Transcript 3-Weathering.ppt

Weathering
The processes that lead
to the surface and subsurface disintegration and
dissolution of rock.
Weathering
Alteration of rocks and minerals by processes
acting at or near Earth's surface
• i) causes net mass flux from a spot
and/or
• ii) causes changes in factors such as strength,
permeability, or particle size that in turn influence
the rate of mass flux from a site
Major controls on weathering
Thermodynamics - materials and
reactions (parent materials and
reactants)
Kinetics - rates of reactions and
processes (time)
Environmental conditions
(temperature, moisture, ph, plants,
critters)
Weathering
Disintegration, or breakdown of
rock (no transport, that is erosion)
2 types are most important to us:
Mechanical (physical) weathering
Chemical weathering
Biological weathering combination
of the other two
3 kinds of weathering
Chemical:
mostly aqueous geochemistry; changes composition
Mechanical (Physical):
mechanical breakdown of rocks; changes size
Biological:
effects of organisms; can act like either chemical or
physical weathering
Relative importance varies.
Mechanical Weathering
No change in chemical composition-just disintegration into smaller pieces
Chemical Weathering
Breakdown as a result of chemical reactions
• for example:
CaCO3+CO2+H2O  Ca2+ + 2HCO3-orcalcite (limestone) + carbon dioxide + water
 calcium + bicarbonate
Mechanical Weathering
• Weathering Processes
Exfoliation / Sheeting
Frost Action
Wetting / Drying
Salt Weathering
Thermal Expansion
• Associated Weathering Landforms
Exfoliation Domes
Shattered Rocks
Talus Slopes
Rock Sea (Felsenmeer)
Chemical Weathering
• Weathering Processes
Hydrolyis (secondary mineral formation)
Dissolution
Hydration
Carbonation
Oxidation
• Associated Weathering Landforms
Spheroidal weathering
Dissolution pits (gnama pits)
Karst topography
Tors and Bornhardts
Mechanical weathering
Physical breakup of rock due to:
• pressure release
• freeze & thaw of water
• thermal expansion and contraction
• crystallization of salt(s) within rocks
Mechanical Weathering
Change in volume
i
total volume
ii
void or fissure volume
Mechanical Weathering
Change in Total Volume
Exfoliation
Rocks formed at high pressure deep within Earth
raised to surface through removal of overlying
rocks.
Relieves stress causes expansion at the surface
and results in exoliation sheets or granular
disintegration along grain to grain contacts.
Pressure release - Exfoliation
Rock breaks apart in layers that are parallel
to the earth's surface
– as rock is uncovered, it expands (due to
the lower confining pressure)
Pressure release - Exfoliation
Sheet joints due to
exfoliation
Exfoliation, back side of glacially eroded valley
Half Dome, Yosemite Valley, California
Onion-skin weathering
Repeated cycles of
expansion and contraction in
near-surface environments
can lead to in place
mechanical rounding of
fractured rock without any
downhill transport.
Freeze & Thaw
Frost wedging: rock breakdown caused by
expansion of ice in cracks and joints
Volume
Change
in Voids
Mechanical Weathering
Where water seeps into rock weaknesses during
day, then freezes and expands at night, it can break
the rock apart.
Freeze & Thaw
Shattered rocks are common in cold and alpine
environments where repeated freeze-thaw cycles
gradually pry rocks apart.
Freeze & thaw
Ranges from single
isolated rocks to whole
surfaces covered by rock
fragments, called
felsenmeer (German for
“rock sea”)
Sierra Nevada, CA
Felsenmeer (rock sea)
Mechanical Weathering
Hydration / Dessication
Expansion and contraction
through wetting and drying
Addition of water into rock
or mineral structure
can cause expansion of
minerals (i.e., swelling
clays). Sheet silicates are
particularly susceptible.
Example: biotite hydration
(principle process of
granite decomposition ?)
Crystallization of salt(s)
Salt Weathering
Growth of salt crystals
from hyperconcentrated
solutions in arid
environments
Saline fluids flow in and
evaporate on surfaces
leaving solids that
expand to fracture and
flake off surficial
material, creating pits
on the surface.
Mechanical Weathering
Insolation intensity or variation (heating and cooling)
Thermal expansion and
contraction
i
can cause several
% volume change
at high temps
ii
intense fire can lead
to spalling rocks
Thermal expansion
Shattered rock:
Extreme range of
temperatures in
desert
environments
Repeated swelling
and shrinking of
minerals
Consequences of physical weathering
Reduces rock material to
smaller fragments that
are easier to transport
(more later)
Increases exposed
surface area of rock,
making it more
vulnerable to further
physical (and chemical)
weathering
Chemical weathering mechanisms
Transformation/decomposition of one mineral
into another
Mineral breakdown, for example:
• carbonate (CaCO3) dissolves easily
• primary minerals --> secondary minerals
(mostly clays)
The key is: net loss of elements retained in the rock
many go into solution and are carried away…
Weathering rates
Formation of joints in a rock
create a pathway for water –
they can enhance mechanical
(or chemical)
More weathering
leads to more and faster
weathering
(positive feedback)
Leads to spherical (or
spheroidal) weathering
Sperhical weathering
Diffusion of weathering into lattice creates
corestones…
Spherical Weathering
Corestones result from differential weathering along fractures.
Spherical Weathering
Exposure of corestones at surface produces rounded boulders.
Rates of Chemical Weathering
What controls amount and rates of dissolution?
1
Flow Path:
distance travelled effects ionic strength, pH, etc...
i) overland flow / flow through soil / groundwater flow
ii) sequencing:
soil to soil
soil to weathered bedrock
2
Contact Time:
longer contact = more removal, but rate decreases
as water saturates
expect groundwater to have higher concentrations
of dissolved species
Rates of Chemical Weathering
What controls amount and rates of dissolution?
3
Water Flux
more H2O = more dissolution
4
Mineral Assemblage
i) thermodynamic properties controlling solubility
ii) kinetics - controls rate of reaction
iii) buffering - controls amount of base or acid required to
change pH
iv) sorption - take up of ions on surface of clays; controls
nutritional capacity of soils
Rates of Chemical Weathering
What controls amount and rates of dissolution?
5
Biological Influences
Chelation: partially decomposed organic
material bonds to metals making them soluble
and allowing migration and re-precipitatation.
Important in cold/wet areas.
6
Temperature: influences reaction kinetics
hot = fast
cold = slow
tropics - deep weathering profiles
high latitude - thin weathering profiles
Chemical Weathering
The process that breaks down rock through
chemical changes.
The agents of chemical weathering
– Water
– Oxygen
– Carbon dioxide
– Living organisms
– Acid rain
Chemical weathering mechanisms
Water is the main operator:
Dissolution
Many ionic and organic compounds dissolve in water
(because parts of them do):
– Si, K, Na, Mg, Ca, Cl, CO3, SO4
Acid Reactions
Water + carbon dioxide <---> carbonic acid
Water + sulfur <---> sulfuric acid
H+ effective at breaking down minerals
Carbonation
Water and carbon dioxide react in atmosphere
and organic rich surface horizons to make
carbonic acid via:
H2O + CO2 (g)  H2CO3
carbonic acid
H2CO3  H+ + HCO3Bicarbonate = most abundant ion in natural
waters; controls pH of natural waters.
Carbonation
pCO2 in the atmosphere = 0.03% by volume (3 x 10-4 bar)
pCO2 in soils is higher (i.e., 1 - 2%) due to microbial
decomposition of organic matter
photosynthesis
nCO2 + nH2O  (CH2O)n + nO2
respiration
Soil pH < natural water pH
So soils are more acidic than rainwater and most
weathering happens below ground.
Solution
Partial or complete dissolution or conversion of a solid
into a solute
pH = negative log base 10 concentration of hydrogen
ions in grams/liter
neutral: pH = 7 (10-7 g of H+ / liter)
acid:
pH < 7 (more H+)
alkaline: pH > 7 (less H+)
Rates of solution = f (kinetics, ph, solubility, and water
flow)
Solution
i)
Solubility of some
species changes
radically with small
changes in pH
ii) Low solubility of
silica
Solution
Silicate solution
Silicate rocks dissolve slowly (in general)
concentrations of Si(OH)4 generally 5-20 ppm
Carbonate solution
CaCO3 + H2O + CO2  Ca2+ + 2HCO3calcite
bicarbonate
Carbonate rocks (i.e., limestone) generally dissolve
more quickly than silicates
Solution
Introduction of other ions, especially Sulfur and Nitrogen
can lower runoff pH.
2SO2 + 2H2O + O2  2SO42- + 4H+
"acid" rain partly due to sulpher emissions in Europe and
eastern U.S.
common pH values
"normal" rainfall ≈ 5.7 (mostly due to carbonic acid)
"acid" rain < 5.6
"normal" soil ≈ 4.6
Average Eastern U.S. & Scotland rainfall ≈ 4.0 (low = 2.1 & 2.4)
vinegar = 2.4
coca cola ≈ 2.3
Acid Rain
Compounds from burning coal, oil and gas react
chemically with water forming acids.
Acid rain causes very rapid chemical weathering
Dissolution pits
(gnamas)
Dissolution pits from salt weathering, Oregon Coast
Dissolution
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3Water + carbon dioxide + calcite dissolve
into calcium ion and bicarbonate ion
Dissolution
Processes by which rocks are dissolved by water:
– strongly influenced by pH and temperature
– (why availability of H+ and CO2 are important)
When water becomes saturated, chemicals may
precipitate out forming evaporite deposits.
Calcium carbonate (calcite, limestone), sodium
chloride (salt), and calcium sulfate (gypsum) are
particularly vulnerable to dissolution weathering.
Hydrolysis
Feldspar + carbonic acid
+H2O
= kaolinite (clay)
+ dissolved K (potassium) ion
+ dissolved bicarbonate ion
+ dissolved silica
Clay is a soft,
platy mineral, so
the rock
disintegrates
Mineral Stability and Secondary Minerals
Chemical weathering generally includes
changes in mineral assemblages from
those found in parent rock.
Water must have access to mineral grains
through grain boundaries, micro-cracks,
fractures, etc... (Coarser materials
generally have larger cracks, and hence,
are more vulnerable to weathering).
Mineral Stability and Secondary Minerals
Most general reaction
cation-Al-silicate + H2CO3 ---> HCO3- + H4SiO4 + cation+ + Al-silicate
Removal of cations produces "cleaner" Al-silicates
Recall that H2CO3 is produced from reaction of atmosphere
and water
Weathering consumes H2CO3 and thereby fixes CO2 from the
atmosphere.
Mineral Stability and Secondary Minerals
Relative mobility of cations:
Ca2+ > Mg2+ > Na+ > K+ > Si4+ > Fe3+ > Al3+
Intense leaching tends to leave Fe and Al-silicates, stripped
of Ca, Na.
Many tropical soils are nutrient poor.
Resistance to Weathering
First to
Crystallize
Fast
Weathering
Bowen’s
Reaction
Series
Goldrich
Stability
Series
Last to
Crystallize
Slow
Weathering
oxides
Clay minerals further decompose to aluminum hydroxides
and dissolved silica.
Clay Minerals
Secondary minerals (clays) are:
– chemically altered, or have
undergone chemical
weathering.
– highly reactive materials with
electrically charged surfaces.
– made up of individual clay
sheets and clay properties
result from the arrangement
of these individual clay
particles.
Clay Mineral Structure
Clay particles are composed of
tetrahedral and octahedral layers
stacked on top of each other.
Kaolinite (1:1 clay)
When one layer of kaolinite lies
above a separate layer of
kaolinite, the layers of
alternating tetrahedral and
octahedral sheets are literally
stacked
The hydroxyls of the octahedral
sheet (bottom of the alumina
sheet) are adjacent to the
oxygen's of the tetrahedral
sheet (top of the silica sheet),
which permits adjacent layers to
be bound by hydrogen
bonding.
2:1 clays
2:1 clay minerals have
one octahedral sheet
between two tetrahedral
sheets
These clays include:
Smectite
(Montmorilinite)
Vermiculite
Illite
Chlorite
Shrink/Swell behavior in clays
Oxidation and Oxides
Oxidation = loss of electron
Reduction = gain of electron
Iron in rocks usually Fe2+, oxidation converts to Fe3+
Change in charge balance can increase vulnerability
to other weathering processes.
Oxidation strongly affects color of rocks and soils
Oxidation and Oxides
Oxidation can affect any ironbearing primary or secondary
(clay) mineral to form
hematite [Fe2O3] and limonite
[FeO(OH)•nH2O].
Oxidation and Oxides
Iron combines with
oxygen in the
presence of water in
a processes called
oxidation to produce
iron oxide (rust)
Biological Weathering
Can be both chemical
and mechanical in
nature.
• roots split rocks apart
• roots produce acids
– that dissolve rocks.
• tree throw
• burrowing animals
• makes H+ ions available
(acid)
Plant Roots
Plant Growth
• Root Growth - Root growth extends into rock fractures,
expand and bust the rock apart
Animal Burrowing
Termite mounds, Northern Territory, Australia
Lichens and mosses
grow on rocks
Roots wedge into pores and
crevices. When the roots
grow, the rock splits.
Lichens produce weak acids
that chemically weather rock
Weathering: broader controls
• Climate
– Temperature and moisture characteristics
– Chemical weathering
• Most effective in areas of warm, moist climates – decaying
vegetation creates acids that enhance weathering
• Least effective in polar regions (water is locked up as ice) and
arid regions (little water)
– Mechanical weathering
• Enhanced where there are frequent freeze-thaw cycles
Alaska
Seattle
Amazon
Altiplano
Weathering Products
Dissolved load of rivers
HCO3- is the dominant cation in many natural waters
Cl- important near coasts and in arid areas
SO42- too low to detect in most natural water - increased by pollution
NO3-, PO4- primarily from organic decomposition, fertilizers
Dissolved load typically about 1/3 of the total load of rivers, but ...
Ganges
8%
(lots of debris from
Himalaya)
Amazon
18%
Mississippi
20%
Yukon
28%
Zaire
42%
Volga
64%
St. Lawrence 89%
(low topography,
big lakes as traps)
Weathering >> Erosion
Erosion >> Weathering
Thin soils, little saprolite development
Polar / Alpine (cold, dry)
Thick saprolite development
Tropical (hot, wet)
Weathering of Bent Pyramid, near Cairo, Egypt
Weathering of Bent Pyramid, near Cairo, Egypt
Karst Topography
CO2 dissolves in rain water and creates carbonic
acid.
Carbonic acid easily weathers limestone and
marble.
Weathering-dominated Landforms
Karst Topography - generated through
solution mostly into limestone bedrock
Giant sinkhole, Alabama – 425 feet long and 150 feet deep
House lost to sinkhole, Bartow, Florida (1967)
Sinkholes
Features of Karst: Caves
Features of Karst: Disappearing
Streams
Carlsbad Caverns, New Mexico
Tower karst, Li River, Guilin, China
Pinnacle Karst, Chocolate Hills, Bohol, Philippines
Duricrusts
Definition:hard layers formed by concentration of
particular components during weathering
Importance: source of bauxite (aluminum ore)
evidence of past climates
role in landscape development
Mechanisms of formation:
i leaching
ii accumulation.
Pilbara iron duriscrust, Australia
Duricrusts
Leaching:
High rainfall removes more mobile components,
and leaves behind less mobile components
ferricrete: Fe duricrust
alcrete: Al duricrust
Accumulation
Material mobilized from elsewhere is reprecipitated in concentrated horizons.
silcrete = silica
calcrete = calcium carbonate
gypcrete = gypsum
Causes
depth of rainfall penetration (arid areas)
change in ph (through effect on solubility)
Duricrusts and Inverted Topography
Duricrusts are hard
and once formed can
be more resistant to
erosion than
surrounding areas.
Differential Weathering
Erosion resistant rocks, or those that have been weathering for less time
tend to form steeper slopes and rise to higher local elevations.
Physical weathering on Mt. Whitney in background
Chemical weathering on Alabama Hills in foreground
Formed from the same plutons. Difference is age of exposure,
air temperature, and glaciation.
Differential
Weathering
Delicate Arch,
Arches NP,
Utah
Canyon de Chelly
Crystallization plus
Differential weathering
Weathering Related Landforms
Tors rock outcrops that stand
on all sides from surrounding
slopes
2 models for formation
i deep weathering, then
erosion
ii
differing erosion resistance
Granite Tors
Weathering
and Soils
We know more about
the movement of
celestial bodies than
about the soil underfoot.
- Leonardo da Vinci