Overview
This lecture reviews the types of mass wasting (mass movement), focusing on how earth materials move (falling, sliding, flowing, subsiding), with examples of their impacts both on land and underwater.
Types of Mass Movements
- Mass movements are classified by material type and mode of movement.
- Major types: slides, falls, flows, and subsidence.
- Slides: movement as a coherent mass on a defined surface (slump or translational slides).
- Falls: material free falls vertically, typically rocks or soil from steep slopes.
- Flows: material moves as a viscous fluid, often water-saturated and unconsolidated.
- Subsidence: ground collapses into voids, often forming sinkholes.
Falls
- Rock falls are rapid, mostly vertical movements of rock from steep slopes or cliffs.
- Triggered by weak planes (joints/cracks) in rock, gravity is the main driving force.
- Falls can be deadly and destructive, especially near populated areas.
Flows
- Creep: slow, downslope movement from freeze-thaw or wet-dry cycles, affects structures and vegetation.
- Earth flows: faster than creep, typically in water-saturated, fine-grained materials; destructive to infrastructure.
- Debris flows/mud flows: rapid, highly destructive slurries; mud flows are water/mud dominated, debris flows have more large particles.
- Debris avalanches: fastest and most destructive, nearly all large debris with minimal water.
Slides
- Slides can be translational (flat surface) or rotational (curved surface, forming slump blocks).
- Debris slides break into many smaller blocks.
- Slump blocks rotate backward and create visible scars and messy deposits.
- Slides and slumps are often triggered by heavy rainfall or coastal activity, may generate tsunamis if they enter water.
Mass Wasting Disasters
- Destructive flows and slides have caused significant loss of life and property worldwide (e.g., Oso, WA, Venezuela, Peru).
- Mud and debris flows can kill quickly, especially during storms or volcanic activity (e.g., lahars).
Avalanches
- Snow avalanches occur mainly in mountainous regions after heavy snowfall or warming events.
- Avalanches can be predicted by identifying unstable snow layers or cornices.
- Fast rescue is critical as suffocation and hypothermia are major risks.
Subsidence
- Caused by withdrawal of groundwater, oil, gas, or collapse over voids.
- Damages infrastructure and increases flood risk, especially near coasts or over old mines.
Underwater (Submarine) Landslides
- Occur on both steep and nearly flat submerged slopes.
- Submarine slides can move vast amounts of sediment quickly and cause tsunamis.
- Studied using sonar and seismic reflection data to identify slide types and history.
- Blocky slides (southern Oregon) pose higher tsunami risk than frequent, smaller disintegrative slides (northern Oregon).
- Massive underwater slides have occurred off the Hawaiian Islands, moving blocks the size of small islands.
Key Terms & Definitions
- Mass wasting — Downslope movement of earth material by gravity.
- Slide — Movement along a surface; can be translational (flat) or rotational (curved/slump).
- Fall — Free fall of material, usually rock, from a steep slope.
- Flow — Movement as a viscous, often water-saturated mass (includes creep, mud flow, debris flow).
- Subsidence — Downward settling or collapse of ground, often due to voids or resource extraction.
- Avalanche — Rapid flow of snow (sometimes with debris) down a slope.
- Lahar — Volcanic mudflow.
- Translational slide — Slide along a planar, typically flat, surface.
- Slump (rotational slide) — Slide along a curved surface with backward rotation.
- Debris flow — Rapid, chaotic flow with mostly large particles.
- Creep — Very slow, gradual downslope movement of soil or rock.
- Submarine landslide — Large-scale underwater mass movement of sediment.
Action Items / Next Steps
- Watch recommended videos of mass wasting events for visualization.
- Review past module on groundwater and subsidence for integration.
- Prepare for the next lecture on factors controlling slope stability.