Shaped by flow, shaping the flow: Crystal fabric evolution in glacier ice and rock-forming minerals
The mechanical behavior of polycrystalline materials such as glacier ice and rock-forming minerals is strongly controlled by their crystal orientation fabric. As individual crystals (grains) deform under applied stress, they tend to develop non-random patterns of preferred orientation that, in turn, induce significant mechanical anisotropy. This phenomena is believed to facilitate the strong shear localization observed in ice-stream shear margins and the apparent hardening of ice shelves, with relevance to continental-scale projections of ice-sheet mass loss, but can also affect the deformation of lithospheric plates and interaction between plates and the sublithospheric mantle. Yet, modeling the two-way coupled evolution between fabric and flow remains largely unexplored, primarily due to computational expense and a lack of useful fabric-compatible (anisotropic) constitutive relations. In this talk, I present a new way to efficiently model the coupled problem for ice and olivine at large scale using the finite element method (FEniCS, Elmer), applied to ice shelves and mantle convective cells. The method is based on the development of a new continuum-scale statistical model for representing grain orientations (Fokker-Planck type problem on R3 x S2 or R3 x SO(3)) and allows for easy calculation of fabric-induced mechanical anisotropy. The method makes it feasible to address open problems in glaciology and geodynamics, related to the rheological uncertainty caused by neglecting crystal fabrics in state-of-art flow models, but also permits direct validation against field observations from e.g. ice-penetrating radar that I discuss.
Short bio: Nicholas Rathmann is an assistant professor at the Niels Bohr Institute, University of Copenhagen. He received his PhD in 2018 from the University of Copenhagen, where his work focused on the deformation and rheology of glacier ice and the turbulent energy cascade. His research combines numerical modeling (FEM, spectral methods), microstructural analysis (ice cores), and field surveying (ice-penetrating radar) to investigate the coupled evolution of crystal fabrics and flow in ice and other geological materials. His work aims to improve constitutive descriptions of anisotropic materials and to bridge observations and modeling across scales, and has recently received funding to further transfer his methods to biophysical flow modeling.
To get news, invitations to seminars and more from Njord, please go here to subscribe to our newsletter.