Weathering: 
  • The process of rocks being broken down either by chemical or physical weathering
Chemical Weathering
  • Occurs when minerals dissolve or are altered in a rock
  • Factors which influence chemical weathering are:
    1. Composition - different minerals are weathered at different rates
    2. Different rock structures are more susceptible to breaking and cracking than are other rock structures, allowing the rock to be weathered easier
    3. Solubility - Different minerals are more/less soluble when exposed to water
    4. Climate - The temperature and amount of rainfall affect the rate at which rock is weathered (Typically, rocks weather faster in wet & warm environments!)
    5. Exposure Time - The longer a rock is exposed to elements which will weather it, the more that rock will weather
    6. Soil Cover - Soil increases the rate of weathering because it holds in moisture, heat and bacteria, causing an environment which is prime for erosion
    7. Surface Area - The greater the surface area of a rock, the more it is exposed to weathering
  • Acids increase the rate of weathering
    • A naturally occurring acid is carbonic acid
      • Carbon Dioxide + Water = Carbonic Acid
          • (CO2 + H2O = H2CO3)
        • H2CO3 is again broken down to form H2CO3- + H+
          • The H+ ion is is the source for acidity in water
            • H+ attaches, breaks & weakens bonds between surface atoms and the rest of the crystal rock structure
    • Other naturally occurring acids can come from
      1. Bacteria in the soil
      2. Decay of organic matter in the soil
      3. Reaction of rainwater with sulfur dioxide, nitrogen, and carbon dioxide (mentioned above)
Physical Weathering
  • The physical fragmenting of solid rock in which the chemical composition of the rock is not altered
  • Factors which influence physical weathering are:
    1. Climate - The more the variation in temperature, the more expanding and contracting the rock does, therefore an increased rate of physical weathering
    2. Zones of weakness - areas in a rock which tend to break or crack due to stresses placed on the rock
    3. Frost Wedging - In areas where frost is a factor, the expanding of frozen water in cracks in a rock increases the rate of physical weathering
      • Since ice has a more definite shape and volume, it is not as easily compressed as water is, therefore when water freezes and forms ice, it takes up more room than water and expands the rock, increasing the rate of physical weathering
    4. Crystallizing Minerals - Minerals which are dissolved in water can get trapped in the rock's pores and re-crystalize, causing the rock to expand, which in turn increases the rate of physical weathering
    5. Organisms - Can create crevices or widen cracks which are then open to physical and chemical weathering
    6. Glaciers - When glaciers cut through ice, it carries along with it rock fragments which scrap the underlying rock and cause physical weathering
    7. Root Wedging - Roots which grow into rock can create enough pressure to break the rock apart
Erosion:
  • The removal of broken rock and soil by natural processes such as:
    1. Wind
      • Common in arid climates (little vegetation)
      • Carries smaller particles (< 2mm) such as sand and silt, but leaves behind larger particles such as gravel (> 2mm)
        • Causes sorting to occur
      • This process of removing large amounts of sediment is called DEFLATION
    2. Water
      • Occurs after saturation of the ground
        • The excess water carries loose material with it
        • The size of particle that is carried away depends on the water current
      • Can carve out gullies and meanders in a landscape over time or during a flood, depending on the current
    3. Glaciers
      • Extremely slow movement
      • Picks up and carries any sediment in its path
        • Exposes the Earth down to the bedrock
          • The bedrock is physically weathered when rock imbedded in the ice scraps along the bedrock and breaks it apart
            • That broken rock then is moved by the glacier
      • When the glacier melts, all the sediment that was moved by the glacier is deposited
        • The deposited sediment is then eroded from there by the other three processes mentioned
    4. Waves
      • Cause erosion along beaches and cliffs on the coast
      • More severe in storms because the current is stronger
      • Cuts away rock on the coastline
How do Humans Impact Erosion?
    1. Kill vegetation that would otherwise hold particles in place so they would not be eroded (such as overgrazing)
    2. Cut down trees, shrubs, grass, etc. that serve as wind breaks which slow down erosion
    3. Move land, which in turn exposes rock to be eroded that otherwise would have been protected if humans had not taken off the protective layer of sediment


    Coastal Erosion:

    • Accelerated by hurricanes
    • Slowly erodes shorelines
      • Weathering rate is increased on shorelines due to more elements such as:
        • Wind
        • Wave
        • Water
        • Crystallization (salt water to salt crystals - expands causing physical weathering)
        • Heat & humidity
      • Combined, this creates more material to be eroded
    • With increased erosion, there is an increase in damage and land movement
Why are Erosion and Weathering Considered to be Natural Hazards?
  • Although is is a very slow process, it can reduce or eliminate soil and nutrients necessary to sustain life
    • Significance:
      • Reduced plant life
      • Reduced minerals & nutrients
      • Exposes roots which can cause a plant to die
  • Process of the Earth that poses no immediate threat but can be hazardous over a period of time
Problems and Risks Associated with Erosion and Weathering:
  • Occurs over a long period of time, but its effect can be short and tragic
  • Has many different causes, so it is hard to prevent
  • Hard to warn public about its effects (such as mass movement) because there is little indication to when an area could give way
  • Every area is prone to erosion
    • No where are you safe from it
  • Hard to controlled because of the power and impact it has in land movement
    • Changes the route of rivers, streams, etc.
  • Removes soil necessary to sustain plant life & exposes roots
  • Causes rock to break & crack which causes unstable ground
  • Erosion can fill streams and lakes with eroded materials, causing water levels to rise
What You Should Know & Tips for Safety:
  • Do no build on top of a hill on unconsolidated material
  • If you erect a retaining wall to override the effects of erosion, make sure it has been built according to code
  • If you're looking to live by the ocean, do no build on the oceanfront on the edge of a hill due to high amounts of erosion that could cause the hill to erode or slide
  • Use common sense when it comes to being safe in eroded/weathered areas
Where are Erosion & Weathering Problematic?
  • There are no areas that are safe from weathering and erosion
    • There is an acceleration of weathering on coastlines and in hot, wet areas where the rate is increased
    • Weathering and erosion are more prevalent in areas where there are instances of flowing water, wind, ice, or wave action
  • Potential Erosion in the Western United States
External Links: References:
  • "Erosion," Microsoft® Encarta® Online Encyclopedia 2000.  http://encarta.msn.com© 1997-2000 Microsoft Corporation.  All rights reserved.
  • Press, Frank & Siever, Raymond (2000).  Understanding Earth.  New York:  W. H. Freeman and Company.