Mass Movements Flashcards
weathering
processes at or near the
Earth’s surface that result in the breakdown of exposed rocks to form regolith
driven by solar radiation: an exogenic process
physical weathering:
mechanical break-up of bedrock into small particles through the action of physical forces acting at or near Earth’s surface
chemical weathering:
chemical changes in rock-forming minerals in the presence of water
acidic surface waters penetrate carbonate rocks along fractures and bedding planes and dissolve the rock
The frequency of rock types at the Earth`s surface.
Igneous Rocks
- Granite 15%
- Basalt 3%
Sedimentary Rocks
- Shale 52%
- Sandstone 15%
- Limestone 7%
Other 8%
The frequency of primary rock-forming minerals at the Earth`s surface.
aluminosilicate minerals
- Feldspar - 30%
- Quartz - 28%
ferromagnesian minerals
Mica - 18%
Pyroxene, Amphibole – 1%
Calcite, Dolomite – 9%
Iron Oxides – 4%
Others – 10%
efficacy of physical weathering processes influenced by:
rock structure – size and abundance of fractures; effective porosity and permeability
degree of saturation of pore space by fluids
frequency of freeze-thaw cycles, wet-dry cycles
hydration
involves the adsorption of water into or onto the crystal structure of minerals
e.g., evaporite minerals, clay minerals, iron oxides
CaSO4 anhydrite + 2H2O water = CaSO4.2H2O gypsum
2Fe2O3 hematite + 3H2O water = 2Fe2O3.3H2O limonite
rock structure
size and abundance of fractures; effective porosity and permeability
rock composition
silicate vs. carbonate minerals
efficacy of chemical weathering processes influenced by:
rock structure
rock composition
supply of water and carbon dioxide
degree of saturation of pore space by fluids
ambient temperatures
hydrolysis
H+ displaces metallic cations and silica, forming secondary clay minerals
silica combines with OH- to form soluble minerals
carbonation
the dissolution of limestone (CaCO3)and dolomite (CaMg(CO3)2), creates karst landscapes
dolomite dissolves more slowly limestone
Mafic rocks -
As with the dissolution of limestone, the constituent parts of the mineral are now entirely dissolved in water, leaving no residual mineral.
Iron olivine can react with water and atmospheric oxygen like this:
hematite and goethite are both very insoluble in water: they remain as residual minerals. It is these iron oxides that give many soils their reddish or yellowish color.
bedrock
unweathered rock
regolith
surface layer of rock particles
soil
surface layer containing living organisms and capable of supporting plants
colluvium
transported regolith and soil on hillslopes
Mass Movement
rock, regolith and soil are susceptible to movement on slopes under the force of gravity
defined as the spontaneous, gravity-driven, downslope movement of materials on hillslopes
as the slope angle increases, the force of gravity exerts a greater downslope force
(shear stress > normal stress )
shear strength
movement may occur when internal cohesion and frictional resistance are overcome by shear stress
the shear strength of materials is expressed as:
S = C + (σn-σpw) tanθ
where S = shear strength
C = cohesion
σn = normal stress
σpw = pore water pressure
θ = angle of internal friction
Factor of Safety = shear strength/shear stress
F < 1; slope is unstable and mass movement may occur
F > 1; slope is stable
slope stability is affected by:
slope angle
nature of surficial materials
nature and extent of vegetation cover
pore water pressure
Topographic relief
refers to the height of a hill or mountain above the land below
landslides occur more frequently in areas of high relief
Slope angle
the steeper the slope, the greater the driving force
steep slopes are
associated with falls and topples
moderate slopes are
associated with slides and flows
gentle slopes are associated with creep
planes of weakness
bedding planes in sedimentary rocks, foliation planes in metamorphic rocks, joints in igneous rocks, or zones along which Earth has moved before
degree of consolidation
creep, slumps and flows are common in unconsolidated materials
shape of slip surface
rotational slides or slumps are curved
translational slides are planar
Rock Mass strength
intact rock strength (Schmidt hammer test)
very strong: quartzite, gabbro, basalt
strong: marble, granite, gneiss
moderately strong: sandstone, shale, slate
weak: coal, siltstone, schist
very weak: chalk, rock salt
The Role of Climate
influences the amount and timing of precipitation that infiltrates rock and soil or flows across the surface as overland flow
water saturates soil, lubricating planes of weakness and increasing pore water pressures
water erodes bases of slopes which decreases stability
arid regions are prone to rock falls, debris flows, and soil creep
humid and sub-humid regions are prone to creep, slides, slumps, and debris flows
The Role of Vegetation
plant roots add strength and cohesion to slope materials
vegetation adds weight to slopes
increases the likelihood that the slope will fail
The Role of Time
forces acting on slopes change with time
driving and resisting forces change seasonally as the water table fluctuates
weathering of rocks occurs slowly over time; affects rock strength
Slope stability can be expressed in terms of a
Factor of Safety (F):
F = Shear strength (s) / Shear stress (τ)
Mass Movement Processes Defined
type of earth materials; rock (consolidated), soil//regolith (unconsolidated)
- fine-grained materials – earth/mud
- coarse-grained materials – debris
mechanism of movement; –creep, slide, flow, fall
Common processes:
-soil creep, debris flows, mudflows, rotational slumps (landslides), rockfalls
Creep
very slow downslope movement of rock, regolith or soil; mm or cm per year
Slide
rock, regolith or soil slide on an inclined surface; minimal internal deformation of materials during movement; ~ 1 km per hour
Slump
coherent blocks of rock, regolith or soil slide on a concave surface; minimal internal deformation of materials during movement; ~ 1 km per hour
Flow
rapid downslope movement of saturated rock, regolith or soil; considerable deformation of materials during movement; > 5 km per hour
Fall and Topple
rocks free fall through the air
Heave and Creep
Heaving associated with freeze-thaw cycles (frost creep), wetting-drying cycles in soils and rocks (seasonal creep).
In periglacial environments underlain by permafrost gelifluction contributes to the mass movement of regolith.
Movement of material is limited to depths of less than 1 metre: the rate of movement decreases rapidly with depth.
These processes contribute to the slow, plastic
deformation of soil and rock: 0.1 - 15 mm/year
on vegetated slopes; several 10’s mm/year on
colluvium affected by frost action.
Landforms created by these processes include
terracettes and solifluction lobes.
rate of creep (ΔL) varies directly with slope angle:
ΔL = H tanβ
where: L = creep parallel to the ground surface
H = heave normal (at right angle) to the
ground surface
β = slope angle
translational slides
involve movement along essentially planar, uniformly sloping surfaces.
rotational slides
involve movement along curved, concave surfaces.
a rotational slide (or slump) occurs when, destabilized by undercutting at the toe and/or overloading at the top, the block rotates as it fails, moving upward at the toe.
plane of failure is concave upwards.
involve movement along curved, concave surfaces.
undercutting (e.g., riverbank erosion)
lubrication of the glide plane is almost always involved in a translational slide event.
earthquake activity is also a common factor.
Slides
often develop in homogeneous sediment where accumulations of sandy silt, silt, or clay are undercut by rivers or exposed along coastlines.
transverse (tensional) and radial (extensional) cracks are commonly developed in the unconsolidated sediment.
rotational slides are often transitional to other styles of mass movement (e.g., flows).
classification of flows is usually based on a combination of factors that include the following:
the type of material involved
the downslope speed
the style of internal deformation while the flow is active
influenced greatly by water
Debris Flows
A debris flow involves sediment clasts of varying sizes, in which fine sediment and fluids support large particles during movement.
The resulting deposit is typically a diamicton.
Water content is critical in both the initiation and the viscosity of debris flows.
Debris flows with moderate viscosity and density often move rapidly.
An area susceptible to moderate-density debris flow is often readily identified by the repetitive occurrence of such flows previously at the same site.
Low-viscosity debris flows can move very rapidly (up to 100 km/h).
Low-density debris flows are relatively uncommon and tend to evolve into other types of mass movement as they progress, as a result of changes in viscosity.
Falls and Topples
falls and topples are downward movements of completely detached blocks of material
a topple involves some rotational component; material pivots on a failure point.
topography is a significant factor, with cliffs and steep slopes (at least 30°) being more susceptible.
Undercutting
the removal of underlying rock to produce unstable overhangs, can promote falls and topples.
Climate conditions that lead to water saturation of fractures are significant in forming falls and topples
frost shattering is a significant process responsible for separating blocks from cliffs.
joints and fractures are produced by the application of either tensional or compressive stress to rock.
the number, spacing, and orientation of fractures, joints and faults are the most significant factors involved in the initiation of falls and topples.
multiple fracture systems that intersect to isolate blocks are conducive to triggering rockfalls.
talus
is a wedge of sediment formed by repeated fall and topple events involving boulder-sized and smaller blocks accumulating at the bottom of a slope.
talus slopes are inherently unstable
rock debris gradually moves downslope under a combination of:
slow creep
slopewash during rainstorms
frost heave
Mass Movement Events in Canada
in Canada over the last 150 years, over 600 people have lost their lives to landslides; an average of four lives every year
annual direct and indirect effects from landslides are estimated to be $200 million
most disastrous event was the 1903 Frank Slide in Alberta in which more than 70 people died when 40 million cubic metres of rock failed on eastern flank of Turtle Mountain
Mass Movement in Human Contexts
mass movements become hazards as a result of human choices about where to live and how to utilize areas: there are no hazards without people.
30 people are killed each year on average in North America; damage exceeds $1 billion USD per year
geomorphologists contemplating an area of potential or previous slope failure can recognize if mass movement poses a risk.
communication of that risk can be difficult: it usually will not be possible to say when the failure will occur.
however, communication of information concerning a hazardous condition is still required.
Human Interaction with Landslides
expansion of urban areas and transportation networks, and exploitation of natural resources have increased landslide incidence.
urbanization
removal of anchoring vegetation construction of roads and buildings installing septic systems, watering lawns and gardens cutting the base of slopes placing fill materials on slopes
Identification of Potential Landslides
crescent-shaped crack or terraces on hillside
scalloped or recessed crest of a valley wall
tongue-shaped area of bare soil or rock on hillside
large boulders or talus piles at base of cliff
tilted trees
linear path of cleared
vegetation extending down a hill
exposed bedrock with layering parallel to slope
undulating land surface at base of slope
Personal Adjustments to Landslide Risks
get a geologic evaluation of property; look for surface features
consult local agencies
look for cracks in house walls, leaning retaining walls, doors or windows that stick
do not buy a home that has a landslide hazard
If unstable slopes cannot be avoided, there are numerous engineered solutions to deter landslides including:
improving drainage
reducing the angle of the slope
excavating to unload the top of the slope
building a protective berm or wall to buttress the bottom of the slope
rock bolts and nets to prevent falling rock or earth from hitting roads or structures
sturzstroms
involve an entire mountainside, setting in motion millions of cubic metres of material, and moving much faster than 100 km/hr.
the largest, most rapid, and most potentially deadly form of mass movement.
initial failure may occur by a translational slide or by a debris flow.
As the disturbed material moves downslope, it accelerates rapidly, may incorporate compressed air from pockets on the mountain face, and undergoes rapid changes in velocity and viscosity
Hope Rockslide, British Columbia
Canada’s largest landslide; Jan. 9, 1965; 47 million cubic metres
slide occurred in steeply sloping metamorphic rocks that were deeply weathered – weathering reduced rock strength
buried 3 km of Highway #3 to a depth up to 85 m over a distance of 3 km; 4 motorists were killed
Outram Lake was obliterated by rock debris
rock debris travelled 2200 m downslope and then 150 m up the opposite valley wall
liquefaction
liquefaction occurs when an abrupt disturbance such as an earthquake causes the water in pore spaces to flow rapidly, resulting in rapid collapse of the dewatered sediment, along with any structures built on it.
Liquefaction-affected sediments can be recognized by the presence of:
sand boils
vertical dewatering pipes
disrupted beds
St.-Jean-Vianney Earthflows
On May 4, 1971 following a winter of record snow and rainstorms, a sudden, huge earthflow in Leda clay destroyed 40 homes and killed 31 people.
