will have implications for Earth’s mantle thermal and chemical evolution. The project will contribute to the education and professional development of high school students, undergraduate students, and graduate students. New tools developed through this project will be shared with the research community through workshops. The geodynamic landscape of the deep mantle holds implications for the evolution of chemical and thermal heterogeneity in Earth’s mantle, the heat escaping the outer core and its flow driving the geodynamo, and the large-scale nature of plate tectonics. However, its key characteristics – namely core-mantle boundary topography, its density and viscosity distribution – remain unconstrained and are often considered in isolation. These properties dictate the driving (buoyancy) and resistive (viscous) forces of convection. The difficulty in constraining these properties stems from several issues: for buoyancy variations, seismic waves are largely insensitive to density and therefore few density tomography models exist. Those models that use seismic normal modes apply consequential approximations that are avoided in this project. Because viscosity variations are expected to be several orders of magnitude, constraining 3D variations is computationally challenging. This project takes a holistic view in considering 3D fields self-consistently, using observations that span the seismic (∼seconds) to convection bands (more than 100 million years). The project begins with a 3D density and core-mantle-boundary field produced by a state-of-the-art inversion methodology for seismic normal mode, which enables exploration of the most dynamically consistent 3D viscosity fields using convection data. Finally, results may be confirmed by comparing consistency with variations of Earth’s rotation (∼1-year timescale) and gravitational changes due to adjustment from the last glacial maximum (∼1000-year timescale) – both of which are sensitive to deep mantle viscosity structure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.