Weathering and Porosity Formation in Subsoil Granitic Clasts, Bishop Creek Moraines, California

Porous weathered rock can play an important role in storing water and nutrients that are accessible to plants and microbes, especially in thin or skeletal soils. The objectives of this study were to measure the rate of porosity development in granitic rock fragments and determine how pore morphology changes with time. Total porosity and pore characteristics were measured on granodiorite clasts in soils of the Bishop Creek moraines, eastern Sierra Nevada, California. The soils formed a chronosequence; soil surface age represented the weathering time for the clasts. Clast porosity was estimated to form at a rate of 0.10% per thousand years. Pores were dominantly inter- and intramineral planar voids formed by stress fracturing induced through biotite expansion. Granodiorite clasts in the older moraines had greater total porosity, microporosity, and connectivity of pores, increasing potential for water movement and storage and ease of root and mycorrhizal penetration. Roots and clay films observed in pores inside the weathered clasts indicate water storage and movement from the soil into clasts. The most weathered clasts have significant porosity that contributes to the ecosystem functions of the regolith.