Long-term geological processes involve deformation of Earth materials, such as rocks, minerals, and ice, by a variety of microscopic processes occurring at rates that range from very fast (instantaneous) to very slow. These microscopic processes act as recorders of deformation conditions while also influencing the macroscopic mechanical behavior. This session will explore deformation over a variety of length and timescales, including the extremes of the strain rate spectrum, with an emphasis on microstructural evolution and the role of microstructure in controlling macroscopic processes. We encourage contributions describing laboratory experiments, theoretical developments, and field studies, so as to encourage lively discussion and debate among practitioners with a variety of backgrounds and geophysical expertise.