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MEAM Seminar: “Viewing Earth’s Surface as a Soft Matter Landscape”
January 22, 2019 at 10:45 AM - 11:45 AM
The Earth’s surface is composed of a staggering diversity of particulate-fluid mixtures: dry to wet, dilute to dense, colloidal to granular, attractive to repulsive particles, laminar to turbulent flows, and steady to highly-unsteady forcing. This material variety is matched by the range of relevant stresses and strain rates, from rapid and catastrophic landslides to the slow relaxation of soil over geologic timescales. In this talk I illustrate the commonalities and challenges in understanding geophysical flows by highlighting two problems: gravity-driven downslope soil movement, and fluid-driven particle transport in rivers.
Soil on hillslopes slowly and imperceptibly creeps downhill, but suddenly liquefies to produce landslides. The transition between creeping and flowing is a yield condition, often defined in terms of the shear stress, that depends on the characteristics of the soil and the geologic environment. We show that the nature of this transition, however, is general. Creep is the localized and erratic motion of soil grains below yield; because this kind of fragility is a generic consequence of disorder, soil creep should be similar to amorphous glass. Indeed, we find that the transition from creeping to landsliding is a continuous phase transition that follows predictions from glass transition models. The generality of this transition suggests that the onset of sediment transport in rivers should behave in a similar manner, and we demonstrate that this is the case using laboratory experiments and simulations. Because the sediment transport rate rapidly increases for stresses above yield, many landscapes such as rivers organize to be close to the yield point. In essence, landscapes flicker back and forth across the glass transition. We explore several consequences of these dynamics for the sculpting of landscapes.
Douglas J. Jerolmack
Professor of Earth and Environmental Science, University of Pennsylvania
Doug Jerolmack is Professor and Graduate Chair in Earth and Environmental Science, with a secondary appointment in Mechanical Engineering and Applied Mechanics, at the University of Pennsylvania. His area of study is experimental geophysics, with a focus on geomorphology (the “science of scenery”). Dr. Jerolmack studies the spatial and temporal evolution of patterns that emerge at the interface of fluid and sediment on Earth and planetary surfaces. His group uses laboratory experiments, combined with field work and theory, to elucidate the minimum number of ingredients that are required to explain physical phenomena. Particular foci include: granular physics of fluid-driven (water and wind) sediment transport; landform dynamics including dunes, river channels, deltas and fans; stochastic and nonlinear transport processes; and landscape response to dynamic boundary conditions such as climate. He received a B.S. in Environmental Engineering at Drexel University in 2001, PhD in Geophysics from MIT in 2006, and was a postdoctoral researcher at the Saint Anthony Falls Lab at University of Minnesota 2006-2007. He has been at Penn since 2007.