GEOPHYSICAL INVESTIGATION OF THE ROYAL ARCHES MEADOW ROCK AVALANCHE IN YOSEMITE VALLEY – CA

Since the retreat of the Tioga glacier (~15,000 yr BP), rockfall along with other slope movements have been the major force shaping Yosemite Valley, CA. Rockfalls or rockslides that extend far beyond the cliff where they originate are called rock avalanches. These events are rare, and move large masses of material over long distances, performing in geologically instantaneous time the amount of work accomplished over millennia by more frequent but smaller magnitude surface processes. Yosemite Valley is home to at least ten of those large deposits, with the Royal Arches Meadow Rock Avalanche (RAMRA), situated in the eastern Yosemite Valley, being one of the oldest events (~14,000 yr BP). Because this event occurred shortly after the Last Glacial Maximum (LGM), mapping the interface between this rock avalanche and the underlying valley surface can give us insights about the state of the valley shortly after the LGM. Besides having an unknown thickness, parts of the RAMRA deposit are covered by recent aggradation, which reduces its surface expression. This presents an additional challenge to estimating the extent and depth of the deposit. To map the interface between the RAMRA and the underlying valley surface as well as to estimate the avalanche extent, we used a combination of geophysical methods (Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR)). The strong electrical resistivity contrast between the resistive rock avalanche and the conductive underlying sediments (valley floor deposits) makes both ERT and GPR ideal methods for our purpose, allowing us to obtain estimations for both the extent and thickness of this rock avalanche.