Regolith Weathering, Plastic Deformation and Hydrologic Evolution in a Volcanic Landscape

Landscape level observations of streams on the island of Hawai’i indicate that runoff ratios are essentially zero on the young active volcanic surfaces of windward Mauna Loa and Kilauea volcanoes and increase with age on the older Mauna Kea and Kohala surfaces. Recent work on both permanent and ephemeral stream flows shows that the changes in runoff are associated with decreases in saturated hydraulic conductivity ( k h ) and aquifer thickness ( D ) that change with surface age (Perez-Fodich et al. 2024). The processes that control k in soils are plausibly related to weathering but the mechanisms are complex. Chemical weathering causes mass loss and most weathering reactions have negative ∆ V so intuitively weathering should increase porosity ( f ) and likely k h . Dry bulk density r s of weathered soils initially does decrease with weathering mass losses. However trends in r s reverse with further age, and calculated soil strain becomes negative. A model of poro-plastic deformation where the medium comprises an incompressible matrix with a finite yield stress and a void fraction f that is deformable but has zero yield stress (e.g. Gurson (1977)) can represent the evolution of soil strain if is a function of weathering mass loss. In this model the overall plastic yield limit is a function of both and f . The constitutive relation between chemical alteration and strength of the residual soil matrix is not known in detail. I hypothesize that the yield stress of the residual matrix is a function of the loss of network-forming components such as SiO 2 , and that the change in yield stress can be described with a power law function of an alteration and mass loss index such as enrichment in an immobile element Nb/Nb 0 . The resulting change in soil hydraulic properties diminishes vertical infiltration and promotes lateral flow toward incipient streams. The resulting incision leads to marked landscape level changes in hydrology and geomorphology, enhanced by incising stream capture of groundwater in shallow aquifers.