Renewal: 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 distribution 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. During the first phase of this project, experimental data and observations of the microstructures of deformed rocks provided the basis for a new framework of equations describing how the viscosity of rocks evolves as they flow. However, this work also highlighted important knowledge gaps regarding the fundamental microphysics of flow and how key processes should be mathematically described. 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 delve deeper into the microscopic processes of viscosity evolution in 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 1600 degrees Celsius and will induce viscosity evolution by imposing instantaneous changes in the applied forces, analogous to those imposed by earthquakes. At the same time, we will monitor sound waves emitted by defects in the crystals to characterise the fundamentals of their behaviour during flow. Using a new approach pioneered in the first phase of the project, 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 what controls key effects, such as the rate of viscosity evolution. Using the refined equations describing viscosity evolution, we will begin to explore the impacts of our work for the behaviour of fault zones over the earthquake cycle and rebound of underlying rocks as ice sheets melt. To build understanding of how the relevant materials and systems behave, we will take a multilevel modelling approach. First, we will analyse simplified scenarios designed to capture the key aspects of the flow of deep, hot rocks in the aftermath of major earthquakes or melting ice sheets to gain intuition of the most important effects. Second, we will integrate our new equations into three-dimensional models of Earth’s crust and mantle to simulate specific earthquakes and melting ice sheets to understand how these systems are likely to behave into the future.