Should we? Can we? apply experimental rock physics knowledge to reconsidering soil production functions?
Authors: Marshall Jill , Rinehart Alex , Eppes Martha Cary , Meredith Phillip 
Published: February 2023

Geomorphology context: Earth surface scientists have long posited what controls bedrock to soil conversion rates, which we can now test (assuming steady state) using cosmogenic nuclides. Additionally, near-surface geophysics allows us to image the near-surface with increasing fidelity, such that weathering states from &#8216;fresh&#8217; bedrock to weathered rock to soil can be inferred over hillslope scales. Current models do not always match field data, and we are yet unable to predict soil thickness. Pedologist Hans Jenny's five factors of soil formation (climate, organisms, topography, parent material, and time) complement the factors geomorphologists presume drive soil production rates (and thus thickness). Geomorphology considers soil production rates from the top-down - whereas the existing soil thickness controls the efficacy of climate and organisms in converting bedrock into disaggregated material, and climate, topography and organisms control the transport efficiency necessary to remove soil - thus keeping the boundary between rock and soil thin enough for more top-down weathering. In most settings, we have few observations of in situ physical weathering. Weathering mechanisms (e.g., thermal, ice segregation, wind-driven tree sway, plant water uptake) are cyclic over brief (seconds to minutes), diurnal, or seasonal cycles. Almost all bring water to the crack network. Unlike traditional laboratory experiment conditions, surface rock is buffered by a soil layer and is subject to disturbance agents that can remove loose fragments - thus modifying the stress state and the crack network.&#160;Rock physics context: Laboratory experiments to date only consider bare rock. While frost weathering has a rich history of physical experimentation, we know of no other physical experiments that directly test near-surface weathering conditions specifically. While all near-surface rock is to some degree broken by tectonics, the journey to the surface, or contraction cooling; a threshold density of cracks is necessary for cracks to intersect significantly. Because crack growth rate is a function of the crack length and eventually, degree of stress accommodation due to increasing porosity, crack growth in non-uniform over time and thus physical weathering is non-uniform even if conditions remain constant. In its simplest form, considering only mechanical sources of damage, the 'Kaiser effect' suggests that under conditions of cyclic loading, cracking happens only when the previous maximum stress is exceeded. However, in natural environments, each cycle of opening refreshes water at the crack tip, allowing chemical damage to accrue, and for fracture propagation. Most progressive rock failure experiments are run monotonically, with the fracture under a consistent loading, or with rapid, cyclic loading&#8212;neither replicating conditions experienced in the natural world necessary to estimate material property change through time.Interdisciplinary context: Geomorphologists and soil scientists have generally ignored factors governing fracture propagation, and rock physicists, focused on index properties and detailed process understanding, have not simulated relevant field conditions. Here, we explore such as above in asking if and how the non-uniform nature of subcritical cracking may be a first order control on soil production and bedrock landscapes, and if so, what experiments exist or are needed to arrive at a new type of soil production function?