Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to years. Mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, and other planets.

Classification of Mass Wasting Fall – free-fall of detached particles, slope steep enough that material falls to base Slide – material remains cohesive and moves along a well-defined surface ex. Bedding, cleavage, fault plane. Slump-Movement occurs along a curve surface of ruptures. Flow – material moves downslope as a viscous flow (most are saturated with water) Mudflow: Consists of at least 50% silt and clay particles with 30% water Debris flow: Contains larger sized particles and less water than mudflow Earthflow: Thick, viscous, tongue shaped mass of wet regolith Quick clays: Composed of fine silt and clay particles saturated with water Solifluction: water saturated surface sediment Creep: the slow downslope movement of soil, sediment or rock along more gentle slopes than give rise to the more rapid forms of mass wasting. The surface itself gives little indication that creep is occurring, however features above the surface of the slope provide evidence that slow downslope movement is occurring.  

Rock Fall (Rock Avalanche) Sudden and rapid free fall movement of rock Occur on steep slopes Can grade into a rockslide if material greater a slope greater than the angle of repose May be caused by EQ, explosion

Land-Rock Slide Movement of material along a defined slippage plane Sudden and rapid movement Large blocks of rock detach along bedding planes, joints, fractures, etc. Occur on steep slopes Can be triggered by rain falls or ground vibration Fastest and most destructive type of mass wasting

Slump Slow to rapid movement Material moves as a coherent unit along a curved surface (spoon-shaped) Blocks of material rotate Debris flows commonly associated with sump

What is the difference between Slump and Slide?

Solifluction. Solifluction occurs in cold regions in which the water near the surface freezes and thaws repeatedly. Solifluction is a relatively slow form of mass wasting. When the water near the surface freezes the soil and rock is moved upward and in the downslope direction due to the expansion of the water. When the water thaws the rock and soil moves vertically back to the surface. Although the downslope motion may only be a fraction of an inch for each freeze and thaw cycle, the repeated cycles each day result in significant downslope movement over time.

Creep: Indicators of creep are curved tree trunks, bent fences or retaining walls, tilted poles, and soil ripples. Trees display bent after being tilted by the creep movement. Trees tend to grow vertically, towards the sun, and so when they are tilted by downslope movement.

Summary of mass movement

Requires weathering of rocks or unconsolidated materials (purple block) Primary driving force is GRAVITY (W) Force mechanics: For the rock to move downslope; force (d) has to be greater than the resistance force (L) holding it up, e.g. friction. Slope angle influences force (d); the greater angle the greater force

Factor of Safety FS > 1 ! STABLE; FS < 1 ! UNSTABLE; FS = 1 ! CRITICAL

FS > 1 ! STABLE; FS < 1 ! UNSTABLE; FS = 1 ! CRITICAL

Type of Material Bedrock - Rock Unconsolidated material - Debris Soil Regolith Sediment

d L d L Slope angle influences force (A); the greater the angle the greater the force

Addition of water Lubricates the material (decreases cohesion) Adds weight (increases downslope force) Increases pore pressure (increases downslope force and decreases cohesion)

Exercises