Lecture Outlines
Physical Geology, 12/e
Plummer & Carlson
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mass Wasting
Physical Geology 12/e, Chapter 9
Mass Wasting
• Mass wasting is downhill movement of
masses of bedrock, rock debris or soil,
driven by the pull of gravity
• Landslides have been far more costly
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 all
major geologic hazards
Classification of Mass Wasting
• Types of mass wasting are
classified based on:
– Rate of movement (speed)
• 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)
– Movement Type
• 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
Mass Wasting Chart
• List the three classifications for which mass
wasting is determined. (Not the three types
of movements.)
Some types of Mass Wasting
Page 222
What are the variables? How is the data chart
set up?
How does the chart specify material?
Tell the specific type of mass
wasting:
1. flow, slow velocity, soil =
creep
2. bedrock, fall, fastest =
rockfall
3. debris, flow, varying speeds =
debris flow
4. bedrock, slide, varying speeds =
rockslide
Factors Controlling Mass Wasting
page 226-228
explain each one
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
Factors Controlling Mass Wasting
Common Types of Mass Wasting
• Creep (or soil creep)
– Very slow downslope
movement of soil
– 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
Common 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 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 often 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
page 238 – 243
explain how to prevent mass
wasting
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