Porosity Characteristics of Granitic Clasts in a Chronosequence of Moraines

Weathered granitic rock has been shown to have appreciable porosity and water holding capacity. It plays a significant role in water and plant relationships, especially where soils are thin in arid and semi-arid regions. Granitic clasts from a chronosequence of moraines were used to assess the rate of porosity formation and characterize changes in porosity. The moraines are near Bishop, CA on the east side of the Sierra Nevada. The ages of the moraines, previously measured by cosmogenic surface dating techniques, can be used to approximate the length of time the rocks have been weathering. Clasts were collected from the upper 60 cm of soil on six of the moraines, with estimated ages of 15, 19, 65, 90, 120, and 160 ka. Total porosity was calculated using bulk density and particle density measurements. Water retention curves were measured and used to estimate effective pore size distributions within rock fragments. Thin sections were analyzed to observe pore branching and networking. Total porosity ranged from 2% in the 15 ka moraine to 15% in the 160 ka moraine.