Chapter 15: Mass Wasting: The Work of Gravity

Mass wasting serves as a fundamental bridge between weathering processes and sediment transport by rivers, glaciers, and coastal systems, while simultaneously shaping landscapes through valley widening and debris delivery. The chapter frames mass wasting as both a significant geologic hazard capable of causing catastrophic property damage and loss of life, and as an essential process that redistributes vast quantities of material across Earth's surface over geologic timescales. The chapter establishes that slope stability depends on complex interactions between driving and resisting forces, with water saturation being a critical control that reduces friction, decreases cohesion, and adds weight to slopes. Oversteepened slopes resulting from stream erosion, wave action, or human engineering modifications such as roadcuts represent particularly vulnerable conditions. Vegetation removal through logging, agriculture, or wildfire substantially increases mass wasting susceptibility by eliminating root anchoring systems. Earthquakes function as catastrophic triggers capable of generating hundreds of simultaneous landslides through ground shaking and liquefaction, wherein water-saturated sediments lose strength and behave as fluid-like masses. The chapter presents a comprehensive classification scheme based on material type, motion mechanics, and movement rates. Falls involve free-falling rock fragments that accumulate as talus slopes. Slides occur along weakness zones and are subdivided into rotational slumps with curved failure surfaces and translational rockslides along planar discontinuities. Flows behave as viscous fluids and encompass debris flows, lahars from volcanic regions, and earthflows composed of clay-rich material. The chapter distinguishes rapid processes such as rock avalanches traveling at extreme velocities while riding on air cushions from slow imperceptible processes including soil creep driven by freeze-thaw cycles and solifluction in permafrost environments. The chapter emphasizes that slow mass wasting processes, though lacking dramatic impact, collectively transport more material than catastrophic events and represent dominant processes shaping landscapes over geologic time.