Landslides & Mass Movement

Vocabulary flashcards covering mass movement causes, stresses, weak planes, mass wasting types, case studies, and mitigation techniques.

Yungay, Peru Disaster

A catastrophic mass movement event where an earthquake triggered a massive ice slab to break off a glacier and shoot down a mountain, burying the town in debris within $4$ minutes and resulting in $20{,}000$ deaths.

Steep Slope Angle Threshold

A slope angle greater than $35^\bot$ ($> 35^\bot$) that significantly increases the likelihood of mass movement.

Gentle Slope Angle Threshold

A slope angle of less than $5^\bot$ ($< 5^\bot$).

Downslope Shear Stress ($\sigma_d$)

The downslope driving stress acting on rock and regolith that is driven by gravity.

Normal Stress ($\sigma_n$)

The resistance stress (or shear strength) that resists downslope movement, governed by cohesion and friction.

Parallel Weak Planes

Geological planes of weakness oriented parallel to the slope angle, which increases the probability of surface failure.

Perpendicular Weak Planes

Geological planes of weakness oriented perpendicular to the slope angle, which decreases the probability of surface failure.

Angle of Repose

The steepest angle that granular material can maintain without collapsing downslope.

Talus

Loose rock material accumulated at a cliff's base that forms a steep angle of repose.

Creep

The slow, gradual downslope movement of regolith caused by repeated cycles of expansion and contraction (freezing/thawing, wetting/drying, warming/cooling), causing trees and tombstones to tilt.

Slump

A type of mass wasting where rock and regolith remain mostly coherent as they slide downslope along a failure surface.

Translational Slump

A slump in which rock and regolith move downslope along a planar failure surface.

Rotational Slump

A slump in which rock and regolith move downslope along a concave failure surface.

Portuguese Bend Slump

A massive translational slump in Southern California where clay overlies shale bedrock; activated in $1956$ following a $1950\text{s}$ housing boom, moving at $1\text{ to }2\,\text{cm}$ per day and destroying $150$ homes.

Mudflow

A non-coherent, viscous downslope flow consisting of a slurry of mud saturated with excessive water.

Debris Flow

A non-coherent, viscous downslope flow composed of a slurry mixture of mud and large rocks saturated with excessive water.

Rockslide

The sudden downslope movement of rocks and debris along a non-vertical slope, which breaks into a chaotic jumble and leaves a visible scar on the slope with a debris pile at its base.

Rockfall

A mass wasting event where rock freely falls down a vertical cliff or slope and shatters upon impact; commonly observed in exposed road cuts and during spring due to ice wedging.

Riprap

A layer of loose stones or boulders added to a slope to stabilize it and mitigate mass wasting risk.

Nevado del Ruiz Eruption (1985)

A volcanic eruption in Colombia with a $\text{VEI} = 3$ whose pyroclastic debris melted glacier ice caps, triggering far-reaching lahars that caused over $24{,}000$ deaths.