Chapter 5 Chapter 6 current textbook Weathering and Soil

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Chapter 5 (Chapter 6 current textbook)
Weathering and Soil

• External processes Occur at or near surface,
  energy from solar heat. Part of the Rock Cycle,
  these process form the components of sedimentary
  rock.
• The 3 Processes that break apart rocks and move
  the debris to lower elevations are
• Weathering
• Mass Wasting
• Erosion
• These processes are responsible for shaping the
  land.

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• Weathering
• Mechanical (physical) Rock is broken into
  smaller pieces by various mechanical processes,
  providing more surface area for chemical
  weathering.
• Chemical weathering Mineral grains are
  attacked by water naturally-occurring acids.
  Microfractures fractures provide pathways.

Reactions occur along exposed edges
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• Mass wasting The transfer of rock and soil
  downslope due to the force of gravity overcoming
  friction and rock/soil cohesion. Water is often
  a facilitator in mass wasting, because water is
  heavy and it acts as a lubricant. More in
  Chapter 15.
• Erosion The physical removal of broken down
  rock and mineral material. Water is the primary
  agent of erosion. Wind and glaciers are less
  important.

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• Mechanical Weathering
• Frost wedging freeze-expansion of water spreads
  microfractures, fractures and joints in the rock
  surface. Over time, repeated cycles can produce
  a talus slope (see Figure 6.3)

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• Unloading as erosion removes material above
  a buried rock unit, expansion of the newly
  ex-posed rock results in the separation of
  individual sheets of rock. May happen in
  quarries, also.

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As erosion gradually exposes a large rock
body, as large sheets separate, break apart, and
spall (slide) off, an exfoliation dome forms.
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• Thermal expansion diurnal heating and cooling
  may cause damage to outer surfaces of rock,
  especially if rock surface has been weakened by
  chemical weathering.
• Biological activity tree roots grow into
  existing fractures and during growth, expand the
  fractures (and thereby cause new fractures). On
  a micro scale, the roots of mosses and other
  plants may infiltrate micro-fractures in rock
  surface. Root growth and expansion also results
  in the fracturing of driveways.

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• Chemical weathering H2O CO2 H2CO3 (carbonic
  acid)
• As rainfall infiltrates soil, additional CO2 is
  added from soil bacteria and organics may add
  additional acids.
• Polarity of water molecule increases its chemical
  effects. Hydrogen ions positive. Oxygen ion
  negative.

Negative side attracts positive ions (cations),
Positive side attracts anions.



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• Dissolution Polarity of the water molecule
  affects individual salt molecules, e.g. Halides
  (NaCl, KCl) Sulfates (gypsum).

Most caverns are formed from the dissolution of
calcite in limestone or marble.
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Oxidation Iron (Fe) easily loses electrons
to Oxygen, producing the reddish-brown rind on
the basalt, similar to the rusty steel on page
193.
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• Hydrolysis reaction of minerals with water,
  when acids are present, effect is intensified.
  Hydrogen ion (H) replaces other cations in
  mineral lattice (structure), disrupting the
  original structure decomposition of mineral and
  rock structural integrity. Most important
  silicate minerals (except quartz) are susceptible
  to hydrolysis, with the dark silicates most
  susceptible. Basalt in slide 10 lost silica to
  hydrolysis, which facilitated oxidation of Fe-Mg
  minerals.

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• 2KAlSi3O8 2(H HCO3-) H2O
  Al2Si2O5(OH)4
• (2K 2HCO3- 4SiO2) in solution.
• K feldspar (orthoclase) kaolinite (clay)
• Hydrolysis products are listed in Table 5.1.
  Hydrolysis produces the spheroidal (rounded)
  weathered surface, by attacking corners first.
  
• When hydrolysis destroys the structure of a rock,
  but traces of the structures remain this is
  saprolite.

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Over time, with Hydrolysis, the biotite gneiss
(left) becomes the saprolite (right). When all
traces of structure are lost, the saprolite
becomes residuum (Ga. Red clay).
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Differential Weathering - Outcrop at right
shows differential weathering between quartz-rich
ledges and other silicate minerals.
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• Rates of weathering dependent on
• Rock Characteristics Climate
• Rock Characteristics quality of chemical bonds
  in minerals affects solubility. Quartz strong
  bonds, stable. Calcite weaker bonds less
  stable.
• See Figure 6.15 refer back to Bowen Reaction
  Series. Minerals that crystallize early
  (olivine, pyroxene, etc.) have a less organized
  structure and weather more easily.

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• Climate Frequency of freeze-thaw cycles in
  colder climates result in more frost wedging,
  i.e., colder climates may favor mechanical
  weathering. Cold climates keep water locked up
  in ice.
• Chemical weathering proceeds more rapidly in
  warm, wetter climates. Temp. is important
  every 100 C increase in temp. doubling of
  chemical reaction rates. Increases in
  atmospheric nitrogen and sulfur compounds (from
  pollutants) can increase the rate of chemical
  weathering.

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• Plants may play a role in chemical weathering, as
  acids from decaying plant matter may assist in
  the breakdown of minerals. The results of
  physical and chemical weathering are subject to
  downslope movement by water (erosion), unless
  held in place by vegetation or soil cohesion.
  The eroded material becomes sediment in streams
  and is transported during floods until it reaches
  it final site of deposition, usually the ocean.
  The materials that remain behind become part of
  the soil profile.

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Soil Its more than just dirt.
The results of physical and chemical weathering
provide for plants
A medium for root infiltration plant support
Nutrients from degraded minerals
Pore space for water and oxygen
The quality of the soil affects the Resiliency
of an ecosystem, i.e., its ability to bounce back
from disturbances.
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As stated, rate of physical and chemical
weathering depends on rainfall and temperature.
Desert climates lack of rainfall inhibits
development of regolith and humus on slopes.
Flatter areas may accumulate a thin layer of
mineral grains, but the lack of moisture prevents
nutrient cycling of organics. Because dried
organics tend to be light, they are prone to wind
erosion. Because of poor soil development, desert
ecosystems have poor resiliency.
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UT El Paso campus desert climate 8 inches
of rain/year poor soil development
GUC campus humid, temperate climate 44 inches
of rain/year good soil development
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Controls of Soil Formation Parent material,
Time, Climate, Plants and animals,
Slope. PARENT MATERIAL Initially, may be most
important. Residual soils form on parent
material. Transported soils from upon
unconsolidated material deposited from elsewhere,
e.g., soils on river bottoms. Transported soils
develop from partially weathered materials and
are often more fertile.
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• Mineral types in parent rock play a role. Pure
  quartz sandstones or quartzites have little that
  can be chemically weathered to produce nutrients
  vs. other silicates, etc..
• Volcanic ash flows form excellent soils
  pulverized nature of ash more surface area.
  Glassy nature of ash breaks down easily.
• CLIMATE Over time contributes more to soil
  character.
• Rate of weathering is dependent on available
  moisture and temperatures. Also important in
  nutrient cycling of humus.

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PLANTS AND ANIMALS - Plants contribute most of
organic matter (humus) to soil, while
micro-organisms some small invertebrates
contribute to breakdown of organics. Earthworms,
ants, etc., play a role in mixing of organics and
regolith. SLOPE - Steep slopes may inhibit water
retention, plant growth, retention of humus.
Also, orienta-tion of slope plays a role. In
mid-latitudes in E-W oriented valleys,
south-facing slope receives more sunlight,
especially during winter. In some desert
mountain ranges, east-facing slopes and valleys
may retain more moisture than those on the
western slopes. Ex McKittrick Canyon, NM
Aguirre Springs, NM
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• Description of soil profile (idealized) pp.
  200 - 201.
• O Layer mostly organics
• A Layer mineral grains, with up to 30 organics
• E Layer light colored horizon, materials
  removed by eluviation washing out of smaller
  grains and leaching of soluble minerals
• B Layer deposition of smaller grains from E
  layer (subsoil)
• C Layer Partially altered parent material

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O A E B solum true soil Well
developed profile suggests environmental
stability over long period of time, i.e., they
are mature soils. Soils w/o profiles may be
termed immature soils Table 6.2 gives summary
of Soil Types Pedalfers - our soil B horizon
Fe oxides and Al clays, leaching of solubles,
more acidic.
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Pedocals El Pasos soil drier climate,
less leaching of solubles, less clay, more
calcium, more alkaline. Caliche the result of
the redeposition of calcite when soil dries in
desert climate. Laterites Tropical soils Fe
oxides and Al clays, below thin O and A layers.
In forest, high rainfall rapid nutrient
cycling, thin O layer and leaching of minerals
thin A layer. With loss of tree layer, thin O and
A layers are gone within a few years poor
recovery.
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• Process of Erosion
• Raindrops dislodge soil particles which are then
  moved by sheetwash. Surface irregularities
  concentrate some of flow into threads of
  current which form small channels or rills.
  Rills combine to form gullies. Rate of erosion
  is dependent on quantity of water, slope angle,
  eroded material, local base level, and other
  conditions.
• Gullies erode by down-cutting by water. May
  widen by sheetwash, rill erosion, and/or by
  mass-wasting.

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Some Georgia Erosion issues
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Stewart Co. Providence Canyons. Thin layer of
red residuum of Clayton Fm. (limestone) over
deltaic sands of Providence Fm..
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Dooly Co. Local erosion in county sand pit,
Perry Sand (shoreline sand) below clayey soil and
alluvial terrace deposits. Alluvial fan forms
with rapid deposition of sand with decrease in
water velocity.
When a soft, permeable sand is present in the
shallow subsurface, this may affect placement of
landfills, septic tanks, surface farming.
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• Problems caused by excessive erosion
• Loss of O and A layer of soil
• Loss of arable farm land when O and A layers are
  damaged
• Silting-up of streams, i.e., excessive mud
  hinders filter feeders (clams, etc.), gills of
  fish, etc.. Sand, silt clog stream channel.
• Growth of deltas into lakes diminishes the water
  capacity of the lakes, i.e., less water for
  drinking, recreation, irrigation, electric
  generation.