GEOL 1340 Unit 6 Mass Wasting

Definition of mass wasting

Downslope movement of rock, regolith, and soil driven directly by gravity.

Two major roles of mass wasting in the Earth system

Initial step in sediment transport within the rock cycle and the most rapid means of modifying slope shapes.

Balance shaping landscapes

Internal tectonic forces (build landforms) versus external surficial forces (shape landforms).

Four primary criteria for classifying mass wasting

Rate of movement, type of material, nature of movement, and environment.

Description of creep

Extremely slow movement ($\text{mm}$ to $\text{cm}$ per year) affecting the upper several centimeters of soil.

Visual evidence of creep

Curved tree trunks, tilted telephone poles and gravestones, broken retaining walls, and sagging foundations.

Definition of solifluction

Gradual downslope movement of soil and regolith driven by freeze-thaw activity in cold environments.

Description of a slump

Downward movement of a coherent mass of loosely consolidated material along a concave-upward or planar failure surface.

Key structural features of a slump

Steep head scarp at the top and a toe at the bottom.

Description of debris flows and mudflows

Rapid, water-saturated slurries of soil, water, and fragmented rock funneling into stream channels.

What is a lahar?

A specialized mudflow composed of volcanic ash mixed with glacial meltwater on volcanic slopes.

Description of rockfalls and rockslides

Rapid to extremely fast falls or slides of individual rocks or large slide blocks moving along pre-existing planes of weakness.

Historic example of a major rock avalanche

The 1903 Frank Slide in Alberta, where 44 million cubic meters of limestone collapsed off Turtle Mountain in 100 seconds at $120\text{ km/h}$.

Examples of submarine mass wasting

Submarine slumps, submarine debris flows, and turbulent turbidity currents.

Two baseline conditions required for mass movement

Initial fracturing and weathering, and an unstable slope.

Formula condition for slope failure

Downslope force ($F_d$) exceeds resisting force ($F_r$) / shear stress exceeds shear strength ($F_d > F_r$).

Definition of angle of repose

The maximum stable angle at which loose, unconsolidated sediment can remain stable without sliding.

Angle of repose for fine sand vs coarse sand vs angular pebbles

$\sim 35^\circ$ for fine sand, $40^\circ$ for coarse sand, and up to $45^\circ$ for angular pebbles.

Common structural planes of weakness parallel to hillslopes

Bedding planes in sedimentary rocks, exfoliation joints in granites, and foliation planes in metamorphic rocks.

Dual role of water in slope stability

Small amounts increase surface tension and cohesion; larger amounts increase speed, and oversaturation destroys cohesion, adds heavy overburden weight, and increases pore water pressure.

Common triggers for mass movement

Oversteepening slopes, reducing slope strength, and shocks/vibrations (earthquakes, traffic, construction blasts).

Why regions like British Columbia and California are highly susceptible to landslides

Steep topography, tectonic activity, and heavy seasonal wet periods.

Tool for property buyers to check landslide risk

Landslide hazard maps.

Engineering methods to prevent undercutting

Installing riprap (heavy rock armor) along shorelines or rivers and setting up drainage diversions.

Function of revegetation in slope stability

Root systems bind soil layers and stabilize potential failure planes.

Structural protections used against mass wasting

Rock bolts, soil nailing, shotcrete coatings, flexible wire mesh nets, debris barriers, debris chutes, and concrete avalanche sheds.