THE SHAPE OF HALF DOME: 3D STUDIES OF MORPHOLOGY AND DIKING

The iconic landforms of Half Dome (HD) and El Capitan in Yosemite Valley, California, capture the attention of every visitor. They are positioned at opposite ends of the valley and offer outstanding 3D views into plutonic bodies. To investigate the origin and architecture of HD, we constructed 3D point cloud and mesh models from over 350 low-altitude aerial photos (6 cm/pixel average resolution). The models show excellent detail on all faces except the vertical Northwest Face, which is highly water-stained and was shadowed during photography. They will be used as a base for 3D mapping of dikes and other features.

Unlike El Capitan, where eight plutonic rock units exist, HD consists only of Half Dome Granodiorite (HDG) in which internal variations correlate with geomorphic and climbing characteristics (Putnam et al., this meeting). The granodiorite on HD is slightly more silicic than average HDG, and this correlates with greater resistance to weathering and increased local relief (Bartley et al., this meeting). Two types of dikes that are cogenetic with the HDG have different spatial distributions on HD. Layered granodiorite dikes are found around the summit and on the northeast shoulder (cable route) but die out toward the southwest. These dikes are somewhat less erosion-resistant than the HDG and their topographic expression is generally weak. In contrast, aplite dikes are sparse on the northeast shoulder but increasingly abundant southwestward. Aplite is more resistant than host HDG, and the southwest (Snake Dike) face exposes abundant aplite dikes that stand out in strong relief. The spatial pattern of dike compositions correlates with a gradual decrease in mafic content of the HDG. We interpret this as an upward transect through one of several mafic-felsic cycles mapped in HDG to the northeast by Coleman et al. (2012), who inferred that the cycles have been tilted down to the west in this area and record upward migration of silicic melt in a crystal mush. The southwest face of HD, the adjacent Diving Board, and the white leucogranite that forms the Porcelain Wall below define the top of this cycle. Parallel changes in lithology of both HDG and cogenetic dikes suggest that melt ascended both in fracture conduits and by Darcian flow in the surrounding mush during assembly and consolidation of the HDG.