Greg M. Stock

High-resolution three-dimensional imaging and analysis of rock falls in Yosemite Valley, California

We present quantitative analyses of recent large rock falls in Yosemite Valley, California, using integrated high-resolution imaging techniques. Rock falls commonly occur from the glacially sculpted granitic walls of Yosemite Valley, modifying this iconic landscape but also posing signifi cant potential hazards and risks. Two large rock falls occurred from the cliff beneath Glacier Point in eastern Yosemite Valley on 7 and 8 October 2008, causing minor injuries and damaging structures in a developed area. We used a combination of gigapixel photography, airborne laser scanning (ALS) data, and ground-based terrestrial laser scanning (TLS) data to characterize the rock-fall detachment surface and adjacent cliff area, quantify the rock-fall volume, evaluate the geologic structure that contributed to failure, and assess the likely failure mode. We merged the ALS and TLS data to resolve the complex, vertical to overhanging topography of the Glacier Point area in three dimensions, and integrated these data with gigapixel photographs to fully image the cliff face in high resolution. Three-dimensional analysis of repeat TLS data reveals that the cumulative failure consisted of a near-planar rock slab with a maximum length of 69.0 m, a mean thickness of 2.1 m, a detachment surface area of 2750 m 2 , and a volume of 5663 ± 36 m 3 . Failure occurred along a surfaceparallel , vertically oriented sheeting joint in a clear example of granitic exfoliation. Stress concentration at crack tips likely propagated fractures through the partially attached slab, leading to failure. Our results demonstrate the utility of high-resolution imaging techniques for quantifying far-range (>1 km) rock falls occurring from the largely inaccessible, vertical rock faces of Yosemite Valley, and for providing highly accurate and precise data needed for rock-fall hazard assessment.

Assessing rockfall susceptibility in steep and overhanging slopes using three-dimensional analysis of failure mechanisms

Rockfalls strongly influence the evolution of steep rocky landscapes and represent a significant hazard in mountainous areas. Defining the most probable future rockfall source areas is of primary importance for both geomorphological investigations and hazard assessment. Thus, a need exists to understand which areas of a steep cliff are more likely to be affected by a rockfall. An important analytical gap exists between regional rockfall susceptibility studies and block-specific geomechanical calculations. Here we present methods for quantifying rockfall susceptibility at the cliff scale, which is suitable for sub-regional hazard assessment (hundreds to thousands of square meters). Our methods use three-dimensional point clouds acquired by terrestrial laser scanning to quantify the fracture patterns and compute failure mechanisms for planar, wedge, and toppling failures on vertical and overhanging rock walls. As a part of this work, we developed a rockfall susceptibility index for each type of failure mechanism according to the interaction between the discontinuities and the local cliff orientation. The susceptibility for slope parallel exfoliation-type failures, which are generally hard to identify, is partly captured by planar and toppling susceptibility indexes. We tested the methods for detecting the most susceptible rockfall source areas on two famously steep landscapes, Yosemite Valley (California, USA) and the Drus in the Mont-Blanc massif (France). Our rockfall susceptibility models show good correspondence with active rockfall sources. The methods offer new tools for investigating rockfall hazard and improving our understanding of rockfall processes.

The coating layer of glacial polish

Glacial polish has previously been thought to form by removal of material by glacier abrasion. Here we identify a micrometer-scale coating layer that suggests that the uppermost interface between ice and rock forms by accreting material to the abraded surface. Bent and broken crystals in a damage zone beneath the coating layer provide evidence for abrasion at the nanoscale, which generates the fragments and amorphous matrix that ultimately compose the coating layer. Flow and shear textures within the coating suggest that this composite material is smeared over the damage zone during ice sliding, forming a smooth surface. The coating can potentially change the shear resistance and erosion rates at the bed of temperate glaciers and likely explains the relative resistance of glacial polish to postglacial weathering.

Mechanics of relative and absolute displacements across normal faults, and implications for uplift and subsidence along the eastern escarpment of the Sierra Nevada, California

The magnitude of late Cenozoic rock uplift of the Sierra Nevada (California, USA) remains unresolved despite more than a century of investigation, with estimates ranging from essentially zero to ∼3 km of uplift at the range crest. Two sets of two-dimensional end-member mechanical models bracket how normal faulting along the eastern escarpment of the Sierra Nevada contributed to uplift of the range over a time span of millions of years. The short-term models are based on dislocations in an elastic half-space. The long-term models involve thin elastic beams resting on an inviscid fluid. Both sets of models predict that if the regional topography were entirely a response to faulting along the eastern escarpment, then the bedrock floors immediately east of the range should consistently lie thousands of meters below sea level, instead of thousands of meters above sea level as they generally do. Both sets of analyses indicate that although faulting would lift the range crest, it would drop the rock east of the range-front faults at least as much, and perhaps much more; model results suggest that ∼66%–85% of the current escarpment relief stems from subsidence of the grabens east of the Sierra Nevada, with only ∼15%–34% resulting from crestal uplift. Our results strongly indicate that range-front faulting in the last 3–10 m.y. uplifted rock at the Sierra Nevada crest by hundreds of meters to as much as 1 km, and that this uplift was superposed on high topography that predated the origin of the eastern escarpment. These conclusions are compatible with diverse geologic observations and measurements.

Thermal influences on spontaneous rock dome exfoliation

Rock domes, with their onion-skin layers of exfoliation sheets, are among the most captivating landforms on Earth. Long recognized as integral in shaping domes, the exact mechanism(s) by which exfoliation occurs remains enigmatic, mainly due to the lack of direct observations of natural events. In August 2014, during the hottest days of summer, a granitic dome in California, USA, spontaneously exfoliated; witnesses observed extensive cracking, including a ~8000 kg sheet popping into the air. Subsequent exfoliation episodes during the following two summers were recorded by instrumentation that captured—for the first time—exfoliation deformation and stress conditions. Here we show that thermal cycling and cumulative dome surface heating can induce subcritical cracking that culminates in seemingly spontaneous exfoliation. Our results indicate that thermal stresses—largely discounted in dome formation literature—can play a key role in triggering exfoliation and therefore may be an important control for shaping domes worldwide.Thermal triggering of rock exfoliation has long been discounted as relevant to the evolution of rock domes. Here, the authors documented and measured recent fracturing events in California, USA to show that hot summer periods can lead to thermal stresses and cause seemingly spontaneous rock exfoliation.