Louis and Elizabeth Scherck Distinguished Professor of Geological Sciences, Brown University.
Understanding the Asthenosphere
The asthenosphere plays a fundamental role in plate tectonics given that its low viscosity strongly mediates how mantle convection is expressed at the surface. However, the origin of the asthenosphere and the role of partial melt in reducing its viscosity remain debated. In regions such as Anatolia and much of Alaska, seismic data (scattered body waves and surface waves) indicate that a rapid increase in velocity with depth occurs at depths of ~150 km, marking the base of a low velocity asthenospheric layer, whereas the top of the asthenosphere corresponds to a vertically-localized decrease in velocity with depth. At globally distributed stations, scattered seismic waves show that the sharp positive gradient at ~150 km depth is common in regions with elevated upper mantle temperatures and is best explained as the base of a layer containing partial melt. This layer could be present beneath ~40% of Earth’s surface. Geodynamic models indicate that if the partial melt significantly reduces mantle viscosity, deformation and anisotropy in seismic velocities should be enhanced within the layer. However, the geographic distribution of the observed melt-bearing layer does not correlate with higher radial anisotropy, indicating that it has relatively little impact on large-scale viscosity. Rather, the low-velocity partially molten layer is embedded within a thicker low-viscosity asthenosphere, whose lower boundary is primarily controlled by gradual pressure and temperature variations with depth.
Karen M. Fischer is a seismologist who studies the structure and dynamics of Earth's interior. Her work focuses on understanding the lithosphere and asthenosphere, how these layers are created and evolve over time, and their roles in plate tectonics and mantle convection.She is a Fellow of the American Geophysical Union (AGU), a recipient of the Harry Fielding Reid Medal from the Seismological Society of America, and the 2016 AGU Beno Gutenberg Lecturer. She has served as the President of the AGU Seismology Section. At Brown, her work has been recognized with the Royce Family Professorship in Teaching Excellence and the Karen T. Romer Award for Undergraduate Advising and Mentoring. She earned her B.S. from Yale University and her Ph.D. from the Massachusetts Institute of Technology, and she was a Postdoctoral Fellow at the Lamont-Doherty Earth Observatory of Columbia University.