Granular materials are ubiquitous in the Earth system: gouge in fault cores; fragmented volcanic rock, tephra and crystal mush; and soils, sediments and debris on hillslopes and in river channels. Although these settings span orders of magnitude in pressure, strain rate and scale, their mechanical behaviour is governed by shared grain-scale processes: shear localisation, force chains, dilatancy and compaction, grain breakage, cohesion, and coupling with pore-fluid pressure. They let granular media switch abruptly between solid-like and fluid-like states, controlling how failure starts, evolves and stops.
In the solid Earth, granular mechanics sets the frictional strength and slip stability of fault gouge, and thus the nucleation, arrest and healing of earthquakes and slow slip, and fault reactivation by natural or induced pore-pressure changes. In volcanic systems, it shapes crystal-rich magma rheology, frictional sliding along conduit plugs and lava domes, flank and dome collapse, and pyroclastic density currents.
At the surface, the same physics governs soil shear strength and its sensitivity to moisture and suction, landslide initiation and creep, rock avalanche runout, slope failures evolving into debris flows and lahars, and sediment transport and erosion. Rainfall, seismic shaking, snowmelt and freeze-thaw act through the same grain-fluid couplings that operate at depth.
We invite laboratory experiments (shear, flume, centrifuge and shake-table tests, grain-scale imaging), field observations and remote sensing, numerical modelling (discrete element, coupled fluid-particle and continuum) and theory (rate-and-state friction, critical state, μ(I) and non-local rheologies). We especially welcome work crossing the solid Earth and surface divide: controls of grain size, shape, comminution and cohesion on friction and dilatancy; hydro-mechanical and thermal coupling (dilatancy hardening, thermal pressurisation, liquefaction); transitions from creep to catastrophic failure; triggering by pore pressure, cyclic loading and seismic waves; and precursors of failure. Interface topics such as earthquake-triggered landslides and coseismic liquefaction are a natural focus.
By bringing together fault mechanics, volcanology, geomorphology, soil mechanics and natural hazards, the session aims to identify common mechanisms and transferable models, and to discuss how grain-scale insights upscale to natural systems and hazard assessment.