DOCUMENTING HOW ROCK PHYSICAL PROPERTIES CHANGE AS A FUNCTION OF EXPOSURE AGE

Rock physical properties are rarely considered in the context of exposure age or erosion rate of the outcrops from which they are sampled, despite the fact that the measurable properties – like unconfined compressive strength (UCS) - of ‘weathered’ samples are known to vary from those of more ‘fresh’ samples (e.g. Cuccuru et al., 2012). Here we use a space-for-time approach to quantify the systematic change in material properties that may occur over geologic time (10^1-10^4 yrs) as rocks are subaerially exposed and weather. We sampled five ~25 cm diameter boulders that were deposited on alluvial fan surfaces of known age along the eastern flank of the Sierra Nevada Mountains in Owens Valley, California. One additional sample was collected from an unvegetated bar in an active channel – with the idea that abrasion during transport would ‘reset’ any major fracturing or weathering on the surface of the boulder. Depending on recovery, on varying numbers of specimens from each boulder we measured: Tensile Strength, porosity, UCS, static and dynamic Youngs Modulus (E), p- and s-wave velocities, fracture toughness (K) and the subcritical cracking index (n). Data are internally consistent between these metrics for the samples - for example as porosity increases, velocity decreases suggesting that tests results are valid. With the exception of the modern wash sample, most data show strong and consistent time-trends. Our results have implications for understanding and interpreting a broad range of surface process and rock physics data.