EAS 222 Midterm

3 Types of weathering

Mechanical, Chemical and Biogenic

Paleosols

Ancient soils that have become preserved in the strata as a part of the fossil record

Clastic Sedimentary rock

Made up of clasts of pre-existing rocks. Pieces of rock loosened by weathering then transported to a basin/depression where sediment is trapped and compacted/cemented

Biochemical (Organic) Sedimentary rocks

Formed from cemented pieces of once living organisms. Chert from diatoms, limestone from calcareous organisms/precipitation, and coal from water-laden fossilized plants

Chemical Inorganic Sedimentary Rocks

Mineral constituents in solution become supersaturated and inorganically precipitate. Most commonly evaporites: Gypsum, halite, etc

Subaqueous and Subaereal

Subaqueous is weathering that occurs under water, and subaerial is weathering that occurs out of water

Mechanical Weathering (Definition)

Produces smaller constituents without altered chemistry or mineralogy, usually by exceeding the tensile strength of the rock

Stress-release weathering

Freeze-thaw weathering

Physical breakdown of rocks by the invasion of water into cracks or planes of weakness and expansion when water freezes

Insolation Weathering

Sun-heated rock surfaces expand and then contract in the sun's absence, weakening bounding planes and grain boundaries

Salt weathering

Generation of internal pressure when high-salt concentrated water invades the pores or weakness planes of a rock and forms salt evaporates

Wetting and Drying weathering

Weakening of tensile stresses by causing negative pore pressure in the process of removing water and swelling pressures in the process of adding water to a rock.

Hydration weathering

Expanding of clay minerals due to absorption of water

Stress-release weathering

When buried rocks under high pressure suddenly experience offloading (a portion of the rock weight above is removed) and the compacted stones will begin to expand and may experience brittle pressures

Exfoliation weathering

Flaking of the outer layers of sediment in a bed. Mainly caused by repeated cycles of heating and cooling.

Chemical weathering

Changes in composition and mineralogy with a net loss of material and/or a change in particle size.

3 types of chemical weathering

Congruent dissolution

Hydrolysis

Oxidation and Reduction (Redox)

Congruent Dissolution weathering

Completely dissolving a mineral into solution without precipitation of it or any other mineral out of solution. Often due to rainwater.

hydrolysis weathering

Chemical reaction between silicate minerals and acids leading to the mineral's breakdown along with release of metal ions from solution.

Redox weathering

Chemical alteration of metal ions (ie iron and manganese) in silicate minerals due to interactions of oxygen in contacting water. Oxidation (Loss of electron) of the metal and reduction of the water will cause the metal cation to fall out of the crystal lattice and may cause the crystal lattice to become unstable.

Hydration/dehydration weathering

Hydration: Water molecules are added to a mineral's crystal structure to form a new mineral

Dehydration: Water molecules removed from a mineral's crystal structure to form a new mineral

Bowen's Reaction Series (In regards to weathering)

Minerals that form at high temperatures, such as olivine, will have significantly less stable at surface conditions than minerals formed at low temperatures such as quartz.

Products of subaerial weathering

Source rock residues, secondary minerals and soluble constituents

Source rock residues

Found in immature soils

Includes igneous and metamorphic rocks, fragments and assemblages of unstable rocks such as olivine, hornblende, calcitic plagioclase, etc. Mature soils would only include the most stable minerals.

Secondary minerals

New minerals formed in the same area during chemical weathering

Soluble materials

Extracted from rocks by chemical weathering and removed from the weathering site by soil or groundwater. End deposition site will be the ocean.

Processes of subaqueous weathering

Exposed seafloor rocks will experience chemical reactions with sea water.

Temperature percolation through cracks in the oceanic plate may bring water to high-temperature/pressure forming minerals and allow them to dissolve said minerals. When the water is heated and returns to the surface, the minerals are deposited out of solution in vents of sulfide, sulfate, and oxide deposits.

Soil forming factors

Parent material, time, climate, relief, organisms

Soil Profile Horizons

O: Organic

A: Topsoil

E: Eluviated horizon

B: Subsoil

C: Parent material

R: Bedrock

Bioturbation

Disturbance of sedimentary deposits by organisms

mass wasting

the downhill movement of a large mass of rocks or soil due to gravity

Fluid Density

Mass per unit volume.

Affects forces that act in a fluid: temperature, rate of particle settling, response to gravity, etc.

Fluid Viscosity

Ability of a fluid to flow.

Low flows easily, high flows with difficulty. Affects turbulence and changes with temperature.

Turbulent Fluids

Support Mechanism: Turbulent fluid

Observed Type of Flow: Turbidity Flow

Newtonian Fluids

Support mechanism: Upwards escape of intergranular fluid

Observed type of flow: Liquefied flow

Non-Newtonian Fluids

Support mechanism: Grain interaction through dispersive pressures

Observed type of flow: Grain flow

Bingham Plastic Fluids

Support mechanism: Matrix

Observed type of flow: Mud or debris flow

laminar flow

Streamlines are parallel and don't intertwine, predictable flow lines.

turbulent flow

Streamlines twist and turn and cross each other. There is significant movement in directions other than the overall direction of the flow

Eddies

swirling fluid in the reverse direction of a current created in a turbulent flow regime

Reynolds number

Dimensionless numbers used to determine if a flow is laminar or turbulent. Used to assess the magnitude and turbulence (low Re= laminar, high Re= turbulent).

Transitional Flow

A flow that alternates between being laminar and turbulent.

Boundary Layers

Layers of a flowing fluid in contact with a surface, such as a riverbed. From bottom to top there is an increase in flow velocity followed by a zone of consistent velocity.

Laminar/viscous sublayer

Present in smooth beds, characterized by an unmoving section at the bed boundary where viscous forces dominate. Within the underlying layer, flow is laminar and is traced by overlying layers

Froude Number

Dimensionless number defined as the ratio of the flow inertia to the gravitational forces used to measure flow regimes of open channel flow

Entrainment (of Sediments)

The process by which sediment is picked up from a surface and carried some distance by a overpassing fluid flow. Fluid drag and fluid lift push the sediment into suspension, while gravitational forces will pull it out of suspension.

critical threshold

The passing of positive entrainment forces beyond the negative entrainment forces that will result in pushing a particle into suspension

Bernoulli Effect

When a fluid is overpassing the top of an exposed particle, the pressure above it will be lower than the pressure below it due to the difference in fluid speeds. This will allow the particle to move off the surface more easily.

Hjulstrom diagram

A diagram of critical flow velocity against mean grain size.

Velocity decreases with increase of size from Clay to fine sand.

Velocity then increases with the increase of size from fine sand onwards

Kurtosis

A measure of the skewness of a graph. When a grain sized graph is positively skewed, The mode sediment is coarse and when it is negatively skewed, the mode sediment is fine.

Stoke's Law of Settling

An object falling through a fluid experiences the force of gravity, buoyancy and drag.

Bed load, Suspended load and Dissolved load

Bed Load: Portion that cannot be kept in constant suspension but still moves with the current through either saltation or traction

Suspended Load: low enough density/small enough size that is kept in contact suspension by electrostatic and viscous interactions with the surrounding water

Dissolved load: Ions that are dissolved in moving solution

Wash load

clay-sized portion of suspended load

Sediment Transport by Wind

Surface creep, saltation, and suspension. Due to air's low density, much more positive entrainment forces are required to lift clasts, and clasts will always be smaller than those transported by water.

Sediment transport by glacial ice

Less positive entrainment forces required to suspend clasts, and the size threshold of clasts that can be transported is significantly larger than water due to ice's high density.

Rock Fall

Free falling movement of massive rocks from a cliff or steep slopes

Slides

Mass movements of large portions of sediment due to shear failure with little to no internal deformation

Sediment Gravity Flow

A mass movement mimicking fluid flow where sediment undergoes mass internal deformation

Turbidity current

A subaqueous downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension

Turbidites

Graded layers of sand interbedded with finer pelagic sediments typical of the deep sea floor.

Bouma Sequence

An idealised sequence of sediments and sedimentary structures that could be seen in a turbidite deposit

Liquified flow

Sediment-gravity flows in which grains within the flow are kept suspension by the upward movement of fluid.

Normal vs Reverse grading

Normal grading: A transition from coarse to fine grained sediments moving up in a depositional unit.

Reverse grading: A transition from fine grained to coarse sediments moving up in a depositional unit. This is much more uncommon and typically only occurs due to kinetic seiving.

Debris flow

A downslope movement of mud mixed with large rock fragments and biological material

Udden-Wentworth Scale

A grade scale for classifying the diameters of sediments.

Particles larger than 64 mm in diameter are classified as cobbles.

Smaller particles are pebbles (16mm), granules(2-4mm), sand(2-1/16mm) and silt (1/16-1/265.)

Those smaller than 1/256 mm are clay.

Phi Scale

Expresses grain size on a logarithmic scale. Phi values increase arithmetically as the grain size decreases geometrically.

• 64 mm= cobbles, -6 phi

• 32-4mm= gravel, -5 to -2 phi

• 2mm-125um= sand, -1 to 3 phi

• 63-4um=silt, 4-8 phi

• Below 4um=clay, 9+ phi

Measuring Grain size

Thin sections, estimations or measurements (larger clasts), sieving of loose sediment

Particle Form

Sphericity or "Particle shape". From equant to platy.

• oblate(disc)

• Bladed (asymmetrical)

• Prolate (roller)