Weathering Processes and Sediments Flashcards
What is the geomorphic significance of phys and chem breakdown of surface materials?
- 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)
Why is weathering a disequilibrium response of rock bodies or sediment
- Conditions at the surface (or transport/deposition) are very different from those of rock formation
- ie. heat, pressure, chemistry, mechanical action
What are the 2 main processes of weathering?
- Chemical decomposition
- Physical disintegration
- both work simultaneously often
What are types of physical weathering?
- pressure release
- freeze-thaw
- thermal expansion
- salt crystal growth
- biotic
- hydration
What are types of chemical weathering?
- Solution
- Ion Exchange
- Hydrolysis
- Oxidation
Mechanical Weathering
- No change in chemical composition, just disintegration into smaller pieces
- Increases total surface area which makes it more vulnerable to further breakdown/weathering
Chemical Weathering
- Breakdown a a result of chemical reactions/ alterations
- Limestone or marble
Role of Phys/Mech Weathering
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
Pressure Release
- Uncovered rock expands due to lower confining pressure
- Results in Exfoliation
Exfoliation
- 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
Freeze-Thaw/ Frost Wedging
- 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
What is freeze-thaw/frost-wedging controlled by?
- Moisture content
- Rate of temperature change
- Freeze-thaw cycles
- Rock type (porosity etc.)
- Structure (fractured/jointedness of rock)
Thermal Expansion
- 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
Salt Weathering
- Evaporation of salt brine
- Grain by grain eventually produces cracks in the surf zone
Tafoni
- Salt weathering that creates a honeycomb like effect in rock surface
- Locally common in Nanaimo group sandstones
Biotic Weathering
- Root splitting: At large scales seedlings sprouting in a crevice
- Plant roots exert physical pressure
Hydration and Swelling
- 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
Expansive Clays
- Bentonite
- Expand 2:1
- Absorb 140x mass of water exert up to 55000 kmm^-2 pressure
- Quick glaciomarine clays
Resistance to weathering
Rock strength, composition, structure
- The form/density of fractures is controlled by rock type
- Biota (sort of)
- Porosity/cementation %
- Grain size (fine = more resistance)
Driving force of phys weathering
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
Rock characteristics that control phys/mech weathering
- Mineral compostion and solubility
- Surface Area and grain size (clay has large and therefore absorbs more water)
- Joints, faults = natural pathways
Definition of chem weathering/decomposition
- 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
Solution
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
Ion Exchange
- 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)
Hydrolysis
- 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
Oxidation
- 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
What is the main driver of chemical weathering?
WATER
- Ionic and organic compounds dissolve in water = dissolution
- Hydration and Hydrolysis both require water
- Acid reactions require water
Does chem weathering enhance phys/mech weathering?
Yes, enhances opportunity for more phys/mech weathering
Which minerals are most resistant to weathering? Least resistant?
- 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)
Why is sand so prevalent on Earth’s surface?
- Sand mostly consists of quartz
- Quartz is relatively stable at surface and resistant to weathering (quartz ridges tend to stick out)
Fracturing
- Disintegration caused by mech weathering exposing more surface area
- Reduces volume per grain
- Greater surface area = more places for chem action
What is the primary way that weathered bedrock is churned up to form soil?
Biota
Chemical weathering is strongest with?
Increased Precip and temp
Phys weathering is strongest with?
Decreased temp and moderate precip
What important by-products does mech and chem weathering produce?
- Soils
- Sediments
- Landforms, karst topo, tafoni
- Secondary minerals, clays
What are the major controls on weathering processes?
- Climate
- Lithology
- Topography
- Vegetation
What do deposits form?
A matrix of solid particles and water and air and organic matter
What is the geomorphic significance of sediments?
- 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
Inherent properties of sediments?
- Mineralogy, texture, shape
Bulk properties of sediments?
Arrangement, packing, porosity, strength, fabric, structures
Relevance and applications of sediments
- Describing and interpreting sed units
- Transporting agents and depositional envrs
- Geomorphic behaviour properties (Rheology, Hydrology)
- Resource potential
- Geotechnical (bridges etc.)
- Hazards
What does Rheology of seds refer to?
Strength, Strain, Deformation
What does Hydrology of seds refer to?
Permeability, Porosity, Transmissivity
Rheological responses
- Response to applied stress (Force per unit area, N m^-2) causing strain or deformation (L in m)
What is resistance?
Shear Strength
Stress
Force applied to a surface area
Strain
Deformation (change in shape or volume) of a material caused by stress
What are the 3 types of stress?
1) Tensile (pulling)
2) Compressive (Crushing, collapsive)
3) Shearing (sliding, tangential)
3 common strain-stress responses
1) Elastic
2) Plastic
3) Viscous Fluid
Elastic stress-strain response
Linear response, reversible to elastic limit, then failure
Plastic stress-strain response
Deform after yield strength (k), then uniform strain
Viscous fluid stress-strain response
Continuous, limitless, unrecoverable strain
- Newtonian fluid (stress = derivative of du/dy) controlled by viscosity
Dilatent
- Rheopectic
- Shear thickening
- Temporarily more resistant to stress
- Can drop to a ‘residual’ state after max stress is reached
- Drumlins may form this way
Thixotropic
- Pseudo-plastic
- Shear thinning
- Very susceptible to stress
Sensitive Quick Clays
- 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
Quick clay sensitivity index
= undisturbed S/ disturbed S
1 for hard (over consolidated,
2-4 for most clays,
>8-16 for failure prone clays
Direct shear test (DST)
- 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
Shrinkage Limit (SL)
Mc below which no volume reduction
- Non-plastic
Plastic Limit (PL)
Mc when material transitions from plastic to solid or crumbles
- Deforms Plasticly
Liquid Limit (LL)
Mc from plastic to liquid behaviour
- Flows like a viscous liquid
Plasticity Index (PL)
- 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)
Phi Classes
- Particle size classification scale
- Logarithmic scale to get reasonable number in a scale
- Exponential scale
- Use V-axis to get length
Boulders
- > 256mm
-
Cobbles
64 to 256,
-8.0 to -6.0
Gravel (pebble)
2 to 64,
-6.0 to -2.0
Sand
- 064 to 2,
- 1.0 to 4.0
- Last visible range before silt/clay
Silt
- 002 to 0.064,
4. 0 to 8.0
Clay
<0.002,
>8.0
- clay grains can’t be seen so hand techniques used to ID grains size in the field
Grain Size (Ternary) Distributions
- 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
How to describe the shape of the distribution of grain size
- Mean, SD, Skewness, Kurtosis
Why are most seds not normally distributed (skewness)?
- 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.
Sorting
- Second moment, SD
- Degree of scatter about mean or range of sizes in a sample
Skewness
- 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
Positive skewness
Mean greater than median
- Fine particle tail
Negative skewness
Median greater than mean,
- Coarse particle tail
Kurtosis
- 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
Shape
= f, original shape, mineralogy, transport history, burial processes and rates
- 3 major axes proportions (platy, rod-like, rounded)
What are the 3 major axes proportions of shape
- Platy
- Rod-like
- Rounded
Roundedness
- Measure of angularity
- reflects duration and distance of transport
- Related but not the same as sphericity
Sediment fabric
- grain-grain orientation and packing indicative of transport, sorting, and/or deposition processes
- imbrication can indicate sediment paleoflow based on dip angle etc.
Porosity and pore water pressure
- Mc is critical for determining bulk strength of sediments
Mc
Moisture content
Porosity (n)
volume of voids, bulk volume
- controls moisture storage and tansmissivity
Which has higher porosity, clay or sand? Which has higher permeability?
- Clay has higher porosity and lower permeability
Pore water pressure (PWP)
- Negative suction, positive hydrostatic
- Promotes strength or instability, depending on moisture content
What does mean size of sediment indicate?
- Energy of transporting/depositing envr
- Competence of process to move certain size = f (velocity)
- Hjulstrom relation, beach sands, and wave energy
What do size variations of sediment indicate?
- Reflect change in process over time/space
- ie upward fining of floodplain seds, downstream fining in rivers
What does sorting of sediments indicate?
- Range in size reflects changes in energy, magnitude and intensity, or availability of sizes to process
- ie aeolian sands vs. glacial tills
What does sediment form indicate?
- Shape reveals transport and reworking processes
