Mass Wasting Chapter 13 Slide 2 What is mass wasting? Basic part
of the Rock Cycle Breakdown of solid bedrock and redistribution of
weathered components Downslope movement of masses of rocks by
gravity Slide 3 Downslope Movement Downslope journeys -
imperceptibly slow, very fast, or somewhere in-between Gravity
tends to pull any mass on a sloping surface downhill As slope angle
increases, gravity exerts a pull in the downslope direction. Slide
4 The Role of Water Not always a factor, but In some types of mass
wasting is very significant factor Unconsolidated sediments behave
differently when wet Example: water and sand drawn together by a
force called capillary attraction (caused by surface tension) Slide
5 The Role of Water (2) Capillary force holds wet sand together BUT
- Saturation creates a slurry reducing friction - easily flows away
Large rock mass movement attributed to effects of increased water
pressure within voids Result - sudden failure (collapse of rock
mass due to reduced friction) Slide 6 Slope Failure CR - large
slope failure in glaciomarine silt, Churchill River, Labrador.
Slide 7 Mass Wasting Processes Landslide any perceptible downslope
movement of a mass of bedrock, regolith*, or both Two broad
categories: Sudden failure of a slope: relatively coherent masses
of rock or rock debris Downslope flow of mixtures of solid
material, water, and air *regolith any solid materials, such as
rock fragments or soil, lying on top of bedrock Slide 8 Slope
Failures Collapse of rock or sediment mass All hill slopes and
mountain cliffs are susceptible due to constant pull of gravity
Rock debris transferred downslope and more stable slope condition
is reestablished Several types of slope failure: Slumps, fails, and
slides Slide 9 Small rock slope failure in limestone on a strike
slope in Jasper National Park, Canadian Rocky Mountains. Slide 10
Slumps Failure in which a downward and outward rotational movement
of rock or regolith occurs along a curved concave-up surface Most
common May occur singly or in groups Range 1-2 meters, or 100-1000s
meters Numerous along roads and highways Relate to construction
activity Slide 11 Slump Aerial of the La Conchita slump. Santa
Barbara County, California. March 1995. Classic slump clearly
detailing the scarp, slump and flow at base of the movement. An
orchard is at the top of the slope. Slide 12 La Conchita slump.
September 1995. Ground level perspective of the slump. Slide 13 La
Conchita slump. September 1995. Destroyed home is still in place in
2002. Slide 14 Slump 1. The top of the displaced block usually is
tilted backward, producing a reversed slope. 2. Slumps are
frequently associated with heavy rains or sudden shocks such as
earthquakes. 3. Distinct episodes of slumping may be related to
changing climatic conditions. 4.Slumping may recur seasonally and
be associated with seepage of water into the ground during the
rainy season. Slide 15 A large slump on a high gravel terrace
beside the Yakima River in central Washington has broken up a major
highway and displaced it more than 100m laterally into the river
channel. Slide 16 Collapse of slope to stand in the way of stream
making a pond, located at Draix - French South Alps. Slide 17
Rockfalls and Debris Falls Free falling of detached bodies of
bedrock from a cliff or steep slope May be single rock fragment or
a huge mass of rock Debris fall similar to rockfall but consists of
a mixture of rock material as well as vegetation Slide 18 Forms due
to gravity in the Alps (Dolomites, Italy): a debris cone (left side
of the picture) and a landslide (rock-fall, right side of the
picture in the foreground). Scale is given by some persons in the
lower left corner. Slide 19 Rockfalls and Debris Falls As the rock
material falls off it gathers speed until it breaks on impact into
a vast number of smaller pieces. These pieces continue to bounce,
roll, and slide down- slope before friction and decreasing slope
angle bring them to a halt. Slide 20 Rockfall Rockfall due to wedge
failures, Satluj Valley Higher Himalayas, India. Slide 21 Debris
flow influenced channel with levees, Payette River drainage of
Idaho, USA. Debris flows are a common occurrence following fire on
granitic soils. Slide 22 Debris flow influenced first order channel
creating a fan and rapid as it enters a higher order channel,
Payette River drainage of Idaho USA. Slide 23 Debris fan of Poucet
torrent after a debris flow, located in the Maurienne Valley -
French North Alps. Slide 24 Rockslides and Debris Slides Rockslide
the sudden downslope movement of a detached mass of bedrock (or of
debris in a debris slide) along an inclined surface, such as a
bedding plane Common in high mountains with steep slopes Rock
debris range in size: sand to boulder Slide 25 When large rock
slides occur, the resulting deposit generally is a chaotic, jumbled
mass of rock, with individual boulders that may measure tens of
meters across. Slide 26 Rock Debris Talus accumulation of angular
rock fragments at the bases of steep cliffs; sloping outward from
the cliff that supplies it Angle of repose the steepest angle at
which the debris remains stable; typically lies between 30 0 and 37
0. Slide 27 Talus cone at gravel pit, northern MI. Slide 28 Talus A
talus at the base of a steep mountain slope in the Brooks Range, in
northern Alaska. Rock fall debris moving down a steep gully spreads
at its base to form conical talus. Slide 29 Angle of repose Coarse,
angular limestone blocks stand at the angle of repose (about 30 0 )
in a talus below steep cliffs in the central Brooks Range, Alaska.
Slide 30 Sediment Flows Mass wasting process in which particles
move in such a way that the overall mass can be described as a
flowing motion Mixture of solid particles, water, and sometimes air
Slide 31 Sediment Flows Classification of sediment flows on the
basis of their velocity and sediment concentration. The transition
from a sediment-laden stream to a slurry flow occurs when the
sediment concentration becomes so high that the stream no longer
acts as a transporting agent; instead the direct action of gravity
becomes the primary force causing the saturated sediment to flow.
Slide 32 Factors Controlling Flow Relative proportions of solids,
water, and air Physical & chemical properties of sediment Two
classes of sediment flows: Slurry flow moving mass of
water-saturated sediment Granular flow a mixture of sediment, air,
and water; subdivided by flow velocity into: - creep, and debris
avalanche Slide 33 Slurry (wet) Flows Slurry - moving mass of
water-saturated sediment Slide 34 Slurry Flows Sediment mixture
often very dense Boulders too large may roll with the flow Flow
cease results in poorly sorted (or even non-sorted) sediment Three
types of slurry flows Solifluction Debris flows Mudflows Slide 35
Solifluction Solifluction the very slow downslope movement of
saturated soil and regolith Slide 36 Solifluction This process lies
at the lower end of the velocity scale for flowing sediment-water
mixtures Results in distinctive surfaces; lobes and sheets of
debris Occurs on hilltops in temperate and tropical latitudes Slide
37 Two solifluction lobes, 1-2m thick, have crept slowly downslope
and covered glacial deposits on the floor of the Orgiere Valley in
the Italian Alps. Slide 38 Debris Flows - Involves the downslope
movement of unconsolidated regolith, the greater part being coarser
than sand, at rates ranging from only about 1 m/yr to as much as
100 km/h Some begin with a slump or debris slide and continue to
flow downslope Commonly have a tongue-like front Very irregular
surfaces Associated with heavy rainfall Slide 39 Debris Flow Slide
40 Mudflows a rapidly moving debris flow with a water content
sufficient to make it highly fluid Predominant particles no coarser
than sand Velocity at upper end of debris flow Highly mobile Travel
rapidly along valley floors and spread laterally when no longer
contained by valleys Slide 41 Mudflows Slide 42 The bouldery front
of a muddy debris flow advances from left to right along a stream
channel in the wake of an earlier surge of muddy debris. The steep
bouldery front, about 2 m high and advancing at 1.3 m/s, acts as a
moving dam, holding back the flow of muddy sediment upstream.
Passage of muddy debris flow along a canyon bottom near Farmington,
Utah in June 1983. Slide 43 Mt. St. Helens Mudflows - During the
1980 eruption of Mount St. Helens in Washington, volcanic mudflows
were channeled down valleys west and east of the mountain. Some
mudflows reached the Columbia River, having traveled more than 90
km. Flow velocities were a high as 40 m/s and averaged 7 m/s. Slide
44 Mudflow Deposit Slide 45 Granular Flows Largely dry with air
filling the pores Or, may be initially saturated with water but
have a range of grain sizes and shapes that allows water to escape
easily Types of granular flow: Creep and colluvium Earthflows
Grainflows Debris avalanches Slide 46 Granular (dry) Flows
Earthflow Debris Avalanche Slide 47 Creep and Colluvium The
imperceptibly slow downslope movement of regolith Steeply inclined
rock strata are sometimes bent over in the downslope direction
Occurs at a rate too slow to be seen Rates higher on steeper slopes
than on gentle slopes Rates increase with soil moisture increase
Slide 48 Creep Effects of creep on surface features and on bedrock.
Steeply inclined strata have been deformed near the surface by
differential creep, so they appear folded. Telephone poles and
fence posts affected by creep are tilted, stone fences are
deformed, roadbeds are locally displaced, tree trunks may be bent,
and gravestones are tilted or fallen. Slide 49 Copyright Richard
Hanson 2003 Downslope tilting of steeply dipping sedimentary layers
due to creep (near Marathon, West Texas). Slide 50 Hill slope
processes (presumably sheet wash and soil creep) have removed
approximately 1 meter of soil to expose the root system of the
tree. On the other hand, the roots look pretty grizzly and uniform,
so there may have been a landslide episode decades ago that
stripped the topsoil off the hill. (Photo taken in the mountains
immediately north of Lake Arrowhead, California.) Slide 51 Shale
Overturned by Creep Beds of shale have been overturned by slow
downslope creep on a hillside in the Laramie Basin, Wyoming. Slide
52 Slump blocks in colluvium north of Pittsburgh, PA. Slide 53
Earthflows Among the more common mass-wasting features on the
landscape are earthflows, granular flows having a velocity in the
range of about 1 cm/day to several hundred m/h. May remain active
for several days, months, or even years Made of weak regolith,
predominantly silt or clay-sized particles Occur on gentle to
moderate steep slopes Occur where ground is saturated Slide 54
Earthflows At the top of a typical earthflow is a steep scarp,
which is a cliff formed where slide material has moved away from
undisturbed ground upslope. An earthflow generally has a narrow,
tongue -like shape and a rounded bulging front. They may range in
size from several meters long and wide and less than a meter deep,
to as much as several hundred meters wide, more than 1 km long, and
more than 10 m deep. Slide 55 A special Earthflow A special earth
flow occurs in wet, highly porous sediments. These sediments may
weaken if shaken suddenly, as by an earthquake. An abrupt shock
increases shear stress and may cause a momentary buildup of water
pressure in pore spaces. The result is rapid fluidation of sediment
and abrupt failure, a process known as liquefaction. Slide 56 Grain
Flow Involves movement of a dry or nearly dry granular sediment
with air filling the pore spaces Occurs naturally when accumulating
sand grains produce a slope that exceeds the angle of repose,
leading to failure Slide 57 Demonstration of grain flow in sand
Slide 58 Debris Avalanche A large, rapidly moving debris avalanche
commonly constitutes a huge mass of falling rock and debris that
breaks up, pulverizes on impact, and then continues to travel
downslope, often for great distances. Slide 59 Avalanche Recent
rock avalanche on Anderson Lake, southwestern B.C., Canada. Slide
60 Avalanche Small rock avalanche in north Vancouver Island, B.C.,
Canada. Slide 61 Debris Avalanche, Mt. Shasta A massive prehistoric
debris avalanche from the north- western flank of Mount Shasta
volcano in northern California left a chaotic deposit (hills in
middle distance) that extends 34 km from the volcano and covers at
least 450 km 2. Slide 62 Mass Wasting in Cold Climates Mass wasting
is especially active at high latitudes and high altitudes where
temperatures are very low Regions where much of the landscape is
underlain by perennially frozen ground and where frost action
occurs Frost heaving and creep Gelifluction Rock glaciers Slide 63
Anatomy of Frost Heaving Frost heaving on a street in Sweden. Slide
64 Freeze-Thaw Cycle Stone moved downslope by alternate freezing
and thawing of the ground. As freezing occurs (B), the stone is
raised perpendicular to the ground surface, which also rises. When
the ground thaws and settles (C), gravity pulls the stone down
approximately vertically, giving it a small but significant
component of movement downslope. (A-C) Slide 65 Mass Wasting in
Cold Climates Gelifluction as gravity pulls the thawed sediment
slowly downslope, distinctive lobes and sheets of debris are
produced. ( This process is similar to solifluction in temperate
and topical climates) Rock glacier is a tongue or lobe of ice-
cemented rock debris that moves slowly downslope in a manner
similar to glaciers Slide 66 Front of active rock glacier, Kluane
National Park, Yukon Territory Slide 67 Rock glacier -
tongue-shaped rock glacier on the NW flank of Mount Cumulus, Never
Summer Mountains, CO, fed by friable volcanic rocks. Note adjacent
valley-wall rock glaciers and talus and penetration to and below
treeline. Slide 68 Assignment Answer Questions for Review # 1-12
page 350 - DUE 11/17 Prepare for Vocabulary quiz 11/17 study The
Language of Geology 23 vocabulary terms for Chapter 13 on page
350
