Assignments:

1. Chapters 5, 14 and 15 in Smith and Punn
2. Earth Revealed: movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view#15 Weathering and Soils and #16 Mass Wasting.

Terms: disintegration, decompostion, detrital, clastic, unloading joints, sheeting, exfoliation, solution, hydroloysis, oxidation, regolith, soil, mass-wasting, landslide, heave, creep, slump, liquefaction, sinkhole, deadman, gabbion

Weathering

Significance:

• Releases nutrients from rock
• Produces soils which supports life
• Reason why all natural and man made objects lack permanence
• Important component of the rock cycle
• Creates economic resources (e.g. bauxite, iron ore, rock salt, etc.).

Processes:

A. Mechanical (Physical) Weathering: Also refered to as disintegration--produces detrital clastic sediment by fracturing rock

1. Growth of foreign crystals

• ice: frost wedging
• salts

2. Expansion

• Unloading (unloading joints, sheeting)
• Hydration (spheroidal weathering): volume expansion related to chemical weathering
• Temperature (spalling)

3. Brittle tectonic deformation

• faulting and jointing

4. Organic activity

• plant wedging
• burrowing

5. Geologic agents (abrasion)

• glaciers
• water: waves, rivers
• wind

6. Man

• blasting, mining, etc

Importance: Mechanical weathering prepares the rock for chemical weather by increasing the exposed surface area.

B. Chemical Weathering: Also refered to as decomposition--produces new minerals and dissolved solids

1. Simple solution: solution by water only

product=dissolved ions (e.g. halite: product=dissolved Cl^- and Na⁺)

2. Acid reactions (acids release H+ that replace other cations in minerals)

3. Hydrolysis: weathering accomplished by acidic water

• Carbonation: reaction with carbonic acid (H₂CO₃ ions)
• Other acids: sulfuric acid, nitric acid, organic acids
• acid rain: increased acidity of water caused primarily by the burning of fossil fuels

Oxidation: Breakdown by reaction with oxygen (product= oxide minerals + dissolved ions)

Byproducts of weathering are:

• rock detritus
• new residual minerals (e.g. clays, such as kaolinite (Al), illite (K), montmorillinite (Na/Ca) and oxides, such as hematite, geothite, limonite)
• dissolved ions, which later becomes chemically precipitated as rocks or cement (halite, gypsum, anhydrite, calcite, silica).
• Significance: These byproducts are the ingredients needed in the formation of sedimentary rocks.

Factors that control depth of weathering

• Rates of weathering
  • Parent Material: No all minerals have the same stability (c.f. Bowen's reaction series)
  • Climate
    • Temperature: Controls rates of reaction
    • Precipitation: Water is the universal solvent
  • Organic activity (controlled by climate): increases acid production
  • Degree of fragmentation: Mechanical weathering increases exposed surface area
    • Sediments (glacial till, alluvium, talus, etc.) weather faster than rock
• Time: Length of time surface materials are exposed to weathering
  • Govern by by erosion rate

Erosion

The removal of weathered material one or more various geologic agents.

Agents of erosion

• Water: overland flow, streams, waves
• Ice: Glaciers
• Wind
• Gravity: Mass-wasting
• Man: Mining and development

Regolith/overburden and soil

Transported and Residual (insitu) regolith

• Transported regolith is that deposited by some geologic agent
• insitu regolith is that derived from the underlying bedrock through weathering

Soil--the upper portion of the regolith that supports plant life

• Soil horizons
  • O horizon: organic zone
  • A horizon: mixed zone of organic and mineral material
  • E horizon: zone of leaching
  • B horizon: zone of accumulation
  • C horizon: parent material (e.g. bedrock or transported regolith)
• Not all soils may develop all the horizons listed above. Why?
• Factors controlling soil formation are essentially the same as those controlling depth of weathering:
  • Parent material
  • Slope: controls local erosion rates
  • Time
  • Climate: controls precipitation, temperature, and vegetation
• Some soil types: alkali soils (arid), pedicals (semiarid), pedalfers (temperate), laterites (tropical)
  

Concepts: Weathering and Soils

1. What is the significance of weathering?
2. How does weathering affect man?
3. How does man affect weathering?
4. What class of rocks would not exist if it were not for weathering. Explain.
5. How and why do soils differ in tropical, arid, semiarid, and temperate (e.g. New England) regions
6. what resources are mined form tropical soils?
7. Why are insitu soils rare in New England?

Mass Wasting

Mass wasting is the downslope movement of material under the influence of gravity alone. Sediment that is being mass wasted is not being transported by a geologic agent, such as a glacier or stream. Mass wasting is one of the most important processes affecting slopes.

Assigment: Read The liquid Earth (1999, by Brenda Bell)

1. Mechanism of movement
   • Heave: rachet-like movement resulting from expansion and contraction
   • Flow: movent takes place throughout the moving body
   • Slide: movement occurs along planar surfaces
   • Fall: Material falls through the air
2. Speed
   • fast or slow rates of movement
3. Composition (rock, soil, ice, or mixed debris) and water content

Types of mass wasting phenomena

A. Slow movement

1. soil or rock creep (heave)
2. solifluction (shallow flow of a saturated layer over an impermeable layer)

B. Moderate to fast moving mass-wasting events (Landslides)

1. soil, rock or debris flow
2. Slump
   1. Upper portion is a slide that moves along a curved rupture
   2. Lower portion or toe is an earthflow
3. soil, rock or debris slide
4. Rock fall
5. Avalanche: very fast flow, generally initiated by a fall
6. Mudflow
   Lahar: flow of saturated volcanic ash (volcanic mudflows)

Factors that influence the stability of slopes

A. Setting the stage for failure:

1. Topography: high relief
2. Geology: Material and structure conducive to failure -- these can be further weakened by weathering
3. Proximity to Geologic Agent: waves (coast), stream, glacier
4. Removal of Vegetation: fire, drought (Post wildfire landslide hazards/Preventing slope failure (Anaheim))

B. Triggers: Factors that initiate movement

1. Water: Rainstorm, snowmelt, irrigation, etc.
2. Storm related erosion: undercutting by waves, river, etc  Removal of lateral support
3. Ground shaking: earthquake, eruption, etc.

C. How does man contributes to slope failure

1. addition of water: irrigation, waste disposal,
2. ground shaking: blasting
3. Undercutting and over steepening: road cuts, terrancing
4. Addition of weight: over building
5. Creating unstable slopes from fill or mine tailings (1966 landslide in Aberfan, South Wales)

Surface subsidence

Causes

• Underground mining
• Solution of underlying bedrock (eg. karst terrains: read pp. 232-328)
  • example: Winter Park, FLA (USGS)
• Overpumping of aquifers or oil well fields
  • Causes of land subsidence from ground-water pumbing (USGS)

 Internet Sites to Explore

USGS Landslide Hazard Program / Landslides in the News Current Events

Mass movement from Earth Science Australia

Megatsunami and Landslides

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