I. Mass Wasting & Landscape Development

A. Landscape Development Concepts

• "geomorphology" - the science of landforms: their origin, evolution, form, and spatial distribution

• "denudation" (defn) - all processes that cause a degradation (wearing down) of the landscape: weathering, mass movement, erosion, & transport

• ideally, endogenic processes build and create initial landscapes, whereas exogenic processes work toward low relief, an ultimate condition of little change, and the stability of sequential landscapes

• (Geologist, John Wesley Powell (1834-1902) - base level:

• a hypothetical level below which a stream cannot erode its valley; i.o.w., the lowest level of denudation - represented by sea level extending back under the landscape

• "local" base levels may control the lower limit of local streams - due to lakes, resistant rocks, and human-made dams

• as physical factors fluctuate in an area, the surface constantly responds in search of equilibrium

• the "dynamic equilibrium model":

• landscapes in a dynamic equilibrium show ongoing adaptions to the ever-changing conditions of rock structure, climate, local relief, and elevation

• the pattern over time:

• a) equilibrium stability; b) destabilizing event; c)period of adjustment; & d) a new and different condition of equilibrium stability

B. Slopes

• defn - are curved, inclined surfaces that bound landforms

• slopes are the most common elements in our physical landscape

• while most slopes appear stable and unchanging, they are dynamic features because gravity results in the downslope movement of material

• principal elements of slopes:

• waxing ("increasing") (convex) slope; free face, debris slope (receives materials), waning (concave) slope

• slopes are shaped by the relationship between rates of weathering and the rates of mass movement and erosion of slope materials

• a slope is stable if its strength is greater than the denudation processes; & unstable if its materials are weaker than than these processes

C. Mass Wasting

• all mass wasting (movement) occurs on slopes

• weathering (physical & chemical) weakens surface rock & makes it more susceptible to the force of gravity - & thus downslope movement

• mass wasting is the step that follows weathering

• weathering by itself does not produce significant landforms

defn - "mass wasting": all movement of materials propelled & controlled by gravity

• the downslope movement of rock, regolith, and soil under the influence of gravity
• mass wasting does not require a transporting medium (e.g., wind, water, or ice)

• for material to move downslope, the forces of erosion must overcome friction, particle cohesion, & inertia

• movement can range from very gradual to an avalanche of material

mass movement can range from:

• dry to wet ....... slow to fast ..... small to large ....... free-falling to gradual or intermittent ......

• the greater the slope angle, the more susceptible the surface material is to mass movement

• mass movement factors (controls):

• gravity (driving force) weight, size, & shape of surface material

• available moisture (frozen or fluid)
• lubricating effect
• adds weight
• reduces cohesion among particles
• earthquakes may trigger mass movement by generating more driving force

• oversteepened slopes
• human activities create oversteepened and unstable slopes - slopes primed for mass wasting
• vegetation
• the root systems of plants bind soil and regolith together to protect against mass wasting and erosion
• where plants are lacking, mass wasting is enhanced
• vegetation can be removed by construction, fire, farming, etc.
• vegetation contributes to slope stability
• mass movement processes:

• classified on the basis of:

(1) type of material

rock vs. debris/earth/mud

(2) the type of motion displayed:

• fall: the movement involves the free-fall of detached individual pieces - common on steep slopes

• slide: occur whenever material remains fairly coherent and moves along a well defined surface

• flow: material moves downslope as a viscous fluid; are typically saturated with water

(3) velocity of the movement:

• the most rapid type of mass movement is a "rock avalanche"
• most mass movements are much slower, almost imperceptible

The Events/Processes:

• rockfall - the free-falling movement of debris from a cliff or steep slope, generally falling straight down or bounding downslope - "talus cones" form at the base of the steep slope

• debris avalanche - a mass of falling & tumbling rock, debris, and soil - reach tremendous velocities due to the presence of ice & water

• slump - the downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved (concave) surface - the material does not travel far or fast - common in thick accumulations of cohesive material

• rockslide - a sudden rapid downslope movement (via a sliding motion) of a cohesive mass of regolith and/or bedrock that is not saturated (smaller velocity than debris avalanche)

• if the material involved is largely unconsolidated, it is termed a "debris slide"

• mudflow - the downslope movement in channels - as a unit - of very wet soil & rock - common in semiarid mountainous regions - "lahars" when on the slopes of volcanoes

• earthflow - the downslope movement - as a unit - of very wet soil & rock - common on hillsides in humid areas after heavy rain and snow melt - leaves a scar on the hillslope shaped like a "tongue"

• soil creep - the persistent mass movement of surface soil - the individual soil particles are lifted & disturbed by:

• the expansion of soil moisture as it freezes, by cycles of moistness & dryness, by diurnal temperature variations, & by grazing livestock & the digging of animals

• Permafrost Landscapes