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Geog375Gmphlgy > Weathering Processes and Sediments > Flashcards

Weathering Processes and Sediments Flashcards

1
Q

What is the geomorphic significance of phys and chem breakdown of surface materials?

A
  • Easily manipulated materials that can form new landscapes and landforms
  • Production of seds, sed landforms, by-products (cements), soils
  • Linked to control of other geo processes (karst topo, mass wasting, colluviation, aeolian landforms, glacio/fluvial processes)
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2
Q

Why is weathering a disequilibrium response of rock bodies or sediment

A
  • Conditions at the surface (or transport/deposition) are very different from those of rock formation
  • ie. heat, pressure, chemistry, mechanical action
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3
Q

What are the 2 main processes of weathering?

A
  • Chemical decomposition
  • Physical disintegration
  • both work simultaneously often
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4
Q

What are types of physical weathering?

A
  • pressure release
  • freeze-thaw
  • thermal expansion
  • salt crystal growth
  • biotic
  • hydration
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5
Q

What are types of chemical weathering?

A
  • Solution
  • Ion Exchange
  • Hydrolysis
  • Oxidation
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6
Q

Mechanical Weathering

A
  • No change in chemical composition, just disintegration into smaller pieces
  • Increases total surface area which makes it more vulnerable to further breakdown/weathering
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7
Q

Chemical Weathering

A
  • Breakdown a a result of chemical reactions/ alterations

- Limestone or marble

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8
Q

Role of Phys/Mech Weathering

A

1) Reduces rock material to smaller fragments that are easier to transport
2) Increases exposed surface area making rock more vulnerable to further physical and them weathering

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9
Q

Pressure Release

A
  • Uncovered rock expands due to lower confining pressure

- Results in Exfoliation

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10
Q

Exfoliation

A
  • Igneous rocks formed at depth exfoliate at surface
  • Rock breaks apart in layers that are parallel to the earth’s surface
  • Creates slab-like morphology
  • Slabs sheet off in onion-like layers
  • Enhances weathering and erosion, risk in mines
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11
Q

Freeze-Thaw/ Frost Wedging

A
  • Rock breakdown by expansion of ice in cracks and joints
  • Water expands 9% when frozen, up to 20million kgm^-2 pressure
  • Significant in periglacial and seasonal climates
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12
Q

What is freeze-thaw/frost-wedging controlled by?

A
  • Moisture content
  • Rate of temperature change
  • Freeze-thaw cycles
  • Rock type (porosity etc.)
  • Structure (fractured/jointedness of rock)
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13
Q

Thermal Expansion

A
  • Due to extreme range of temps that can shatter rocks
  • Desert envrs
  • Repeated swelling and shrinking of minerals with different expansion rates also shatters rocks
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14
Q

Salt Weathering

A
  • Evaporation of salt brine

- Grain by grain eventually produces cracks in the surf zone

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15
Q

Tafoni

A
  • Salt weathering that creates a honeycomb like effect in rock surface
  • Locally common in Nanaimo group sandstones
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16
Q

Biotic Weathering

A
  • Root splitting: At large scales seedlings sprouting in a crevice
  • Plant roots exert physical pressure
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17
Q

Hydration and Swelling

A
  • Hydration: Attachment (adsorption) of water molecules to crystalline structure of a rock
  • Causes expansion and weakness
  • Mineral example: Anhydrite plus water = Gypsum
  • Not chemical b/c easily reversed, no chem change
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18
Q

Expansive Clays

A
  • Bentonite
  • Expand 2:1
  • Absorb 140x mass of water exert up to 55000 kmm^-2 pressure
  • Quick glaciomarine clays
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19
Q

Resistance to weathering

A

Rock strength, composition, structure

  • The form/density of fractures is controlled by rock type
  • Biota (sort of)
  • Porosity/cementation %
  • Grain size (fine = more resistance)
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20
Q

Driving force of phys weathering

A

1) ice crystallization requires water
2) salt crystallization requires water
3) Biota growth requires water
- Temperature
- Joints are pathway for water that enhance mech weathering

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21
Q

Rock characteristics that control phys/mech weathering

A
  • Mineral compostion and solubility
  • Surface Area and grain size (clay has large and therefore absorbs more water)
  • Joints, faults = natural pathways
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22
Q

Definition of chem weathering/decomposition

A
  • Transformation/ decompostion (chem alteration) of one mineral into another
  • Direct effect of atm or bio produced chems
  • Almost always with water
  • Weakens structure at mineral level
  • Produces soluble materials and insoluble sediments transported in leachate, groundwater and/or runoff
  • Synthesis of new secondary minerals, clay, oxides, precipitates
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23
Q

Solution

A

Process by which rock is dissolved in water (carbonation of limestone)

  • Bio activity in soils generates CO2
  • Bicarbonate is dominant ion in surface runoff (rivers)
  • When water becomes saturated, chemicals may precipitate out forming evaporite deposits
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24
Q

Ion Exchange

A
  • Substitution of ions in sol’n for ions in minerals
  • Surfaces of clay particles (unsatisfied elemental charges, exposed hydroxyl groups, isomorphous substitution common)
  • Isomorphous substitution (replacement by similar sized ions i.e. Si for Al or Al for Mg, no alteration in form of mineral)
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25
Q

Hydrolysis

A
  • H and O in water with rock form new substances
  • Carbonation is essentially same reaction but w/ CO2 instead of H+
  • Feldspar weathering to kaolinite
  • Most common mechanism for clay formation
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26
Q

Oxidation

A
  • O dissolved in water promotes oxidation of sulphides, ferrous oxides, native metals
  • Often cause of red soils
  • Olivine with water and O to Hematite and dissolved silica acid
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27
Q

What is the main driver of chemical weathering?

A

WATER

  • Ionic and organic compounds dissolve in water = dissolution
  • Hydration and Hydrolysis both require water
  • Acid reactions require water
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28
Q

Does chem weathering enhance phys/mech weathering?

A

Yes, enhances opportunity for more phys/mech weathering

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29
Q

Which minerals are most resistant to weathering? Least resistant?

A
  • Most = Late stage minerals (Bowen’s Reaction Series) that are more stable at surface conditions
  • Least = First to form (Olivines, pyroxenes etc.)
  • (first = fastest, last = slowest)
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30
Q

Why is sand so prevalent on Earth’s surface?

A
  • Sand mostly consists of quartz

- Quartz is relatively stable at surface and resistant to weathering (quartz ridges tend to stick out)

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31
Q

Fracturing

A
  • Disintegration caused by mech weathering exposing more surface area
  • Reduces volume per grain
  • Greater surface area = more places for chem action
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32
Q

What is the primary way that weathered bedrock is churned up to form soil?

A

Biota

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33
Q

Chemical weathering is strongest with?

A

Increased Precip and temp

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34
Q

Phys weathering is strongest with?

A

Decreased temp and moderate precip

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35
Q

What important by-products does mech and chem weathering produce?

A
  • Soils
  • Sediments
  • Landforms, karst topo, tafoni
  • Secondary minerals, clays
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36
Q

What are the major controls on weathering processes?

A
  • Climate
  • Lithology
  • Topography
  • Vegetation
37
Q

What do deposits form?

A

A matrix of solid particles and water and air and organic matter

38
Q

What is the geomorphic significance of sediments?

A
  • Transfer of mass through erosion, transport and deposition
  • Deposits form a matrix
  • Reflect history of weathering, erosion, transport, and deposition processes
  • Properties control response to applied forces
39
Q

Inherent properties of sediments?

A
  • Mineralogy, texture, shape
40
Q

Bulk properties of sediments?

A

Arrangement, packing, porosity, strength, fabric, structures

41
Q

Relevance and applications of sediments

A
  • Describing and interpreting sed units
  • Transporting agents and depositional envrs
  • Geomorphic behaviour properties (Rheology, Hydrology)
  • Resource potential
  • Geotechnical (bridges etc.)
  • Hazards
42
Q

What does Rheology of seds refer to?

A

Strength, Strain, Deformation

43
Q

What does Hydrology of seds refer to?

A

Permeability, Porosity, Transmissivity

44
Q

Rheological responses

A
  • Response to applied stress (Force per unit area, N m^-2) causing strain or deformation (L in m)
45
Q

What is resistance?

A

Shear Strength

46
Q

Stress

A

Force applied to a surface area

47
Q

Strain

A

Deformation (change in shape or volume) of a material caused by stress

48
Q

What are the 3 types of stress?

A

1) Tensile (pulling)
2) Compressive (Crushing, collapsive)
3) Shearing (sliding, tangential)

49
Q

3 common strain-stress responses

A

1) Elastic
2) Plastic
3) Viscous Fluid

50
Q

Elastic stress-strain response

A

Linear response, reversible to elastic limit, then failure

51
Q

Plastic stress-strain response

A

Deform after yield strength (k), then uniform strain

52
Q

Viscous fluid stress-strain response

A

Continuous, limitless, unrecoverable strain

- Newtonian fluid (stress = derivative of du/dy) controlled by viscosity

53
Q

Dilatent

A
  • Rheopectic
  • Shear thickening
  • Temporarily more resistant to stress
  • Can drop to a ‘residual’ state after max stress is reached
  • Drumlins may form this way
54
Q

Thixotropic

A
  • Pseudo-plastic
  • Shear thinning
  • Very susceptible to stress
55
Q

Sensitive Quick Clays

A
  • Marine clays with positive salt charges
  • Artificially larger particles bc of flocculation
  • Leach out salts and become precariously unstable
  • Shake leached clays and major failure occurs like a toppling house of cards
  • Structure can flow with increasing moisture, weight (normal load), and/or removal of salt bonds
56
Q

Quick clay sensitivity index

A

= undisturbed S/ disturbed S
1 for hard (over consolidated,
2-4 for most clays,
>8-16 for failure prone clays

57
Q

Direct shear test (DST)

A
  • Used to measure Strength in cohesionless sediments
  • When mechanical force (T) along shear plane > S
  • Stress vs. strain curve
  • Tmax vs. Tresidual
  • Residual surfaces can reactivate at lower stress
58
Q

Shrinkage Limit (SL)

A

Mc below which no volume reduction

- Non-plastic

59
Q

Plastic Limit (PL)

A

Mc when material transitions from plastic to solid or crumbles
- Deforms Plasticly

60
Q

Liquid Limit (LL)

A

Mc from plastic to liquid behaviour

- Flows like a viscous liquid

61
Q

Plasticity Index (PL)

A 
- Deformation potential
PL = LL - PL
- Range in wc where sediment behaves as a deformable plastic
- 0 - 3 (non-plastic)
- 3 - 15 (slightly P)
- 15 - 30 (Med P)
- >30 (Highly P)
62
Q

Phi Classes

A
  • Particle size classification scale
  • Logarithmic scale to get reasonable number in a scale
  • Exponential scale
  • Use V-axis to get length
63
Q

Boulders

A
  • > 256mm

-

64
Q

Cobbles

A

64 to 256,

-8.0 to -6.0

65
Q

Gravel (pebble)

A

2 to 64,

-6.0 to -2.0

66
Q

Sand

A
  1. 064 to 2,
    - 1.0 to 4.0
    - Last visible range before silt/clay
67
Q

Silt

A
  1. 002 to 0.064,

4. 0 to 8.0

68
Q

Clay

A

<0.002,
>8.0
- clay grains can’t be seen so hand techniques used to ID grains size in the field

69
Q

Grain Size (Ternary) Distributions

A
  • Plots to ID grain size based on modal percentages of sand, silt, clay
  • sandy clay, muddy sand, sandy silt etc.
  • plots are different depending on application
70
Q

How to describe the shape of the distribution of grain size

A
  • Mean, SD, Skewness, Kurtosis
71
Q

Why are most seds not normally distributed (skewness)?

A
  • Environment energies are different, ie. streams can only move up to a certain size of particle depending on flow velocity
  • Some are bi or poly-modal for grain rise distribution (desert pavement of sand matrix w/ large pebble pavement.
72
Q

Sorting

A
  • Second moment, SD

- Degree of scatter about mean or range of sizes in a sample

73
Q

Skewness

A
  • Third moment, sk
  • Asymmetry in shape of distribution, sorting toward the tails
  • Most seds are not normally distributed
  • > 0.3 = strongly finely skewed,
  • ## 0.1 to -0.1 = near symmetrical
74
Q

Positive skewness

A

Mean greater than median

- Fine particle tail

75
Q

Negative skewness

A

Median greater than mean,

- Coarse particle tail

76
Q

Kurtosis

A
  • Fourth moment
  • Peakedness in shape of distribution
  • Platykurtic = flat peak with wide distribution
  • Leptokurtic = very pointed peak with high distribution of a single type of sed
77
Q

Shape

A

= f, original shape, mineralogy, transport history, burial processes and rates
- 3 major axes proportions (platy, rod-like, rounded)

78
Q

What are the 3 major axes proportions of shape

A
  • Platy
  • Rod-like
  • Rounded
79
Q

Roundedness

A
  • Measure of angularity
  • reflects duration and distance of transport
  • Related but not the same as sphericity
80
Q

Sediment fabric

A
  • grain-grain orientation and packing indicative of transport, sorting, and/or deposition processes
  • imbrication can indicate sediment paleoflow based on dip angle etc.
81
Q

Porosity and pore water pressure

A
  • Mc is critical for determining bulk strength of sediments
82
Q

Mc

A

Moisture content

83
Q

Porosity (n)

A

volume of voids, bulk volume

- controls moisture storage and tansmissivity

84
Q

Which has higher porosity, clay or sand? Which has higher permeability?

A
  • Clay has higher porosity and lower permeability
85
Q

Pore water pressure (PWP)

A
  • Negative suction, positive hydrostatic

- Promotes strength or instability, depending on moisture content

86
Q

What does mean size of sediment indicate?

A
  • Energy of transporting/depositing envr
  • Competence of process to move certain size = f (velocity)
  • Hjulstrom relation, beach sands, and wave energy
87
Q

What do size variations of sediment indicate?

A
  • Reflect change in process over time/space

- ie upward fining of floodplain seds, downstream fining in rivers

88
Q

What does sorting of sediments indicate?

A
  • Range in size reflects changes in energy, magnitude and intensity, or availability of sizes to process
  • ie aeolian sands vs. glacial tills
89
Q

What does sediment form indicate?

A
  • Shape reveals transport and reworking processes

Geog375Gmphlgy (9 decks)

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