Weathering - University at Buffalo · Disintegration physical weathering; to de-integrate the...
Transcript of Weathering - University at Buffalo · Disintegration physical weathering; to de-integrate the...
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Weathering
M. I. Bursik
ublearns.buffalo.edu
September 14, 2008
Contents
1 Weathering 5
2 Overview of the forces of destruction 5
3 Weathering 5
4 Granular disintegration and decomposition 6
5 Disintegration 6
6 Freezing 7
7 Heating and Cooling 7
8 Types 7
9 Spallation by fire 8
10 Spallation by fire 8
11 Spallation by fire 9
12 Deep spallation and granulation, possibly diurnal 10
13 Unloading 11
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14 Exfoliation example 11
15 Exfoliation 12
16 Abrasion 13
17 Glacial abrasion 13
18 Ventifaction 13
19 Organic destruction 14
20 Spallation by plants 14
21 Salt Weathering 15
22 Decomposition 15
23 Hydrolysis 16
24 Oxidation 16
25 Dehydration 16
26 Dissolution 16
27 Leaching 17
28 Decomposition 17
29 Decomposition 18
30 Decomposition 19
31 Disintegration and decomposition working together 19
32 Rate of Weathering 20
33 Rate of Weathering (cont.) 20
34 Results 21
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35 Soil and Regolith 21
36 Factors 21
37 Soil profile 22
38 O Horizon 22
39 A Horizon 22
40 Leaching factor 22
41 B Horizon 23
42 Bt Horizon 23
43 Bk Horizon 23
44 C Horizon 23
45 Soil Maturity 24
46 Paleosols 24
47 Soil Classification 24
48 Processes, Stratigraphy and Landforms 25
49 Flow, Erosion and Deposition, Landforms 25
50 Shear Stress 25
51 Landforms from Erosion and Deposition 26
52 Differences between Quaternary and bedrock stratig-raphy 26
53 Quaternary stratigraphic columns 26
54 How to record and study Quaternary stratigraphy . . . 27
55 Mass Wasting and Landslides 28
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56 Sliding Block Model 28
57 Shear Strength 29
58 Shear Strength 29
59 Internal Friction 30
60 Effective Normal Stress 31
61 Plasticity 31
62 Cohesion 32
63 Extending the sliding block model 32
64 Extending the sliding block model (cont.) 3264.1 What we have . . . . . . . . . . . . . . . . . . . . 33
65 Cohesion 33
66 History 33
67 A simple expression for s 34
68 Factor of Safety 34
69 Mass Wasting Landforms 34
70 Types of Landslides 35
71 Matrix of Types 36
72 Landslide Movement 37
73 La Conchita, CA, 2005 38
74 Vaiont Dam, Italy, 1963 40
75 Three Gorges Dam, China 41
76 Shiveluch Volcano, Kamchatka, Russia 41
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77 Shiveluch Volcano, Kamchatka, Russia 42
78 Shiveluch Volcano, Kamchatka, Russia 42
1 Weathering
Remember we are starting with forces of destruction, sowe need to begin with:
Rocks have been uplifted by tectonic forces, or eruptedfrom volcanoes, and exposed
These solid, coherent materials need to be readied for ero-sion
They are readied for erosion by the process of weath-ering
2 Overview of the forces of destruction
Rocks Uplifted and Exposed Weathering Erosion (and erosional landforms, such as valleys) Deposition (and depositional landforms, such as deltas)
Lithification, see the courses in petrology
3 Weathering
Igneous, metamorphic and sedimentary rocks form at depthor at high temperature, in an environment where they arestable. If these rocks are brought to the Earths surfaceand/or cooled, they will become mechanically and chemi-cally unstable
Weathering The disintegration and decomposition of rocks ator near the surface of the Earth
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Disintegration physical weathering; to de-integrate theconstituent parts (minerals) of a rock
Decomposition chemical weathering; to de-compose thechemical structure of the minerals themselves
4 Granular disintegration anddecomposition
Wheeler Crest granodiorite deeply weathering in pits due togranular disintegration and differential decomposition.
5 Disintegration
Does not change the chemical composition of the rock
Caused by freezing water
heating and cooling
unloading
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abrasion
organic processes
6 Freezing
Most effective where there are many freeze-thaw cycles(like here)
Water expands 9% in volume when it freezes
Called frost wedging
7 Heating and Cooling
Fire probably responsible for most of the mass removed byheating and cooling in many areas (particularly semiaridWestern U.S.)
Expansion during the day from insolation and heating alsomay contribute
Contraction at night with cooling
Diurnal cycle
8 Types
Each mineral has a different coefficient of thermal ex-pansion
Granular disintegration
Heating on outside of the rock (differential heating) Spalling or spallation
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9 Spallation by fire
Fire spallation of a granitic boulder following the Old Fire, Oct,2003
10 Spallation by fire
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Fire spallation of a granitic boulder following the Old Fire, Oct,2003. Close up view.
11 Spallation by fire
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Fire spallation of a granitic boulder following the Old Fire, Oct,2003. Note chips on ground ready for erosion.
12 Deep spallation and granulation,possibly diurnal
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Deeply spalled and granulated granite boulder in Mono Basin.Possibly this type of spallation is caused by diurnal temperaturechanges in an area of vast diurnal temperature variation.
13 Unloading
Unloading Release of pressure as rocks near surface from depth
Expansion of rock in planes parallel to surface
Tensional stress perpendicular to surface
Brittle failure Exfoliation or sheeting
14 Exfoliation example
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Royal Arches, Yosemite Valley, showing sheeting of granite
15 Exfoliation
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Face of Half Dome, Yosemite Valley, CA, showing granitic sheet-ing
16 Abrasion
Running water, ice and wind carry particles These particles strike surfaces breaking off other par-
ticles Sandblasting or abrasion
17 Glacial abrasion
Glacially abraded and striated granitic boulder near AlgonquinProvincial Park, ONT
18 Ventifaction
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Ventifacted Wheeler Crest granodiorite, Sierra Nevada, CA
Ventifaction weathered by aeolian abrasion.
19 Organic destruction
Roots pry rocks apart with growth
20 Spallation by plants
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Small scale spallation caused by the activity of plants
21 Salt Weathering
Salt is carried in solution (salt spray) and crystallizes, ex-panding in cracks
22 Decomposition
Mostly effected by water
As water falls through air and flows through soil, carbondioxide is dissolved within it:
H2O + CO2 H2CO3 H+ +HCO3 (1)
Hydrogen (hydronium) and bicarbonate ions are effectiveat attacking minerals
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23 Hydrolysis
Hydrolysis replacement of ions by H+ or OH ions from water
What do we note in a rocks color?
Hydrolysis can change the chemical composition of a rock:
4KAlSi3O8+4H++2H2O 4K
++Al4Si4O10(OH)8+8SiO2(2)
Potassium feldspar + Hydronium + Water Ionic K +Clay + Silica
24 Oxidation
Oxidation loss of an electron
Iron is perhaps the most readily oxidized of the common,rock-forming cations, i.e. Fe2+ Fe3+
4FeO + 2H2O +O2 4FeO(OH) (3)
Magnetite + Water + Oxygen Goethite
25 Dehydration
Dehydration loss of water
commonly results in formation of hematite (Fe2O3)from goethite
Goethite = yellowish, hematite = reddish
26 Dissolution
Dissolution breaking into constituent ions and holding in so-lution
CaCO3 +H2CO3 Ca2+ + 2HCO3 (4)
Limestone + Carbonic acid Ionic Ca + Bicarbonate
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Typical reaction for limestone, which is common rock typein western New York
Can result in unusual topography called karst There are some karstic features along Main Street
(near throughway)
27 Leaching
Leaching the removal of soluble matter by aqueous solutions
Not so much a type of chemical reaction, but impor-tant to chemical weathering
There is a loss of mass with leaching
e.g., in the rock in the preceding example, many ofthe K+ ions released by hydrolysis have subsequentlybeen taken into solution and removed by leaching
28 Decomposition
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Pit formation in Wheeler Crest granodiorite at June Lake, CA,caused by differential mineral decomposition
29 Decomposition
Differential decomposition of syenite, Princess Sodalite Mine,Bancroft, ONT
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30 Decomposition
Differential decomposition of syenite, Princess Sodalite Mine,Bancroft, ONT. Close up view.
31 Disintegration and decompositionworking together
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Decomposition and disintegration work together in BandelierNational Momument to form distinct weathering features
32 Rate of Weathering
Clearly, weathering procedes with time (as measurementssuggest)
As it procedes, it is influenced by
Rock structure degree of fracturing
Rock type e.g., minerals lower on Bowens reaction series
weather more slowly
33 Rate of Weathering (cont.)
Climate
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wet fast decomposition; variable precipitation fast disintegration
warm fast decomposition; variable temperaturehigh diurnal variation fast disintegration
Topography steep little retention of water; flat great reten-
tion of water
34 Results
Broken down rock; rock available for erosion
Movement of ions
Soils and Regolith
35 Soil and Regolith
Regolith unlithified deposits at the Earths surface producedby weathering
very little non-soil regolith in New York, but common inWestern U.S.
Soil A weathering residue that has become differentiated withdepth into horizons. The combination of mineral, organicmatter, water and air SUPPORTING THE GROWTH OFPLANTS.
Soil is the synapsis of Earth, air, water and life
36 Factors
Climate is probably most important. Soils formed on dif-ferent rock but under the same climate are the same. Soilsformed on the same rock but under different climate differ
Time - Soils differentiate more with time
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37 Soil profile
Layers differentiated by weathering lying above the par-ent material
Soil layers parallel the local slope, unlike stratigraphiclayers
Caused by: accumulation of organics in upper horizon leaching of ions, hence minerals
accumulation of weathering products at depth
The layers have different names
38 O Horizon
Organic horizon
Decomposing organic material at surface with very littlemineral content
Very dark, thin (cms) Can be subdivided based on degree of decomposition
39 A Horizon
Zone of eluviation or leaching
Organic acids carried in percolating rain water remove ionsfrom this layer
Dark colored, loose and friable (easily broken apart) Plants aid leaching in friability
40 Leaching factor
A measure of the transport of ions out of the A Hori-zon: Leaching factor = ((K2O+Na2O)/SiO2) weathered /((K2O+Na2O)/SiO2) parent
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41 B Horizon
Zone of illuviation or accumulation
Material that was leached from the A Horizon accumulatesin the B Horizon
More chemical weathering than in underlying layer, so al-though the organic content is low, the nature of the parentmaterial is difficult to recognize
42 Bt Horizon
A subtype of B HorizonBt Horizon clay + hydrated Fe and Al oxides coat parti-
cles, fill spaces, and thus form a relatively imperme-able layer
Clay content noticeably high
Can swell when wetted and crack when dried intocolumns
43 Bk Horizon
Bk Horizon B Horizon with noticeable accumulation of CaCO3
Generally in arid or semiarid regions
K Horizon, Caliche or Calcrete Bk Horizon with> 50%CaCO3
Ca++ (from rocks) and HCO3 (from plants) ions precipi-tate in B Horizon because of low water content in B Hori-zon
44 C Horizon
Slightly decomposed rock
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Less leaching and/or accumulation than overlying layers
Parent material identifiable in angular blocks, grus, orspheroidal core-stones on unweathered parent (R Horizon)
Cox Horizon noticeably oxidized C Horizon
45 Soil Maturity
A function of time of weathering
Immature soil may not look much different from parentmaterial
Thus immature soils of different parents will look dif-ferent
Mature soil has well-defined horizons Mature soils of different parents will look the same
under the same climatic conditions
46 Paleosols
Paleosol Ancient soils that have been removed from the zone ofsoil formation, often through burial by younger sediments
Paleosols are extremely important geological indicators asthey represent times of surface stability
They have played a major role in understanding thehistory of glaciation
47 Soil Classification
Many systems, none are completely accepted worldwide
In the U.S., the Department of Agriculture (Soil Conserva-tion Service, now Natural Resources Conservation Service)developed the Seventh Approximation, which is compli-cated, and based on combining Greek and Latin roots withmodifying prefixes
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48 Processes, Stratigraphy andLandforms
Learn about the dynamics of . . . Mass Wasting
Iceflow and granular flow
Streamflow
Wind
Waves and currents
49 Flow, Erosion and Deposition,Landforms
Couple dynamics of flow with sedimentological concepts oferosion and deposition . . .
Resulting in the sculpting of the landscape through surfaceflows acting to erode and deposit,
And construct, destroy and modify landforms
Fluid flow Erosion/Deposition Deposits and land-forms
50 Shear Stress
The flow of air, water, ice and earth exerts a shear stresson the bed material
This stress works against the strength or the weight of thematerial
To cause erosion
As the fluids slow down, they lose the energy needed tosustain particles in motion
And deposition occurs
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51 Landforms from Erosion andDeposition
All landform elements are made up of some combinationof erosional and depositional features caused by the actionof surficial fluids
Landforms Erosion and deposition result in 3-D forms (not just
horizontal strata and unconformities)
Thus they result in the development of landforms
Landforms can be thought of as the containers for the workof sedimentary processes
52 Differences between Quaternary andbedrock stratigraphy
We are looking at deposits not rocks
In sedimentology/stratigraphy/structure, virtually all sed-imentary units are treated as originally semi-infinite, per-fectly flat-lying bodies (Steno)
In this class, it is clear that many deposits that we seetoday are contained in landforms having complex shape
Why the difference? Many subaerial, surficial deposits are ephemeral, and
are not preserved for long times in the stratigraphicrecord
53 Quaternary stratigraphic columns
In many localities, particularly when doing environmentalor volcanic studies, it is necessary to record the shallow(Quaternary) stratigraphy
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It is also important to study the Quaternary stratigraphyto understand the nature of the landforms
Sometimes the stratigraphy will provide diagnosticevidence about landform genesis
54 How to record and study Quaternarystratigraphy . . .
There are conventions that a Quaternary stratigraphic col-umn must follow
Time scale on far left
Height scale next - column must be to scale
Rock (deposit) column
Width related to grain size Schematic representation of sedimentary struc-
tures
Description of units, with interpretation
An example of an acceptable stratigraphic column.
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55 Mass Wasting and Landslides
Glacier
River
Sea or Lake
Slide
Dunes
Mountains Plains
We are going to follow earth materials from source regions tobase level, looking at processes, deposits and landforms alongthe way
Mass wasting the movement of materials down slope underthe influence of gravity
Water may or may not be present
We are interested in how materials on slopes begin motion The basic problem in this area is the sliding block
56 Sliding Block Model
Chalkboard
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57 Shear Strength
Coefficient of friction exists only for single particles (blocks)on slopes
Real earth materials have a shear strength , which isdirectly analogous to the coefficient of friction. Shearstrength, s, is defined as follows:
s = f (, C(p, , vegetation, N, , T, history, . . .))(5)
58 Shear Strength
Shear Strength the aggregate properties of a material that re-sist shearing stress
A function of internal friction, cohesion and effectivenormal stress A shear strength, S, is dependent oncohesion, c, effective normal stress, (sigma prime),and internal friction angle, , as:
S = c+ tan (6)
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59 Internal Friction
Used for geologicalmaterials comprisedof separate fragments,known as granularmaterials
Caused by the planargliding and interlock-ing of constituent frag-ments of the material
Measured by referenceto an angle, (phi),at which failure (move-ment) occurs
F < 1
Slip plane
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60 Effective Normal Stress
Normal stress force/area resolved in the direction perpendic-ular to a plane of interest (failure plane)
Total normal stress, , is supported by both grain-to-graincontacts and by fluid pressure, :
= + (7)
So, if part of the weight (total normal stress) on amaterial piece of earth is supported by an interstitialfluid (groundwater), then the effective normal stressis decreased
tan = S/
0 10 20 30 40 50 60 70Normal stress at failure (Pa)
5
10
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Bas
al f
rict
ion
angl
e (d
egre
es)
61 Plasticity
A perfect plastic material does not deform at all untilits yield strength is reached, then it deforms at a rateproportional to the shear stress
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62 Cohesion
Cohesion solid rock and clay-rich materials have cohesion, whichis residual shear strength in the absence of effective normalstress
In clays, absorption of ions and polar molecules (wa-ter) by the clay particles creates weak chemical bonds
Plastic limit (PL; Atterberg) moisture content atwhich a clay-rich soil begins to actic as a plas-tic or becomes slippery, given as the ratio of theweight of contained water to the weight of thesoil element
Liquid limit (LL; Atterberg) moisture content at whicha clay-rich soil loses all cohesion and acts like afluid, given as the ratio of the weight of containedwater to the weight of the soil element
63 Extending the sliding block model
We want to extend what we have learned about the slidingblock in two directions
First, we want to use some extension of the prob-lem to predict landslide potential for various realisticsituations
Method of Infinite Slope and Swedish Methodof Slices
64 Extending the sliding block model(cont.)
Second, we want to understand what happens during move-ment for something that is not a rigid particle
e.g., something that doesnt have a front and a back,that could potentially break apart during movementand flow, earth flow, debris flow
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a continuum
how does this move?
64.1 What we have . . .
We know thatFg = mg sin (8)
for driving forces, andFr = mg cos (9)
for resisting forces, andFgFr
> 1 (10)
for the block to move
65 Cohesion
C is the cohesion , a function of the strength of the chem-ical bonds and capillary forces holding a material together(whereas is dependent on mechanical locking)
where there is not much clay, C is relatively unim-portant
p is pore fluid pressure ; is slope angle; N is normalforce; is strain (strain rate may also be a factor); T istemperature
66 History
History becomes important because of the phenomenon ofdilatancy - an increase in porosity (decrease in contact)that often makes it so that failure occurs where it hasoccurred before
Where material properties are variable, dilatancy be-comes less important
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67 A simple expression for s
Fr = N
for a sliding blockN N Area
(N is the normal stress )
s = N,failure + C (11)
(for shear strength on a plane in a continuum. Compare with1.117.27)
Then if shear stress along a plane, > s FAILURE!
68 Factor of Safety
The ratio of the shear strength to the shear stress is anestimator of whether failure will occur on a slope
It is one example of a Factor of Safety , F :
F fresistingfdriving
(12)
Often for a continuum, the factor of safety is closer to 1.3than to 1
Use F = 1.3 for all landslide problems as the criticalfactor of safety
69 Mass Wasting Landforms
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Nomenclature of the different parts of a landslide.
70 Types of Landslides
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The different types of landslides are differentiated by mode ofmovement and material.
71 Matrix of Types
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72 Landslide Movement
Landslide Animation 1
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http://wwwgeology.nsm.buffalo.edu/mib/Figures/MassWasting/Landslides_Animation.gif
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(Loading)
Landslide Animation 2
73 La Conchita, CA, 2005
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movie_big_vol.mpgMedia File (video/mpeg)
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Open-File Report
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http://pubs.usgs.gov/of/2005/1067/508of05-1067.html
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74 Vaiont Dam, Italy, 1963
by D. Petley
Download the PDF sheet on Vaiont from UBLearns (fromUCSB)
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75 Three Gorges Dam, China
News Story
76 Shiveluch Volcano, Kamchatka,Russia
from http://www.kscnet.ru/ivs/
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http://www.sacbee.com/111/story/430151.htmlhttp://www.kscnet.ru/ivs/
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77 Shiveluch Volcano, Kamchatka,Russia
78 Shiveluch Volcano, Kamchatka,Russia
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WeatheringOverview of the forces of destructionWeathering Granular disintegration and decompositionDisintegration Freezing Heating and Cooling TypesSpallation by fireSpallation by fireSpallation by fireDeep spallation and granulation, possibly diurnalUnloading Exfoliation exampleExfoliationAbrasionGlacial abrasionVentifactionOrganic destructionSpallation by plantsSalt WeatheringDecomposition HydrolysisOxidationDehydrationDissolution Leaching DecompositionDecompositionDecompositionDisintegration and decomposition working togetherRate of WeatheringRate of Weathering (cont.)ResultsSoil and RegolithFactorsSoil profileO HorizonA Horizon Leaching factorB HorizonBt HorizonBk HorizonC HorizonSoil MaturityPaleosolsSoil ClassificationProcesses, Stratigraphy and LandformsFlow, Erosion and Deposition, LandformsShear StressLandforms from Erosion and DepositionDifferences between Quaternary and bedrock stratigraphyQuaternary stratigraphic columnsHow to record and study Quaternary stratigraphy Mass Wasting and LandslidesSliding Block ModelShear StrengthShear StrengthInternal FrictionEffective Normal StressPlasticityCohesionExtending the sliding block modelExtending the sliding block model (cont.) What we have
CohesionHistory A simple expression for sFactor of SafetyMass Wasting LandformsTypes of LandslidesMatrix of TypesLandslide MovementLa Conchita, CA, 2005Vaiont Dam, Italy, 1963Three Gorges Dam, ChinaShiveluch Volcano, Kamchatka, RussiaShiveluch Volcano, Kamchatka, RussiaShiveluch Volcano, Kamchatka, Russia