Geology 1001-section 4
    Dr. Dutrow
 

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MASS WASTING (abbreviated versions of overheads)

· Mass movement = process that transports quantities of Earth's material downslope under the pull of gravity without

 
 

- economic impact ca $75 billion in damages
 

· steady-state process

i.e. input of material on a slope =

 

· Mass wasting = process by which mass movements occur and erode land surface.

- consequence of

 

 

Factors that affect movement

1. Steepness and instability of slope -
 
 

2. Nature of slope materials;

        -unconsolidated
        - consolidates

    friction between grains must be greater than

- angle of repose - the maximum angle at which

 
· depends on
 

        more stable

        less stable
 

· depends on amount of

- increases surface tension -
 
 

* Vegetation
 

 

3. Role of Gravity

(a) Gravity tends to move particles downslope but there are two competing forces.

i.Shear Stress - force acting to cause movement
 
ii. Shear Strength - resistance to

 

A. frictional resistance - particles are not

 

B. particle cohesion
C. vegetation binding: materials bound together (b) if shear stress <shear strength =

(c) Shear stress can increase by:

i.

ii.

 (d) Shear strength can decrease by: i.
ii.
iii.
4. Role of water (a) small amounts of water increase the strength of sediments due to

(b) too much water

 

(c) Large rock masses may move due to

 

 
(d) Water reduces shear strength by:
i. reduction of

 

ii. reduction of
 
MASS-WASTING PROCESS

· Classified based on:

1. nature of material (e.g.

 

2. speed of movement (e.g. cm/yr - km/sec)
 
3. manner of movement
  · Takes place on slopes by a number of possible processes:
1. Sudden Failure- downslope transfer of coherent masses e.g. Yosemite
2. Downslope flow - internal movement of masses
 
Slow Mass Movement - FAILURE

1. CREEP - slowest, mm-cm/yr
 

-         loose material is

-         e.g.
 
 

2. SOLIFLUCTION - downslope movement of
 

    occurs only in cold regions when upper
 
 

RAPID Mass Movement
-km/hr
-classified by type of motion
 
 

1. SLUMP - downward and outward rotation of curved, concave up surface

e.g. oversteepened road cuts or river banks - (check out the banks along of the stream drainage next to University Lakes)
- commonly associated with
    Produces a SCARP

2. FALLS and SLIDES

· Rockfall - free falling rocks, often slabs broken loose by weathering

· Debris fall - free falling
 

SLIDES

· Rockslide - detached rock mass that slides down slope as a

 

· Debris slide - detached
 

· Rock avalanche - rock flows rather than

  · Accumulations produce a talus apron

 

SEDIMENT FLOWS

(a) Flow of mixtures of rock, water and air
(b) Factors that determine the way sediment flows depend on:
i. relative proportions of
ii. physical/chemical properties of

 

(c) TYPES of sediment flows:
i. Slurry Flow - moving mass of

 

ii. Granular flow - grain-supported mixture of

 

 
(d) SLURRY Flows:
i. Solifluction - downslope movement of

 

- occurs only in cold regions when

 

 

ii. Debris flows - unconsolidated regolith -

 

iii. Mud flows - unconsolidated masses of mixed

 

 
(e) GRANULAR Flows
i. Creep - very slow downslope movement

 

depends on
hockey stick trees
 
ii. Earthflows - fluid mass movements that travel slightly faster than
 
iii. Liquefaction - water saturated soil transformed into

 

   

Factors responsible for mass movements

(a) oversteepening of

 

(b) water content

    - overirrigation of slopes
 

(c) structural attitude of
 

(d) earthquakes

(e) volcanic eruptions

(f) clear cutting of trees

(h)mining