Mass Movements

Question 1

weathering

Answer 1

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

Question 2

physical weathering:

Answer 2

mechanical break-up of bedrock into small particles through the action of physical forces acting at or near Earth’s surface

Question 3

chemical weathering:

Answer 3

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

Question 4

The frequency of rock types at the Earth`s surface.

Answer 4

Igneous Rocks

• Granite 15%
• Basalt 3%

Sedimentary Rocks

• Shale 52%
• Sandstone 15%
• Limestone 7%

Other 8%

Question 5

The frequency of primary rock-forming minerals at the Earth`s surface.

Answer 5

aluminosilicate minerals

• Feldspar - 30%
• Quartz - 28%

ferromagnesian minerals
Mica - 18%
Pyroxene, Amphibole – 1%

Calcite, Dolomite – 9%
Iron Oxides – 4%
Others – 10%

Question 6

efficacy of physical weathering processes influenced by:

Answer 6

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

Question 7

hydration

Answer 7

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

Question 8

rock structure

Answer 8

size and abundance of fractures; effective porosity and permeability

Question 9

rock composition

Answer 9

silicate vs. carbonate minerals

Question 10

efficacy of chemical weathering processes influenced by:

Answer 10

rock structure

rock composition

supply of water and carbon dioxide

degree of saturation of pore space by fluids

ambient temperatures

Question 11

hydrolysis

Answer 11

H+ displaces metallic cations and silica, forming secondary clay minerals

silica combines with OH- to form soluble minerals

Question 12

carbonation

Answer 12

the dissolution of limestone (CaCO3)and dolomite (CaMg(CO3)2), creates karst landscapes

dolomite dissolves more slowly limestone

Question 13

Mafic rocks -

Answer 13

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.

Question 14

bedrock

Answer 14

unweathered rock

Question 15

regolith

Answer 15

surface layer of rock particles

Question 16

soil

Answer 16

surface layer containing living organisms and capable of supporting plants

Question 17

colluvium

Answer 17

transported regolith and soil on hillslopes

Question 18

Mass Movement

Answer 18

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 )

Question 19

shear strength

Answer 19

movement may occur when internal cohesion and frictional resistance are overcome by shear stress

Question 20

the shear strength of materials is expressed as:

Answer 20

S = C + (σn-σpw) tanθ
where S = shear strength
C = cohesion
σn = normal stress
σpw = pore water pressure
θ = angle of internal friction

Question 21

Factor of Safety = shear strength/shear stress

Answer 21

F < 1; slope is unstable and mass movement may occur

F > 1; slope is stable

Question 22

slope stability is affected by:

Answer 22

slope angle

nature of surficial materials

nature and extent of vegetation cover

pore water pressure

Question 23

Topographic relief

Answer 23

refers to the height of a hill or mountain above the land below

landslides occur more frequently in areas of high relief

Question 24

Slope angle

Answer 24

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

Question 25

planes of weakness

Answer 25

bedding planes in sedimentary rocks, foliation planes in metamorphic rocks, joints in igneous rocks, or zones along which Earth has moved before

Question 26

degree of consolidation

Answer 26

creep, slumps and flows are common in unconsolidated materials

Question 27

shape of slip surface

Answer 27

rotational slides or slumps are curved translational slides are planar

Question 28

Rock Mass strength

Answer 28

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

Question 29

The Role of Climate

Answer 29

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

Question 30

The Role of Vegetation

Answer 30

plant roots add strength and cohesion to slope materials vegetation adds weight to slopes increases the likelihood that the slope will fail

Question 31

The Role of Time

Answer 31

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

Question 32

Slope stability can be expressed in terms of a

Answer 32

Factor of Safety (F): F = Shear strength (s) / Shear stress (τ)

Question 33

Mass Movement Processes Defined

Answer 33

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

Question 34

Creep

Answer 34

very slow downslope movement of rock, regolith or soil; mm or cm per year

Question 35

Slide

Answer 35

rock, regolith or soil slide on an inclined surface; minimal internal deformation of materials during movement; ~ 1 km per hour

Question 36

Slump

Answer 36

coherent blocks of rock, regolith or soil slide on a concave surface; minimal internal deformation of materials during movement; ~ 1 km per hour

Question 37

Flow

Answer 37

rapid downslope movement of saturated rock, regolith or soil; considerable deformation of materials during movement; > 5 km per hour

Question 38

Fall and Topple

Answer 38

rocks free fall through the air

Question 39

Heave and Creep

Answer 39

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.

Question 40

rate of creep (ΔL) varies directly with slope angle:

Answer 40

ΔL = H tanβ where: L = creep parallel to the ground surface H = heave normal (at right angle) to the ground surface β = slope angle

Question 41

translational slides

Answer 41

involve movement along essentially planar, uniformly sloping surfaces.

Question 42

rotational slides

Answer 42

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.

Question 43

involve movement along curved, concave surfaces.

Answer 43

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.

Question 44

Slides

Answer 44

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).

Question 45

classification of flows is usually based on a combination of factors that include the following:

Answer 45

the type of material involved the downslope speed the style of internal deformation while the flow is active influenced greatly by water

Question 46

Debris Flows

Answer 46

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.

Question 47

Falls and Topples

Answer 47

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.

Question 48

Undercutting

Answer 48

the removal of underlying rock to produce unstable overhangs, can promote falls and topples.

Question 49

Climate conditions that lead to water saturation of fractures are significant in forming falls and topples

Answer 49

frost shattering is a significant process responsible for separating blocks from cliffs.

Question 50

joints and fractures are produced by the application of either tensional or compressive stress to rock.

Answer 50

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.

Question 51

talus

Answer 51

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

Question 52

rock debris gradually moves downslope under a combination of:

Answer 52

slow creep slopewash during rainstorms frost heave

Question 53

Mass Movement Events in Canada

Answer 53

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

Question 54

Mass Movement in Human Contexts

Answer 54

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

Question 55

geomorphologists contemplating an area of potential or previous slope failure can recognize if mass movement poses a risk.

Answer 55

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.

Question 56

Human Interaction with Landslides

Answer 56

expansion of urban areas and transportation networks, and exploitation of natural resources have increased landslide incidence.

Question 57

urbanization

Answer 57

``` 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 ```

Question 58

Identification of Potential Landslides

Answer 58

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

Question 59

Personal Adjustments to Landslide Risks

Answer 59

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

Question 60

If unstable slopes cannot be avoided, there are numerous engineered solutions to deter landslides including:

Answer 60

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

Question 61

sturzstroms

Answer 61

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

Question 62

Hope Rockslide, British Columbia

Answer 62

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

Question 63

liquefaction

Answer 63

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.

Question 64

Liquefaction-affected sediments can be recognized by the presence of:

Answer 64

sand boils vertical dewatering pipes disrupted beds

Question 65

St.-Jean-Vianney Earthflows

Answer 65

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.