Microphysics of evolving rock viscosity in the seismic and glacial cycles

Despite being the epitome of strength, the solid rocks below Earth's surface can flow surprisingly rapidly over human timescales, impacting processes of societal relevance. This project aims to deliver new equations describing this flow based on the underlying processes operating in the rocks.

Major earthquakes and the melting of ice sheets cause deflections of Earth's surface that are facilitated by viscous flow of the hot rocks below. This deformation creates important feedbacks. During the seismic cycle, earthquakes induce viscous flow of rocks beneath the fault zone that impacts the spatial and temporal distributions of future earthquakes. During the glacial cycle, viscous flow of rocks beneath melting ice sheets causes ground uplift that impacts sea-level change. Therefore, modelling these systems requires knowledge of the viscosity of rocks in Earth's lower crust and upper mantle.

Unfortunately, the viscosities of these rocks are not constant but instead undergo a transient evolution whenever there is a change in the applied forces. Whilst we know that this viscosity evolution occurs, we do not know why. Without knowing the microscale processes that control the viscosity evolution, we cannot formulate equations that reliably predict flow of rocks in the Earth over the seismic and glacial cycles. At a time when populations exposed to seismic risk are rapidly expanding and when the modelling of ice-sheet dynamics is of unprecedented importance, it is critical to develop new models for the viscosity evolution of the rocks that underpin these systems.

Deciphering the microphysical processes that control the viscosity evolution of rocks requires an ambitious multidisciplinary approach. Each element of the research will be centred on the novel adaptation of techniques from the forefront of the materials sciences to analyse key geological minerals. Experiments will be conducted at temperatures up to 1500 degrees Celcius and will induce viscosity evolution by imposing instantaneous changes in the applied forces, analogous to those imposed by earthquakes. For the first time, a subset of the experiments will be performed inside a scanning electron microscope allowing the samples to be directly imaged during the tests. The microstructures of the samples will be analysed using state-of-the-art microscopy techniques, pioneered by our group, to measure distortions of the crystal lattices and the forces trapped within them. The combined mechanical data and microstructural observations will provide the new insights necessary to determine the key processes operating in the crystal lattices of the minerals that cause their viscosities to evolve.

The interpretations from the laboratory will be subject to two critical tests. To check the consistency and robustness of the interpretations, we will employ the latest models of deforming crystalline materials. We will adapt these models, developed to simulate metals, to analyse geological materials. The relevance of the laboratory experiments to natural rocks will be tested by comparing the microstructures of minerals from both settings. We will utilise samples from the deep portions of major fault zones that provide direct records of viscous flow in the lower crust and upper mantle.

The critical information gained from experiments, microstructural analyses, and modelling will be used to construct and calibrate new equations describing viscosity evolution. For the first time, the equations will be based on rigorous analyses of the specific underlying processes. These equations will unlock the next generation of large-scale models that incorporate the impacts of viscosity evolution in the seismic and glacial cycles. The mechanical and microstructural data generated in this project will be made freely available, providing a new and unique digital resource. Similarly, the techniques pioneered in this project will open new frontiers in the dynamics of geological materials.