POROSITY AND DEEP WEATHERING OF CRYSTALLINE ROCKS
Here we present results from a compilation of studies that quantify porosity in weathering profiles. With this data we evaluate modes of porosity development in weathered crystalline bedrock. These data point to an important difference in systems where weathering induced fracturing opens fracture porosity and allows for deeper transport of water compared to systems where porosity development occurs at the grain scale through mineral dissolution only. Weathering induced fracturing observed in granitic rocks opens pathway to advect meteoric fluid, promoting deeper weathering. In contrast, in studied mafic rocks that weather without weathering induced fracturing, solute transport is dominantly by diffusion leading to less overall weathering and thinner regolith. Thus, the presence or absence of fracturing during weathering may explain thicker regolith on granitic rocks compared to mafic rocks under similar weathering conditions: an observation that is opposite of what would be predicted from mineral dissolution kinetics. A full understanding of regolith formation will only be possible when we can quantitatively describe the changes in porosity and surface area at the pore scale that occur during weathering, especially deep in the Critical Zone.