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