Weathering - University at Buffalo · Disintegration physical weathering; to de-integrate the...

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



12 Deep spallation and granulation, possibly diurnal 10

13 Unloading 11

1

http://ublearns.buffalo.edu

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

2

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

3

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

4



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

5

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

6

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

7

9 Spallation by fire

Fire spallation of a granitic boulder following the Old Fire, Oct,2003


8

Fire spallation of a granitic boulder following the Old Fire, Oct,2003. Close up view.


9

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

10

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

11

Royal Arches, Yosemite Valley, showing sheeting of granite

15 Exfoliation

12

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

13

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

14

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

15

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

16

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

17

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

18

30 Decomposition

Differential decomposition of syenite, Princess Sodalite Mine,Bancroft, ONT. Close up view.

31 Disintegration and decompositionworking together

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

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.

27

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

28

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)

29

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

30

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

15

20

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

31

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

32

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

33

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

34

Nomenclature of the different parts of a landslide.

70 Types of Landslides

35

The different types of landslides are differentiated by mode ofmovement and material.

71 Matrix of Types

36

72 Landslide Movement

Landslide Animation 1

37

http://wwwgeology.nsm.buffalo.edu/mib/Figures/MassWasting/Landslides_Animation.gif

(Loading)

Landslide Animation 2

73 La Conchita, CA, 2005

38

movie_big_vol.mpgMedia File (video/mpeg)

Open-File Report

39

http://pubs.usgs.gov/of/2005/1067/508of05-1067.html

74 Vaiont Dam, Italy, 1963

by D. Petley

Download the PDF sheet on Vaiont from UBLearns (fromUCSB)

40

75 Three Gorges Dam, China

News Story

76 Shiveluch Volcano, Kamchatka,Russia

from http://www.kscnet.ru/ivs/

41

http://www.sacbee.com/111/story/430151.htmlhttp://www.kscnet.ru/ivs/



42

43

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

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