EES Guest Speaker - Dr. Lars Hansen

This week’s seminar speaker is Dr. Lars Hansen.  Please join us Thursday, December 4th, 2025 at 3pm in room 351 Nat Sci.

Talk title: When rocks push back: Angry dislocations and their influence on seismic properties and geodynamics

The rheological properties of rocks and minerals control a wide variety of large-scale processes in Earth's interior. These processes include—in order of decreasing timescales—mantle convection and the strength of the lithosphere, glacial isostatic uplift and its influence on sea-level rise, postseismic creep and reloading of faults after earthquakes, the attenuation of teleseismic waves, and the frictional stability of fault gouge that controls earthquake nucleation. Fundamentally, those rheological properties are controlled by underlying microphysical processes within the constituent minerals. The primary microphysical process is the movement and interaction of crystal dislocations. We have identified that the elastic interactions of dislocations lead to a directional internal stress, often referred to as a backstress, and we have developed a microphysical model that describes the evolution of backstresses during deformation over a wide range of timescales. Here we describe and validate this model using a range of laboratory experiments that include multianvil compression at a synchrotron, high-temperature uniaxial creep, constant-load nanoindentation, and high-strain-rate impact. This model provides a new framework for interpreting seismological and geodynamic observations in terms of the density of dislocations and suggests that spatial variations of dislocation density in Earth's interior may be more important in some situations than other state variables such as water activity or melt content.