Abstract In December 1991, 11.8 ± 2.4 × 10 6 m 3 of rock and ice fell from Mount Cook (3754 m), the highest peak in New Zealand's Southern Alps. It fell 2720 m in 2 min and traveled 7.5 km (averaging 60 m/s). It generated a magnitude (M L ) 3.9 earthquake, becoming finely comminuted and doubling in mass through erosion. In December 1991, rockfalls began at Mount Fletcher (2467 m) 30 km northwest of Mount Cook. They continued until the ridge north of Mount Fletcher fell in rock avalanches in May and September 1992. They dropped 1440 m along similar 3.8 km paths in 50 s, generating magnitude 2.8 and 2.7 earthquakes. The first displaced 7.8 × 10 6 m 3 of water from a lake, the second, ~5 × 10 6 m 3 . In February 1996, rockfalls of ~10 5 m 3 fell from Mount Thomson (2642 m), 15 km southwest of Mount Cook. It was about an annual event in the park, and a 10-year event at Mueller Glacier. The sequence was similar to earlier phases at Mount Fletcher. The collapses all were of steep, east-facing slopes of intensely fractured rock on the hanging wall of the Main Divide fault. The larger three were of cohesionless, anisotropic materials having weaknesses along steeply dipping joints almost parallel to, but steeper than the slope. Initial failures were on lower, but steeper slopes. Retrogressive failures quickly took the entire slopes. The peaks are at limiting slope equilibrium. Collapse is their major erosion process, maintaining relief in balance with uplift and valley erosion. The hanging wall of the Main Divide fault has the fastest uplift rate, and the highest frequency of failure. The frequency of >10 6 m 3 collapse is 1 per 20–30 yr.

Abstract Very large rock avalanches, involving more than about 10 6 m 3 of rock debris, exhibit anomalously low coefficients of friction. Consequently they travel much farther than conventional slope-stability criteria predict. Such long-runout landslides ( sturz-strom ) include the catastrophic Elm (1881), Frank (1903), and Sherman Glacier (1964) events. Attempts to explain this behavior have considered water or air lubrication, local steam generation, or even the formation of melt layers within the rock debris. Discovery of deposits of such landslides on Mars and the moon, however, appears to rule out the fundamental involvement of volatiles or atmospheric gases in the flow mechanism. It appears that large, high-frequency pressure fluctuations due to irregularities in the flow of the debris may locally relieve overburden stresses in the rock mass and allow rapid pseudoviscous flow of even dry rock debris. If the avalanche volume is large enough, the rate of production of this vibrational (acoustic) energy exceeds its loss rate, and sustained motion is possible. Small-scale laboratory experiments have verified theoretical predictions of the rheology of such acoustically fluidized debris. This rheology is consistent with the rate and pattern of observed large rock avalanches. Although much work remains to be done, acoustic fluidization is the most plausible explanation of the fluidity of large, dry debris avalanches.

Abstract The Martinez Mountain rock avalanche, tentatively dated as early Holocene, comprises approximately 3.8 x 10 8 m 3 of granitic gneiss that broke away from the mountainside at elevation 1,926 m and flowed to elevation 49 m. The length of the slide debris is 7.6 km, and the width ranges from 1,070 to 1,370 m. Rock in the area surrounding the slide is closely jointed, with northwest-trending faults cutting under the slide mass. Two dominant sets of high-angle joints intersect at the crown of the slide and are believed to have controlled formation of the broad V-shaped scarp. A third joint set dips down-slope at 15° to 30°, and it locally controlled the basal surface of the slide. Because of the crystalline nature of the rock involved in sliding, water was probably not a significant factor in the triggering mechanism; rather, it is thought that a major seismic event caused the slide. Such seismic triggering is consistent with historic occurrences of massive rockslides and in the seismically active Coachella Valley such an event would provide the energy needed to disaggregate the rock and start it moving. An understanding of the structural geology and probable triggering mechanism provides a basis for comparison with similar geologic terranes and assessment of the potential for catastrophic landsliding that might affect urban developments.