Lect 14 FINAL CORNETT Weathering & Mass Movement

8/19/2019 Lect 14 FINAL CORNETT Weathering & Mass Movement

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WEATHERING, EROSION & SUDDEN MASS MOVEM


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Learning Objectives• What is weathering and how does it occur?• What factors influence erosion?•

What is mass movement and what types occurin nature?• How can you calculate mass movements?•

What are the implications of mass transport?


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Weathering

• Weathering involves the physical breakdownand chemical alteration of rock at or nearEarth’s surface

– Two types:• Mechanical weathering – physical forces breaking

rocks into smaller pieces• Chemical weathering – chemical transformation of

rock into new compounds – Both types work simultaneously and reinforce

each other


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Mechanical Weathering IncreasesSurface Area


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Mechanical Weathering• Types of Mechanical Weathering

– Frost wedging – Sheeting/Unloading – Biological activity – Salt crystal growth


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Mechanical Weathering – Frost wedging – Water works its way into cracks in rocks and the freezing enlarges the cracks

in the rocks – Lenses of ice grow larger as they attract liquid water from surrounding

areas


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Unloading Leads to SheetingLarge masses of igneous rock are exposed by erosion and glaciation & concentricslabs break loose

An exfoliation dome is formed after continued weathering causes the slabsto separate and spall offJoints are fractures produced by contraction during the crystallization ofmagma


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Mechanical Weathering

Biological activity• Plant roots grow into fractures in

a rock, causing the cracks toexpand

• Burrowing animals break downrocks by moving fresh material tothe surface, enhancing physicaland chemical weathering


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Chemical Weathering

• The Most Important Agent Is Water – Responsible for transport of ions and

molecules involved in chemical processes• Types of Chemical Weathering

–

Dissolution – Oxidation – Hydrolysis – Spheroidal weathering


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Chemical WeatheringDissolution• Certain minerals dissolve in water

– Halite (NaCl) is one of the most water-soluble minerals• A small amount of acid in water increases the corrosive force of water,

causing dissolution – Carbonic acid is created when carbon dioxide dissolves in raindrops –

Calcite (CaCO3) is easily attacked by weakly acidic solutions


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Chemical WeatheringOxidation• Oxygen combines with iron to form iron oxide• Water increases the speed of the reaction• Important in decomposing ferromagnesium minerals like olivine, pyroxene,

hornblende, and biotite


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Chemical Weathering

Hydrolysis• The reaction of any substance with water• A hydrogen ion attacks and replaces another ion• Silicates primarily decompose by hydrolysis

– Clay minerals are the most abundant product of weathering – Clay minerals are usually very stable under surface conditions


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The Formation of Rounded Boulders

Spheroidal weathering


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Rates of Weathering• The rate of weathering is influenced by rock type

(composition) and climate – Different minerals weather at different rates – Silicate minerals weather in the same order as

crystallization (Bowen’s reaction series) – Warm, moist climates enhance (and cold, dry climates

inhibit) chemical weathering – Uneven weathering of the rock due to different mineral

composition• Called differential weathering


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Monuments to Weathering:Examples of Differential Weathering


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Soil – A Product of Weathering – Earth’s land surface is covered by a layer of rock and

mineral fragments produced by weathering, calledregolith

– Soil is a combination of mineral and organic matter,water, and air and is the portion of the regolith thatsupports the growth of plants


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Examples of Soil Conservation


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Weathering and Erosion

• Slow processes (the snail analogy)• Very important in agriculture•

Very important in global processes (e.g. salinity inthe ocean)

Sudden Mass Movement• The ‘destructive forces (the fast rabbit analogy)


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What is mass movement?Plate tectonics→ topographyRelief→ planed out

Particle motion – one by one OR, under certain conditions,can also move'en masse‘.

Mass movement (def): movement in which bedrock, rock debris,or soil move downslope in bulk because ofgravity.

Mt. Yamnuska

Main driving force → … Erosion agents: …

Processes: weathering, erosion, etc.


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Why study mass movements?

Landslides (one type of mass mvt) are Canada's most

destructive natural hazard.

In Canada, landslides cause between $100 – 200 million indamage annually, and have caused> 600 deaths since 1840.

Better understanding of the geology, and the risk factors can

limit the damage and save lives.


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April 2008 - evacuation on rue Lafrance, Gatineau

Why study mass movements?

Maybe if some of these house owners had taken GEO1111they would not have built their homes at that location ??


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What makes a slope fail?

Gravity!GRAVITATIONAL DRIVING FORCE

Fdriv = ma = mg


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Gravity !GRAVITATIONAL DRIVING FORCE

Fdriv = ma = mg

Friction and CohesionRESISTING FORCE

Fres=μ fric mg cosθ

What keeps a slope in place?


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The Slope Experiment

A simplified example of mass movement


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Force analysis

DRIVING FORCE (Fdriv = mg sin θ)

RESISTING FORCE (Fres = μ fric N =μ fric mg cos θ)


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Force analysis

DRIVING FORCE (Fdriv = mg sin θ)

RESISTINGFORCE(Fres = μfric N =μfric mg cos θ)

1) No motion;Fdriv < or = Fres

Two scenarios:

2) Initiation of motion when;Fdriv > Fres

(when critical angle (θc) is exceeded or Fres is decreased)


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A practical Question

Maya Geo is concerned that, a large block of shale is going to slipdown a mountain slope in the upcoming rainy season and thiswill trigger an avalanche and bury the town of Gold Strike. Thegold miners are laughing at her concern. Maya made the

measurements listed below. Should the miners laugh or shouldthey get out of town beer-in-hand?• Length of mountain slope = 1 km• Height of block of shale above the town = 500m• Weight of block of shale = 100,000 kg• Frictional resistance of the shale block = 0.55


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Maya Geo’s Calculation

Height 500 mLength 1000 mWeight 100000 kgg 9.8 m/s^2Force gravity m g sin(Ang)Angle 30 degreessin (ang) 0.5Force down 490000 N

Force Resist Uf mg cos(Ang) Ncos (angle) 0.866Ufric 0.55 NForce Resist is 466774 N


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l ld d


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Rockslide – The Frank Slide (1903), Canadian Cordillera (Alberta)Real World Tragedy


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What makes a slope fail?Force analysis – additional variables

Rock can move in one solid block (slide) asin the diagram of the previous slide, but rockdebris and soil can alsodeforminternally(flow) under the influence of gravity.

shear deformation

Force analysis

This internal resistance to deformation (orinternal strength) is aresisting forceand can be added to our previous equation.

Therefore,Fres = basal friction + internal strength


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What makes a slope fail?Factors (4) that influenceinternal strength of rock debrissediment or soil:

1) electrostatic forces (e.g. clay particles)2) friction at grain contact = f(angularity, grain size)

Angle of repose ↑ with:

45

40

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a) Grain size & b) Grain angularity..

finesand

coarsesand

rounded angular a n g l e o f r e p o s e > →

grain size > →


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What makes a slope fail?Factors that influence internal strength of rock debrissediment or soil (continued)

3) pore pressure

4) surface tensiondamp sand

vsdry sand

vswet sand

Damp sand Dry sand Wet sand

More cohesive Less cohesive

Ideal sandmoisture?

Factor? →


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What makes a slope fail?WhatFACTORS control thesusceptibility to mass movement?

1) slope angle

2) relief

Increase in risk of mass mvt if:

3) bedrock type4) thickness of debris

5) climate:a) iceb) water/moisturec) raind) vegetation

…

…

jointed, fractured, bedded bedrock

thick debris or soil cover

freeze-thaw cycles…

low cover

f


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What makes a slope fail?What TRIGGERS mass movements?

Earthquakes, dynamite, explosions,eruptions, loud sounds

heavy rainfall / snowmelt, add buildingsEvents or activities:

heavy rainfall / snowmelt

excavation, road construction, river erosio


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H d i l d l ?


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How does material move downslope?Classification of mass movements is based on:

1) type of movement:FALL, SLIDE, or FLOW

2) type of material: rock (solid and coherent), debris (unconsolidated)and the water content

3) rate of movement: from less than 1 cm/year to many hundreds of k

FALL SLIDESrotational translational FLOW

H d i l d l ?


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How does material move downslope?

FALL:material falls freelythrough the air

mvt type material name velocity

Rock

Debris

Rockfall

Debris fall

very fast> a few km/h

fast to very fast> 1 km/h

H d i l d l ?


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SLIDE:descending materialmoves along a ruptureor detachment surface


Rock

Debris

Rockslide

Debris slide(translational)

fasta few km/h

slow to moderate< 1 km/h

Slump(rotational)

slow to fast

< a few km/h

H d t i l d l ?


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How does material move downslope?Debris slide – coherent mass of debris moving along a surfacepotential trigger?

H d t i l d l ?


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FLOW:downslope mvt ina viscous fluid


Rock

Debris

Creep

Creep

very slow; < 10 cm/y

very fast; > 10 km/h

Debris flow

Avalanche

EarthflowGelifluction

MudflowQuickclays

Avalanche

i n c r e

a s i n

g

v e l o

c i t y

(marine origin)

(debris or snow)

(permafrost) d e c r e

a

s i n g g r a i n

s i z

e

very fast; > 10 km/h

very slow; < 10 cm/y

moderate to fast1-4 km/h

H d t i l d l ?


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How does material move downslope?Creep – very slow downslope movement

Mechanisms: wet/dry cyclesfreeze/thaw cycles

H d t i l d l ?


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How does material move downslope?Gelifluction – in colder climates;during warmer months, water-saturateddebris moves downslopeover permafrost

Northwestern AklaskaSiberia

H d t i l d l ?


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flow of mixedmud, rock, andsurface debris.

Debris flow – flow where coarse material (boulder, gravel) is predominantand is supported by a matrix of finer sediment

*video

H d t i l d l ?


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How does material move downslope?Mudflow – mixture of debris and water, usually confined to a channel.

• High water content (rain, snow)• Lesser % of coarser debris, mostly mud or ash• Consistency of chocolatemilkshake

• Mudflow are generally faster and flow furtherthan debris flows

Can occur on the flanks of volcanoes, steep deforested mountain sides,over steepened cliffs of unconsolidated fine sediments (eg. glacialdeposits forming the bluffs around Lake Ontario)



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Mudflow – Nevado Del Ruiz, Columbia

Pyroclastic flows (ash, hot gases, sediment) evolve intomudflows (sediment and water) downstream

On this map,two mud flows are shown inred and grey



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Mudflow – Nevado Del Ruiz, Columbia, 1985Thickness of debris:40 m

Maximum flow velocity:50km/hDistance between Nevado Del Ruizand town of Armero:74 kmNumber of people who perished:

23 000 (3/4 of the town)



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How does material move downslope?Quickclays(syn.: sensitive clays) – clays deposited in glacio-marine environmenstability ↓ when salt is leached out by freshwater.

Na tional Research Council

nondisturbeddisturbed

> 30

Strength:



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How does material move downslope?Quickclays – landslides in the St-Lawrence, Ottawa, and Saguenay river va

J Aylsworth

St-Jean Vianney

1971Blue area → Champlain Sea (12000-9500 yrs ago)- at end of Wisconsinan Glaciation

Champlain Seadeposited a quickclay(sensitive clay)referred to as the

Leda clay

Lemieux,1993

Notre-Dame de la Salette1908



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How does material move downslope?Quickclay flow – started as a rotational slide (slump) and turned into a flow

St-Jean Vianney (Saguenay) in May 1971 – 31 dead

R l W ld T d W hi 2014


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Real World Tragedy - Washington 2014


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Summary• Mechanical and Chemical Weathering

– Increase surface area, break up rock and dissolve solubleminerals

– Globally very important• Erosion

– Transports particulate material – Wind and water born particulate material

• Mass Movement – Sudden failure of slope when resistance force is overcome –

Forces can be calculated to assess the risk – Leads to local destruction

Lect 14 FINAL CORNETT Weathering & Mass Movement

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