WHY IS HALF DOME THERE? TOPOGRAPHIC EFFECT OF LITHOLOGIC VARIATION IN THE HALF DOME GRANODIORITE, YOSEMITE NATIONAL PARK, CALIFORNIA

Although subtle, lithologic variation in the Half Dome Granodiorite appears to correlate with topographic expression. The granodiorite contains km-scale cyclical lithologic variations that we interpret to reflect buoyant ascent of rhyolitic pore melt during assembly and consolidation of the pluton (Coleman et al., Geosphere, 2012). Within a cycle, the color index (CI; = % dark minerals) varies from ~20 near the bottom to near 0 at the top. Bulk magnetic susceptibility (BMS) is an effective quantitative proxy for mapping variation in CI in Half Dome Granodiorite. Visual comparison of mapped BMS to topography suggests that CI variation in the Half Dome is a significant factor in the topographic evolution of Yosemite. In particular, topographic highs including Half Dome, Cloud’s Rest, North Dome, Indian Rock, and Mt. Watkins are underlain by rocks with below-average BMS. To test this hypothesis, we compared surface elevation at 1750 sites where we have BMS measurements to the average elevation of the surrounding topographic surface within a 2 km radius. The result is a weak negative correlation between local elevation and BMS, i.e., low-BMS rocks tend to be topographically higher than high BMS-rocks. More significantly, nearly all the sites where elevation is >300 m above the local average have low BMS.

Studies of granite weathering indicate that volumetric expansion of biotite is the principal mechanism by which such rocks are weathered. Half Dome Granodiorite that has lower than average BMS and CI contains less biotite and therefore should be less susceptible to weathering and erosion. If so, the lithologic effect should be stronger in areas above the Last Glacial Maximum trim line where hillslope processes that depend on prior weathering of bedrock are important, than below the trim line where glaciers cut deeply into bedrock and exposed surfaces that are minimally weathered.