Transcript Weathering and Soils • Weathering divided into 2 categories - Physical (disintegration) > breaks rocks and sediments into smaller pieces - Chemical (decomposition) > chemical.

Weathering and Soils
• Weathering divided into 2 categories
- Physical (disintegration)
> breaks rocks and sediments into smaller pieces
- Chemical (decomposition)
> chemical reactions on the surface of minerals
• both are separate processes but usually relate
to each other
- surface area to volume changes as physical processes
break sediments into smaller pieces
- chemical reactions on minerals changes dimensions
of mineral grains and can cause physical
disintegration
Volume to surface area ratios
• Chemical weathering works on the surface. Smaller particle sizes (in
a given volume) have greater surface area for chemical weathering.
Decomposition (chemical weathering)
• Geochemical weathering
- inorganic chemical reactions
- causes rock to get "rotten" while still preserving many
of the original structures created during formation of
the rock
• Creates a bedrock residuum called a Saprolite
• Water is most important element
- facilitates transfer of chemical elements and chemical
reactions
> water often dissociates into OH- and H + which then acts on
electrically imbalanced surfaces on mineral grains
Ion exchange due to water
Abrasion pH
• change in pH of water (or other fluids) by
powdering a mineral and placing in water
- related to ion exchange, and reaches a steady state
Gives a measure of how effective water is at weathering any
given mineral- implies a steady state condition is reached
• H+ is depleted, OH- is enriched relative to H+
• Implication is that the reactions can achieve
some initial maximum rate and extent and then
the rates drop off considerably
Etching
• related to chemical weathering acting and
pieces breaking off along cleavage surfaces
- keeps rates from achieving some steady state
processes of chemical weathering
• Oxidation/Reduction
- a function of available free oxygen and the Eh (redox
potential)
- Oxygen strips electrons away from other atoms
• the process is reversible
- often related to water levels in streams and
groundwater systems
processes of chemical weathering
• solution
- removal of atoms from mineral structures
- causes minerals to fall apart
• related to pH of the solvent
- typically the water
chemical processes
• Hydrolysis
- reactions between a salt and water
> produces Acids and Bases
- H ions replace metallic elements (K in the example below)
- reaction continues until all H atoms have been used up
• 2KAlSi3O8 + 2H + 9H2O
orthoclase
feldspar
H4Al2Si2O9 + 4H4SiO4 + 2K
kaolinite
chemical processes
• Ion exchange
- usually a function of a polarized molecular
arrangement
> causes a propensity to try and balance the charges by
replacing ions with other ions through a process called
adsorption
> esp. Cations
- often measured as CEC (Cation Exchange Capacity)
> a measure of the adsorption ability of a surface
- particulary common in clay minerals
Other factors
• Mobilty-how suceptiple atoms are to movement
due to normal chemical processes
• Leaching
- removal of elements from upper portions of the
weathering profile to locations lower in the weatherin
profile
• Chelation- mobilization of relatively immobile
ions by encapsulating them in a complex molcular
structure
- common way to move things like Aluminum
- usually involves organic compounds that fix the meta
into a mobile compound
Oxidation
• interaction bewteen substances and oxygen (and
in some cases water)
• E.G., 4FeO
+ O2
2Fe2O3
The iron goes from the ferrous reduced
state (Fe2+) to the ferric (Fe3+) state
MgFeSiO4 + 2H2O Mg(OH)2 + H2SiO3 + FeO
olivine
magnesium
hydroxide
2FeS2 + 7H2O + 15O
pyrite
silicic
acid
2Fe(OH) + 4H2SO4
sulfuric
acid
Clay minerals
• occur as layers or sheets arranged in a variety of
ways
- layers consist of sheets of aluminum and silica that
alternate between discrete layers
> 1:1 - clay minerals include Kaolinite, Allophane, and Halloysite
> 2:1 - clay mionerals include Illite, Smectite, Montmorillonite, and
Glauconite
• usually derived from the decomposition of silicate
minerals like feldspars, micas, and hornblende
• Can have unique properties that are important for
good construction
> shrink-swell clays like smectite
Soil Characteristics
What is a soil?
•
naturally occurring 4 dimensional feature (Up/Down;
Left/Right; Front/Back; Time)
•
Consists of layers or horizons that parallel the
earth’s surface
•
Layers have distinctive mineralogical, chemical,
particle size composition that are a function of
pedogenesis acting on the parent material
•
Consists of unconsolidated materials*
•* some horizons act like lithified materials though
•
Some include capable of supporting plant life
Factors influencing soil development
1.
2.
3.
4.
5.
They are the same things that influence weathering
Climate
• Temperature and moisture
Parent Material
• Influences mineral content
Topography
• Includes elevation, slope and aspect
Vegetation
• Dependent on temperature and climate
Time
• Take time for mechanical and chemical processes to
occur
Development with time
• most factors develop with time
- not a steady process though- rates of change vary with time.
Use of soils
•
Relative age tool
- Must isolate all variables to address the effects time
•
Stratigraphic tool
- represents former land surface
•
Paleoenvironmental tool
- certain soil orders only occur in limited climates
> e.g., aridisol, vertisol
- certain characteristics develop best in restricted
climates
Soil Profile
The vertical arrangement of the soil from the surface
to the parent material
•Soils take isotropic materials like basalt or as or
quartzite and make them anisotropic at the surface
The soil
profile follows
the landscape,
and when
conditions
warrant, cuts
across existing
strata
Pedon
The 3 dimension version of a soil profile. It typically
includes an areal dimension lacking in the soil profile
•This is needed to describe the lateral variability in
soils within a given region
•Pedon is the fundamental mappable unit in soil
science
•The pedon is not able to be photographed, and is
commonly poorly described by researches due to
time and monetary constraints.
The Weathering Profile
In areas with extensive moisture and or extensive
permeability, weathering extends very deep
primarily in the form oxidation and reduction
The soil profile is the uppermost part of the
weathering profile
The weathering profile may be only as thick as the soil
profile in many circumstances
Master Horizons - O (organic)
•Common in areas with high
biological productivity
•Typically black or dark
brown
•Usually thin, but can be
quite thick
•Can indicate continued
deposition if very thick
•cumulic O horizon
Can look like A-Horizon
O
Ob-horizon
Commonly, a lab
analysis is needed to
distinguish between
A and O horizons
•O is more than 30%
organics
•A is less than 30%
Master Horizons - A
Dark in color
•often black
•Doesn’t have to be
Mineral content dominant
Can be quite thick
Zone of eluviation
Often difficult to
differentiate between A
and O horizons
QuickTime™ and a
decompressor
are needed to see this picture.
A
2Bt1
3Bt2
Master Horizons E-Horizon
Zone of extreme leaching
of minerals- eluviation
Bleached- a gray color
below the A/O horizons
but above the B
Primary minerals provide
color- rest has been
washed away
Common under conifer
forests and grasslands
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Master Horizons B-Horizon
Zone of Illuviation
Clays and chemicals collect in this horizon
Typically marks the average depth of wetting front
Bt-Horizon
Master horizons K-Horizon
Occurs at the base of the
B-horizon
Common in arid environments
Depth of wetting front
determines location
Caliche creates an
impermeable zone
like concrete
K horizon
Master Horizons C-horizon
Slightly altered parent
materials
Typically oxidized only
Recognizable as parent
material
Master Horizons - subordinate designations (1-3)
“b” is the most common one of these 5 designations
They appear immediately after the Master Horizon
designation and are always lower case
Master Horizons - subordinate designations (2-3)
“k” is the most common of these designations
“ss” commonly occurs in areas with shrink-swell clays
like smektite (e.g., Texas Gulf Coast).
Master Horizons - subordinate designations (3-3)
“x” commonly designates a fragipan, a dense brittle
horizon that slakes in water, but is impermeable
otherwise.
Soil Description
1- Clear off a face so that the color changes are easily
viewable
•Natural light is best; colors can be very subtle
2- Mark any locations where the color changes
•Golf tees work great (expensive), as do plastic
toothpicks (cheap)
3- Use your dirt knife and start poking the soil gently
moving down from the top
•Feel for changes in the resistance to the “poke” and
mark where it changes (except where the change
coincides with already marked locations
4- Stretch a tape measure and record the intervals
between the markers (from the top to the bottom)
•e.g.,0-10 cm; 10-26 cm, etc…
Soil description (continued)
5- Determine the horizon name for the first layer by
describing the following criteria
•Color
* see specific slide
•Structure * see specific slide
•Texture
* see specific slide
6- Describe the character of the bottom contact with
underlying layers
7- Look at samples using a hand lens to determine if
there are clay films or silt coats
•Look in pore spaces and on ped faces
8- Test with hydrochloric acid solution for reactivity
Characteristics- Color
• Described using 3 sets of numbers
- Munsell Soil Color Chart
• First tells the ratio of red to yellow (Hue)
- e.g., 7.5 YR, 5R, 10YR, 2.5YR, 7.5R
- these identify on what page you examine the
next 2 numbers
• Second tells light vs dark (Value)
- denoted as the numerator of a fraction
• Third tells the strength of the color
- deviation from a neutral of the same color
- the denominator of the fraction
Peds
Other factors
• Calcium Carbonate
• Topography of the horizon boundary
- smooth, wavy, irregular
• Distinctiveness of the boundary
- diffuse, gradual, distinct, abrupt
• Mottles
- uniformity of color, "blotchiness"
• Organic matter
• and anything else you can see that doesn't fall
into one of the above categories
Mottles in a soil
Lab determinations
Bulk densitythe weight of the soil per unit volume
A measure of how dense the soil is (grams/cm3)
•Increases as clay content increases
Coat peds in parafin wax and measure weight in
water and weight without wax.
Lab determinations
pH
•Take a small sample and mix with DI water.
Insert the probe and take a reading
- other methods are available
CEC- Cation Exchange Capacity
•Determines the total negative electrical charge on
clay surfaces in meq/100 g of soil (milliequivalents)
Calcium Carbonate content
•Measured using a chittack apparatus
Particle size
•Hydrometer; Pipette; Laser
Carbon
•Loss on ignition
Paleosols
Soils no longer in
the active soil
forming
environment
Multiple paleosols
in loess