Mass Wasting - The Changing Earth: Exploring Geology and Evolution - page 273

INTRODUCTION
Triggered by relentless torrential rains that began in
December 1999, the floods and mudslides that devastated
Venezuela were some of the worst ever to strike that coun-
try. Although an accurate death toll is impossible to deter-
mine, it is estimated that at least 19,000 people were killed, as
many as 150,000 were left homeless, 35,000 to 40,000 homes
were destroyed or buried by mudslides, and between 10 and
20 billion in damage was done before the rains and slides
abated. It is easy to cite the numbers of dead and homeless,
but the human side of the disaster was most vividly brought
home by a mother who described standing helplessly by and
watching her four small children buried alive in the family
car as a raging mudslide carried it away.
Mudslides engulfed and buried not only homes, build-
ings, and roads, but also entire communities. Some areas were
covered with as much as 7 m of mud. In addition, fl ooding
and the accompanying mudslides swept away large parts of
many of Venezuela's northern coastal communities, leaving
huge areas uninhabitable.
This terrible tragedy illustrates how geology affects all of
our lives. The underlying causes of the mudslides in Venezu-
ela can be found anywhere in the world. In fact, landslides (a
general term for mass movements of earth) cause, on average,
between 25 and 50 deaths and more than 2 billion in dam-
age annually in the United States. By being able to recognize
and understand how landslides occur and what the results
may be, we can fi nd ways to reduce hazards and minimize
damage in terms of both human life and property damage.

Mass wasting (also called mass movement ) is defined
as the downslope movement of material under the direct
infl uence of gravity. Most types of mass wasting are aided by
weathering and usually involve surfi cial material. The mate-
rial moves at rates ranging from almost imperceptible, as in
the case of creep, to extremely fast, as in a rockfall or slide.
Although water can play an important role, the relentless
pull of gravity is the major force behind mass wasting.
Mass wasting is an important geologic process that can
occur at any time and almost any place. It is thus important
to study this phenomenon because it affects all of us, no mat-
ter where we live (Table 11.1). Although all major landslides
have natural causes, many smaller ones are the result of hu-
man activity and could have been prevented or their damage
minimized.

FACTORS THAT INFLUENCE
MASS WASTING
When the gravitational force acting on a slope exceeds its
resisting force, slope failure (mass wasting) occurs. The
resisting forces that help maintain slope stability include
the slope material's strength and cohesion, the amount of
internal friction between grains, and any external support
of the slope ( Figure 11.1). These factors collectively defi ne

a slope's shear strength.
Opposing a slope's shear strength is the force of gravity.
Gravity operates vertically, but has a component acting par-
allel to the slope, thereby causing instability (Figure 11.1).

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Selected Landslides, Their Cause, and the Number of People Killed TABLE 11.1

Date Location Type Deaths

218 B . C . Alps (European) Avalanche—destroyed Hannibal's army 18,000

1556 China (Hsian) Landslides—earthquake triggered 1,000,000

1806 Switzerland (Goldau) Rock slide 457

1903 Canada (Frank, Alberta) Rock slide 70

1920 China (Kansu) Landslides—earthquake triggered ~200,000

1941 Peru (Huaraz) Avalanche and mudfl ow 7,000

1962 Peru (Mt. Hauscarán) Ice avalanche and mudfl ow ~4,000

1963 Italy (Vaiont Dam) Landslide—subsequent fl ood ~2,000

1966 United Kingdom (Aberfan, South Wales) Debris fl ow—collapse of mining-waste tip 144

1970 Peru (Mt. Hauscarán) Rockfall and debris avalanche— earthquake triggered 25,000

1981 Indonesia (West Irian) Landslide—earthquake triggered 261

1987 El Salvador (San Salvador) Landslide 1,000

1989 Tadzhikistan Mudfl ow—earthquake triggered 274

1994 Colombia (Paez River Valley) Avalanche—earthquake triggered >300

1999 Venezuela Mudfl ow >19,000

2005 U.S.A. (Southern California) Mudfl ow 10

Source: Data from J. Whittow, Disasters : The Anatomy of Environmental Hazards (Athens: University of Georgia Press, 1979); Geotimes ; Earth ; and U.S.G.S.