Chapter 16: Unsafe Ground: Landslides and Other Mass Movements

Marshak presents a comprehensive classification of mass-movement processes spanning temporal scales from imperceptibly slow creep and solifluction to catastrophic rockfalls, avalanches, and debris flows. Understanding slope stability requires analyzing the mechanical balance between resistance forces—derived from friction, cohesion, and substrate strength—and downslope gravitational forces that increase with slope angle and material weight. The angle of repose, the steepest angle at which loose material remains stable, serves as a fundamental threshold for predicting failure. Slope failure occurs when this equilibrium breaks, triggered by mechanisms including substrate weakening from chemical weathering, saturation from infiltrating water that reduces effective stress, undercutting by fluvial or wave erosion, seismic vibration, and anthropogenic modifications such as slope steepening during construction or deforestation removing stabilizing vegetation. The chapter analyzes landmark disasters—the 1970 Yungay landslide in Peru that destroyed a town, the Vaiont Dam rockslide in Italy that generated catastrophic waves, and the 2014 Oso mudslide in Washington—to illustrate how geological hazards interact with human settlement patterns and infrastructure. Beyond terrestrial processes, submarine mass movements including turbidity currents and seafloor slumps generate tsunamis that pose regional hazards, as demonstrated by the Storegga Slide and 1998 Papua New Guinea events. Modern mitigation strategies employ engineering controls such as drainage systems, slope regrading, rock bolts, and retaining walls alongside ecological approaches including revegetation and avalanche sheds. Geologists employ satellite monitoring, ground-based sensors, and hazard mapping to identify vulnerable regions and inform land-use planning, particularly in tectonically active and mountainous terrain where climate extremes, steep topography, and tectonic activity converge to create elevated mass-movement risk.