A GEOLOGIST-CLIMBER'S PERSPECTIVE ON THE MANY FACES OF HALF DOME, YOSEMITE NATIONAL PARK, CALIFORNIA

Since 1870, when the summit of Half Dome in Yosemite National Park, CA, was declared “perfectly inaccessible” by Josiah Whitney of the California Geological Survey, the summit of the iconic granitic monolith has been a goal for outdoor enthusiasts from around the world. Each of its four main faces presents a different climbing challenge and has a unique geomorphic character because of the bedrock geology and erosion history of the dome and surrounding area. Recent high-resolution geologic mapping and 3D modeling reveal the causes of many of these differences. The overhanging, fractured, rockfall-prone Northwest Face likely owes its existence in part to a fault adjacent to and oriented nearly parallel with the face. The easiest route ascending this steep face is provided by a series of surface (sub)parallel exfoliation sheets that extend the length of the face. The comparatively low slope angles of the northeast, south, and west faces are a product of the local homogeneity of the Half Dome Granodiorite and the lack of significant regional joints or faults in those orientations. Climbers scale these otherwise featureless faces via steeply dipping aplite dikes that protrude up to 0.5 m from the surface of the rock because of differential weathering. The high relief of these dikes, as well as cosmogenic Be-10 exposure ages on boulders in a glacial moraine at the base of the dome, suggest that glaciers reached less than halfway up the face during the Last Glacial Maximum. Therefore, the summit of Half Dome has likely never been glaciated, and owes its rounded shape entirely to exfoliation. Layered dikes of Half Dome Granodiorite exist on the summit yet appear to have little effect on morphology of the dome. Abundant exfoliation joints on the northwest and northeast faces of the dome and aplite dikes on the south and west faces created the differences in difficulty and style of climbing, affected the slope angle of the dome, and influence the risk of rockfall. In this fashion, bedrock geology directly affected the development of Half Dome as one of the most significant features in mountaineering and outdoor recreation culture.