EEPS Colloquium: Lars Hansen

Lars Hansen, Professor of Earth & Environmental Sciences, University of Minnesota

Dislocations and dilation: How microscopic crystal defects control the dynamics of the solid Earth

The mechanical properties of rocks control a range of large-scale, geodynamic processes over a broad set of timescales. Examples include flexure of the lithosphere at subduction zones, fault stability and the depth at which earthquakes nucleate, surface uplift after mass loss from ice sheets, the attenuation of teleseismic waves, and melt migration in and extraction from volcanic systems. Here I described how those large-scale processes intimately depend on the dynamics of microscopic defects within the crystals, specifically, dislocations and surfaces (i.e., cracks or pores). I will present recent data from a wide range of laboratory experiments to calibrate a model specifically for upper-mantle rocks. These experiments reveal that dislocation interactions in crystals set the behavior of the upper mantle over short, intermediate, and long timescales. In addition, dislocation interactions can lead to crack nucleation that may lead to weaker lithospheric strength than previously predicted. In some highly porous rocks, the dislocation dynamics at grain contacts still controls the macroscopic strength. However, at sufficiently high temperatures, low grain sizes, and high porosities, the rocks begin to behave as a granular medium, in which the dilatancy (i.e, increase of porosity or crack density) becomes the strength-controlling factor.

Host: Phil Skemer