GEOG 1350 Chapter 6: CHAPTER 6

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16 Aug 2016
October 6th, 2015. Katy Lemaire
Rock avalanche is a type of landslide involving sudden failure of a large mass of rock that
rapidly fragments and travels as a streaming mass at high speeds.
6.1 Introduction to Landslides
-landslides and mass wasting are terms to describe the downslope movement of rock or
sediment due to gravity
Types of Landslides:
-the variables that underpin the classifications are: the mechanism of the movement, the type
of material, the amount of water present and the rate of movement.
-A fall involves bounding of rock or blocks of sediment from the face of the cliff
-A slide is the downslope movement of a coherent block of rock or sediment along a discrete
failure plane
-A slump is a particular type of slide in which the failure plane is curved upwards
-A flow is the slow to rapid downslope movement of sediment in which particles move semi-
independently of one another, commonly with the aid of water
-Debris Flows are mixtures of mud, debris and water. They range in consistency from thick mud
soups to wet cement, they typically move rapidly
-Creeps are very slow flows of rock or sediment, at rates ranging from millimetres to dozens of
centimetres per year (sackung-slope sag- has been applied to large slow-moving landslides)
-A slow-creep like movement in which a rock mass pivots about a point is a topple
-high velocity flows of fragmented rock are referred to as rock avalanches
 famous landslide in the Cordillera Blanca of Peru (large EQ released an avalanche of rock and
ice fragments which quickly transformed into debris flow
-subaqueous landslides (underwater) submerged slope of a delta or at the edge of a
continental shelf can change into a debris flow or turbidity current (example is the grand banks
landslide of Newfoundland in 1929.)
Forces on Slopes:
-relationship between driving forces move rock or sediment down a slope and resisting forces
that oppose those forces
-the resisting force is the shear strength of the slopes material (resistance to failure)
-slope stability is evaluated by computing a factor of safety (FS) defined as the ratio of the
resisting forces to the driving forces which are determined by variables such as types of
materials, slope angle and topography, climate, vegetation, water and time
(1) Material Type: mineral composition, degree of cementation or consolidation and presence
of planes of weakness, planes of weakness may be sedimentary bedding planes, metamorphic
foliation, joints or zones along which rock or soil has moved before. They are hazardous if they
are on an incline of 15 degrees. Slides have 2 basic patterns of movement, rotation and
translational, rotational has slides and slips and translational has planar slip surfaces.
-rotational produces small topographic benches that tilt upslope which include fractures in all
rock types
-A common type of a translational slide is a debris avalanche which is a very shallow slide of
sediment or soil over bedrock
(2) Slope and Topography: steepness and topographic relief, steeper the slope the greater the
driving forces that promote failure. Topographic relief refers to the height of a hill or mountain
above the land below
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