GLG 362/598 Geomorphology

Mass Wasting processes

Large quantities of material are set im motion all at once. (as opposed to other processes in which individual particles are entrained and thus have a low concentration).
For mass wasting, the medium is not important versus wind and water for aeolian and fluvial processes.
We consider the effect of gravity directly on the particles rather than its effect through the transporting medium.

Initial classification:

Production-limited	Transport-limited
Transport rate depends on materials and may exceed that available from the regolith. So we end up with thin or no regolith and threshold angles (above which failure occurs are importnat). Rockfalls are an example of this type of mass wasting process	Transport rate depends on external force. Production rate > erosion rate from transport so we have thick regolith. Creep is an example of this type of mass wasting process.

Another classification:

Creep/Heave vs. Flow vs. Slide. See Figure 4.26 from Ritter, et al.
Most processes are some combination of these three.
One movement may begin as one process and change as time passes, conditions change, or slope changes.

Creep and heave

Creep-slow, nearly imperceptible downslip of soil debris.
    Continuous (gravity) or seasonal (raindrop impact, depth of motion limited to depth of penetration of weakening effect).
    Reduction of cohesion or strength of material.
    Mass transport rate is usually proportional to local slope.

Seasonal example: Frost heaving

Slope dependent and moisture content dependent: more heave; lower cohesion; depth of penetration or mobile layer important.

Continuous creep

Not restricted to unconsolidated material. Could be soil or could be blocky material.
See figures 4.27, 4.28, and 4.30 from Ritter, et al.

Solifluction

Humid climate, spring snowmelt

Gelifluction

periglacial

Similar to continuous creep above. Material is water saturated, but does not completely detach or flow but "sags" downhill.

Slides

Bedrock or soil material and needs a shear plane (detachment surface). See figure 4.33 from Ritter. Its source is a classic (Varnes, 1958). See also figure from Pewe for a local example.
Well known associations of slides with various characteristics:
    fine-grained porous materials.
    Soluble or structurally weak.
    Weathering state: deep chemical weathering
    Disposition of strata:

Examples

Blackhawk slide reference is from Shreve, 1968. The Blackhawk Landslide. Geological Society of America Special Paper 108.

The Blackhawk Landslide, San Bernardino County, CA. Photo: Kerry Sieh
Blackhawk cross-section.

Overheads:
Blackhawk oblique from Chuang and Greeley, 2000, Large mass movements on Callisto, Journal of Geophysical Research, v. 105, p. 20,227-20,244.
Blawkhawk vertical from Fairchild collection
Blawkhawk vs. other slides and speed from Shreve, 1968.

Emplacement mechanisms

Sometimes these slides turn into dry rock avalanche (or sturstrom "rock storm"). Much material is flowing but in a dry matrix. These flows have very high velocities.

Mt. Huascaran, Peru (1970) 10⁶ m³, slid 4 km at 280 to 400 km/hr:

How do these flows travel so far over low slopes?

Blackhawk emplacement was inferred to be accomodated by an air blanket beneath the slide (assumes that the slide material has very low permeability). Air blanket flows out the sides and then the slide stopped at the front, decelerating upslope and producing the transverse or arrest ridges. Important evidence for the air blanket hypothesis is that some flows have been over snow without disruption and observations of massive air blasts.

Another hypothesis was put out by Hsu in 1975 based upon observations of the Elm slide in Switzerland and further developed by Melosh. Flow resistance is reduce by acoustic fluidization of the granular material. Kinetic energy (vibration) of grains of material so great as to cause fluidization. No great mixing (material in front stays in front). This is an important alternative because of extraterrestrial slides such as large Martian ones (in Gangis Chasm: 120 km movement wiht no air cushion possible). Also our Callisto example would fit this explanation. This kind of flow is similar to one called grainflow by Bagnold.

In comparison with volcanic features (especially pyroclastic flows), heat and degassing are also important in the decrease in flow resistance.

Debris flow or debris avalanche

Unconsolidated debris at the surface rather than bedrock. Commonly associated with massive rainfalls. Move as turbulent flows. Don't go as fast or as far as rock avalanches
Likely circumstances:
Steep slopes
Thick cover of unconsolidated material
Heavy rain
Channelized

Debris flow at Tahoma Creek (Mt. Ranier), July 26, 1988.

Earthflows

Slow flows that are unsaturated (less than 10 percent water). Move relatively slow (hours to days). Unchannelized. May have arcuate scarp at head and lobate toe. Common in humid climates. Slow moving, seasonally reactivated. Often permeable materials over impermeable.

Slumgullion Colorado Earthflow

Small earthflow in roadcut in North Dakota:

Mudflows

Flowing masses of fine-grained material with high fluidity. Much water (up to 60 percent). Much fine grained material (clay size in particular).
Conditions favoring:
Steep slopes
Channelization
Abundant fine grained material
Relatively sparse vegetation
Seasonally high water content
Alpine and semi arid environs and volcanic areas (lahars)

Velocities may be up to 100 km/hr. THey move in surges. House damaged by Mt. St. Helens mudflow

Rotational slides/slumps

See overall structure of Rotational slide (Figure 4.34 from Ritter, et al.). These are usually formed above curved surfaces of failure with intermittent movement commonly involving backward rotation of the rock.