Are some break-ups harder than others? Thoughts on the state of 'crackiness', trees vs. ice, and soft-earth mechanics through time

Earth surface scientists have long posited on what controls the rate that bedrock is converted into disaggregated, potentially mobile material (sediment). The rate and style of sediment production controls the volume and grain size mixture available to transit downhill and often, how this mobile material behaves when shaken by earthquakes or stirred by rainfall. Our inability to predict production rates and associated soil thickness and grain size distributions through time hampers past and present earthcasting. Weathering mechanisms (e.g. thermal, ice segregation, wind-driven tree sway, and plant water uptake) are cyclic over brief (seconds to minutes), diurnal, or seasonal cycles. Each can impart a different cracking pattern. Theoretically, frost-driven cracking can occur anywhere within a rock matrix zone, with areas of concentrated cracking dependent on antecedent rock conditions such as crack patterns, annual temperature amplitudes and harmonics, and water availability. Trees impart a stochastic cracking pattern – dependent on feedbacks between tree spacing, forest structure and wind, with divergence in how conifers vs. broadleaved trees respond to wind and shape flow structures.



All near-surface rock is to some degree broken by tectonics, the journey to the surface, or contraction cooling; however, a threshold density of cracks is necessary for cracks to intersect significantly. Importantly, this crack intersection threshold is a necessary condition for significant physical weathering and eventual detachment. 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. Thus, I suggest physical weathering can be non-uniform even if conditions remain constant. Additionally current ‘soil production functions’ should be recast to consider a climatically-calibrated Middle Earth production function -one that combines eco-geomorphology and often frosty processes from above with non-steady fracture mechanics from below.