804d ago

Weathering, Soil, and Mass Wasting

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Weathering, Soil, and Mass Wasting

Earth’s External Processes

  • Weathering - the disintegration and decomposition of material at or near the surface
  • Mass wasting - the transfer of rock material downslope under the influence of gravity
  • Erosion - the incorporation and transportation of material by a mobile agent, usually water, wind, or ice

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Two Kinds of Weathering

  • Mechanical Weathering - breaking of rocks into smaller pieces
  • Chemical Weathering - chemical transformation of rock into one or more new compounds

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Four Important Processes of Mechanical Weathering

  • Frost wedging - expansion of freezing water

  • Salt Crystal Growth

  • Sheeting - generates onion-like layers and exfoliation domes

  • Biological activity

    • Plants can break rocks

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Chemical Weathering

  • Alters the internal structures of minerals by removing or adding elements

  • Most important agent is water

    • Oxygen dissolved in water oxidizes materials
    • Carbon dioxide (CO2) dissolved in water forms carbonic acid and alters the material
  • Weathering of granite

    • Weathering of potassium feldspar produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution
    • Quartz remains substantially unaltered
    • Weathering of silicate minerals produces insoluble iron oxides and clay minerals
  • Spheroidal Weathering

    • Generates rounded rocks
    • Weathering works inward from exposed surfaces

Rates of Weathering

  • Advanced mechanical weathering aids chemical weathering by increasing the surface area

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Important Factors on Rates of Weathering

  • Rock Characteristics
    • Mineral composition and solubility
    • Physical features such as joints
  • Climate
    • Temperature and moisture are the most crucial factors
    • Chemical weathering is most effective in areas of warm temperatures and abundant moisture
  • Differential weathering
    • Caused by variations in composition
    • Creates unusual and spectacular rock formations and landforms

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Soil

  • An interface in the Earth system

  • Soil is a combination of mineral matter, water, and air

    • Regolith
    • Rock and mineral fragments
    • Supports the growth of plants

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Soil Components

  • Soil texture and structure
    • Texture refers to the proportions of different particle sizes
    • Sand (large size)
    • Silt
    • Clay (small size)
    • Loam (a mixture of all three sizes) is best suited for plant life
  • Structure
    • Soil particles clump together to give a soil its structure
    • Four basic soil structures
    • Platy
    • Prismatic
    • Blocky
    • Spheroidal

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Controls of Soil Formation

  • Parent material

    • Residual soil – parent material is the bedrock
    • Transported soil – parent material has been carried from elsewhere and deposited
  • Time

    • Important in all geologic processes
    • Amount of time to evolve varies for different soils
  • Climate

  • Plants and animals

    • Organisms influence the soil’s physical and chemical properties
    • Furnish organic matter to soil
  • Slope

    • Angle
    • Steep slopes often have poorly developed soils
    • Optimum is a flat-to-undulating upland surface
    • Orientation (direction the slope is facing) influences
    • Soil temperature
    • Moisture

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Soil Profile

  • Soil forming processes operate from the surface downward
  • Horizons – zones or layers of soil
  • Horizons in temperate regions
    • O – organic matter
    • A – organic and mineral matter
    • E – little organic matter
    • Eluviation
    • Leaching
    • B – zone of accumulation
    • C – partially altered parent material
  • Topsoil
    • Zones O and A together
  • Solum
    • “True soil”
    • O, A, E, B together

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Soil

  • Classifying soils - Soil Taxonomy

    • Emphasizes physical and chemical properties of the soil profile
    • Names of the soil units are combinations of syllables of Latin and Greek origin

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Soil Erosion

  • Recycling of Earth materials
    • Natural rates of erosion depend on
    • Soil characteristics
    • Climate
    • Slope
    • Type of vegetation
  • Processes that erode soil
    • Water erosion
    • Raindrops dislodge
    • soil particles
    • Flowing water carries soil away
    • 2/3 of all U.S. soil erosion
    • Wind Erosion
    • Occurs with dry soil conditions
    • Soil erosion and sedimentation can cause
    • Reservoirs to fill with sediment
    • Contamination by pesticides and fertilizers
  • Controlling soil erosion
    • Leave steep slopes undisturbed
    • Terrace crop planting
    • Grassed waterways
    • Tree windbreak barriers

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Mass Wasting

  • The downslope movement of rock, regolith, and soil under the direct influence of gravity
  • Gravity is the controlling force
  • Saturation of the material with water
    • Destroys particle cohesion
    • Water adds weight

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Mass Wasting Triggers

  • Oversteepening of slopes
    • Unconsolidated granular particles assume a stable slope called the angle of repose
    • Stable slope angle is different for various materials
  • Removal of anchoring vegetation
  • Ground vibrations from earthquakes

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Classifying Mass Wasting

  • Each type is defined by

    • The material involved – debris, mud, earth, or rock
    • The movement of the material
    • Fall (free-fall of pieces)
    • Slide (material moves along a well-defined surface)
    • Flow (material moves as a viscous fluid)
    • Each type is defined by
    • The rate of the movement
      • Fast
      • Slow
  • Slump

    • Rapid movement along a curved surface
    • Occur along oversteepened slopes
  • Rockslide

    • Rapid
    • Blocks of bedrock move down a slope
  • Debris flow (mudflow)

    • Rapid flow of debris with water
    • Often confined to channels
    • Debris flows composed mostly of volcanic materials are called lahars
  • Earthflow

    • Rapid
    • Typically occur on hillsides in humid regions
    • Water saturates the soil
    • Liquefaction – a special type of earthflow sometimes associated with earthquakes
  • Creep

    • Slow movement of soil and regolith downhill
    • Causes fences and utility poles to tilt
  • Solifluction

    • Slow movement in areas underlain by permafrost
    • Upper (active) soil layer becomes saturated and slowly flows over a frozen surface below