LANDSLIDES
A. INTRODUCTION
The word "landslide" is used loosely by most people to refer
to a wide variety of slope
movements, some of which are not
truly "slides"
as
geologists define this term. To geologists, a
slide is the movement of a coherent
mass of rock and/or soil
along a defined slip surface (note,
however, that once a slide
mass begins to move, it usually
breaks up as it descends the
slope). Slides are
divided into two types (called block glides
and slumps) based on the nature of the slip surface.
Topography
plays an important role in the occurrence of slides,
so it is not surprising that
mountainous areas have the greatest
susceptibility
to slides. (State by state
risk of landsliding).
B. BLOCK GLIDES
1. DEFINITION
Block glides are the movement of a
rock and/or sediment
mass
along an inclined
plane. Slip planes are inherent in
certain
rock types (e.g., bedding
in sedimentary
rocks or
foliation
in metamorphic rocks), but faults,
fractures and/or
contacts
between two geologic formations can also serve
as
potential slip surfaces (when
they dip toward a valley).
2. CAUSES
In general, block glides occur when driving force due to
gravity
exceeds the forces that resist sliding (friction and
cohesion). The
question is, why would a mass of rock or
soil along a slope become unstable and start to move?
Except in unusual cases, such as rising
magma, the angle
of a slip surface does not get steeper, so something else
must make the slide mass unstable:
a. Possession Beach, Washington
d.
Hebgen
Lake, Montana (photo)
e. Slides with Multiple Causes
C. SLUMPS
1. DEFINITION
Slumps are rotational movements of an earth mass along a
curved slip surface.
They tend to occur in unconsolidated
materials that are relatively uniform in
composition.
2. CAUSES
Slumps occur when the driving force (the mass of the upper
part
of the slope) exceeds the force that resists rotation
(the
mass of
the lower part of the slope). What triggers slumps?
a.
Genesee River, New York
