Quick Announcements

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Quick Announcements

• Next test is March 31st and will cover all
  material from streams to mass wasting.
• March 24th weathering lecture and movie
• March 26th mass wasting lecture and movie
• March 29th review for test
• Last day to turn everything in for unit 2 is
  March 29th.

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Chapter 6 Weathering and Soil
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Earths external processes

• Weathering the physical breakdown
  (disintegration) and chemical alteration
  (decomposition) of rock at Earths surface
• Mass wasting the transfer of rock and soil
  downslope under the influence of gravity
• Erosion the physical removal of material by
  mobile agents such as water, wind, ice, or gravity

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Weathering

• Two types
• Mechanical weathering breaking of rocks into
  smaller pieces
• Types of mechanical weathering
• 1. Frost wedging
• 2. Unloading
• 3. Thermal expansion
• 4. Biological activity

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1. Mechanical weathering- by Frost Wedging
Figure 6.3
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2. Mechanical Weathering by Unloading joints
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Unloading joints or exfoliation on half dome in
Yosemite Nat.Park
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Half dome
The unloading joints or exfoliation sheds layers
of rock from the weakest points or joints and
leaves the face of the rock smooth.
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2b
U-shaped valley carved by glaciers
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2nd type of Weathering

• Chemical weathering
• Breaks down rock components and internal
  structures of minerals
• Most important agent is water
• Responsible for transport of ions and molecules
  involved in chemical processes

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Weathering

• Major processes of chemical weathering
• Dissolution
• Aided by small amounts of acid in the water
• Oxidation
• Any reaction when electrons are lost from one
  element
• Hydrolysis
• The reaction of any substance with water
• Hydrogen ion attacks and replaces other ions

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Weathering

• Alterations caused by chemical weathering
• Decomposition of unstable minerals
• Formation or retention of stable materials
• Physical changes such as the rounding of corners
  or edges

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Combination of Chemical and joint unloading in
Arches National Park
Stages of arch development. Free-standing arches
are developed in fins, isolated by weathering
along parallel joints.  From NPS Arches National
Park, Geology Field Notes. 
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Double Arch
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Tunnel Arch formed by a pothole
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Delicate Arch
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Rates of weathering

• Factors affecting weathering
• Surface area
• Rock characteristics
• Rocks containing calcite (marble and limestone)
  readily dissolve in weakly acidic solutions
• Silicate minerals weather in the same order as
  their order of crystallization

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Weathering of common silicate minerals
Figure 6.15
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Increase in surface area
Figure 6.2
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Rates of weathering

• Climate
• Temperature and moisture characteristics
• Chemical weathering is most effective in areas of
  warm, moist climates
• Differential weathering
• Rocks do not weather uniformly due to regional
  and local factors
• Results in many unusual and spectacular rock
  formations and landforms

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Spheroidal weathering of extensively jointed rock
Figure 6.12
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Joint-controlled weathering
Figure 6.12 D
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Soil we will not be covering soil this semester,
but the slides are here for fun!

• Soil combination of mineral and organic mater,
  water, and air
• Rock and mineral fragments produced by weathering
  (regolith) that supports the growth of plants
• Humus (decayed animal and plant remains) is a
  small, but essential, component

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Composition of a good soil
Figure 6.17
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Soil

• Factors controlling soil formation
• Parent material
• Residual vs. transported soil
• Time
• Amount of time for soil formation varies for
  different soils depending on geologic and
  climatic conditions

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Soil

• Climate
• Most influential control of soil formation
• Key factors are temperature and precipitation
• Plants and animals
• Influence the soils physical and chemical
  properties
• Topography
• Steep slopes often have poorly developed soils
• Optimum terrain is a flat-to-undulating upland
  surface

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Variations in soil development
Figure 6.18
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Soil

• The soil profile
• Soil forming processes operate from the surface
  downward
• Vertical differences are called horizons zones
  or layers of soil
• O horizon organic matter
• A horizon organic and mineral matter
• High biological activity
• O and A horizons make up the topsoil

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Soil

• Soil profile
• E horizon little organic matter
• Zone of eluviation and leaching
• B horizon zone of accumulation
• C horizon partially altered parent material
• Collectively the O, A, E, and B horizons solum,
  or true soil

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Idealized soil profile
Figure 6.21
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Soil profile showing different horizons
Figure 6.20
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Soil

• Classifying soils
• Variations in soil formation over time and
  distances has led to a great variety of
  recognized soil types
• Groups have been established using common
  characteristics
• In the United States a system is used called the
  soil taxonomy

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Soil

• Classifying soils
• Soil taxonomy
• Emphasis is placed on physical and chemical
  properties
• Six hierarchical categories exist
• Descriptive names are derived from Latin and
  Greek
• 12 basic soil orders are recognized

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Soil

• Soil erosion
• Part of the natural recycling of earth materials
• Natural rates of soil erosion depend on
• Soil characteristics
• Climate
• Slope
• Type of vegetation

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Soil

• In many regions soil erosion is much greater than
  the rate of soil formation
• Sedimentation and chemical pollution
• Related to excessive soil erosion
• Occasionally soil particles are contaminated with
  pesticides

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End of Chapter 6