CH 09 MASS WASTING

Lecture Outlines

Physical Geology, 10/e
Plummer, McGeary &
Carlson


Mass Wasting
Physical Geology 10/e, Chapter 9

Steve Kadel, Glendale Community College


Mass Wasting
• Mass wasting is downhill movement of
masses of bedrock, rock debris or soil,
under the pull of gravity
• Landslides are much more costly over
time in the U.S., in terms of both lives
and dollars, than all other geologic and
weather hazards combined
• Mass wasting is, with proper planning,
perhaps the most easily avoidable of the
major geologic hazards


Classification of Mass
Wasting
• Types of mass wasting are
classified based on:
– Rate of movement

• Wide range from < 1cm/year to >100
km/hour

– Type of material
• Did moving mass start out as solid
bedrock or as debris (unconsolidated
material at Earth’s surface)

– Type of movement
• Flow, slide, or fall


•

Classification of Mass
Wasting
Types of movement
– Flow
• Descending mass moves downhill as a viscous
fluid

– Slide
• Descending mass remains relatively intact, and
descends along well-defined surfaces
• Translational slide - movement along plane
parallel to motion
• Rotational slide (slump) - movement along a
curved surface

– Fall

• Material free-falls or bounces down a cliff


Factors Controlling Mass
Wasting
• Factors making mass wasting likely:
– Steep slopes
• Shear forces maximized by gravity

– Large relief
• (large elevation change from top of
mountains/hills to valley floor)

– Thick layer(s) of loose rock,
debris, soil
– Presence of water
• Lubricates moving rocks/debris/soil

– Lack of vegetation
• No roots to hold rock/soil in place

– Seismic (earthquake) activity


Types of Mass Wasting
• Creep
– Very slow downslope movement of
soil or unconsolidated debris
– Major contributing factors include
water in soil and daily freeze-thaw

cycles
– Can be costly to maintain homes,
etc., on creeping ground as
foundations, walls, pipes and
driveways crack and shift
downslope over time


Types of Mass Wasting
• Debris flow - mass wasting in which motion takes
place throughout the moving mass (flow)
– Earthflow - debris moves downslope, slowly or rapidly, as
a viscous fluid
• Commonly occurs on steep hills, with thick debris cover,
after heavy rains
• Solifluction of saturated debris is an example

– Mudflow - flowing mixture of debris and water, usually
down a channel
• Most likely to occur on steep unvegetated slopes with thick
debris cover
• Heavy rains on the slopes of stratocone volcanoes with fresh
ash layers are triggers

– Debris avalanches are very rapid and turbulent
• Can reach speeds of several hundred km/hr


Types of Mass Wasting
• Rockfall - when a block of bedrock breaks free

and falls or bounces down a cliff
– Commonly an apron of fallen rock fragments (talus)
accumulates at cliff base

• Rockslide - the rapid sliding of a mass of
bedrock along an inclined surface of weakness

• Rock avalanche - a very rapidly moving,
turbulent mass of broken-up bedrock

• Debris slide - a coherent mass of debris moving
along a well-defined surface

• Debris fall - a free-falling mass of debris


Preventing Landslides
• Preventing mass wasting of debris
– Construct retaining wall with drains
– Don’t oversteepen slopes during construction

• Preventing rockfalls and rockslides on
highways
– Remove all rock that is prone to sliding
– “Stitch” together outcrop
• Important to know the susceptibility of land to
mass wasting before building any road or
structure

End of Chapter 9