PAK Diagram of Mechanical Weathering (Unloading, Frost Wedging, Tree-Root Wedging, Temperature Changes)

 
Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
WEATHERING AND MASS WASTING
STUDY GUIDE

CHIEF IDEAS:

  1. Weathering is the first stage in the denudation (wearing away) of the landscape. Weathering is the break up of rock material on the spot. Mechanical weathering involves the physical disintegration of rock. Chemical weathering involves the decomposition of minerals by atmospheric gases and water.

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  3. Mechanical weathering occurs when rock breaks into smaller pieces of the original rock. Frost wedging takes place when water in a crack freezes and forces apart the rock. Temperature changes above freezing in places like deserts may cause rocks to break apart. Tree-root wedging may widen cracks in rock. In dry climates, salt crystallization helps to break rock. Unloading of deeply buried rock by erosion of the overlying layers may also cause rock to expand upward and form joints and cracks.

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  5. During chemical weathering, the internal structure of a mineral is altered by reaction with other elements. Oxygen dissolved in water may oxidize minerals that contain iron (oxidation). Carbon dioxide dissolved in water forms a weak carbonic acid that can dissolve limestone or turn the feldspar in granite into clay minerals (carbonation). Water alone also combines with some minerals to form new by-products (hydrolysis). Water may also dissolve some minerals (solution).

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  7. Weathering products such as clay and sand form a blanket or cover of rock debris called regolith. Regolith overlies the bedrock. A surface exposure of unweathered bedrock is an outcrop.

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  9. Weathering tends to produce rounded rocks. Unloading produces a sheeting structure or exfoliation domes.

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  11. Rocks weather at different rates according to mineral composition, texture, and climate. Rocks weather rapidly in hot, moist climates but slowly in dry climates.

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  13. Mass wasting is the movement of regolith downhill under the influence of gravity. Downslope movements may take the form of a slow soil creep that moves fences, posts, and house foundations. This is the most widespread and least spectacular form of mass wasting. Faster movements lubricated by water take place after heavy rain.

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  15. Mudflows have the consistency of wet concrete and can move very rapidly down steep mountain canyons such as those above Los Angeles. Sudden collapse movements may produce earth slumps, alpine debris avalanches, and landslides. Earth slumps are common in the Oakland and Berkeley hills after heavy winter rains. Earth slumps involve a combined collapse and flow movement. Alpine debris avalanches occur in snow-covered mountains. A landslide or rockslide is a rapid sliding of masses of bedrock. In high latitudes, thawed topsoil may flow over a frozen subsoil (solifluction). All these processes change the landscape over a long period of time.

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  17. Mass wasting speeds up when people change the landscape by adding weight to a slope, undercutting the base of the slope, or removing the natural vegetation cover.

STUDY OBJECTIVES

  1. What is denudation? What is the first step in the wearing away of the land? Define weathering. Distinguish between mechanical weathering and chemical weathering.
  2. Describe four different kinds of mechanical weathering.
  3. Describe three different ways in which rocks can be chemically weathered.
  4. List the products of weathering. Define regolith and outcrop.
  5. Describe exfoliation domes.
  6. Describe the factors that influence the rate of weathering. In which climate is chemical weathering most active?
  7. What is mass wasting? What is the slowest form of mass wasting?
  8. Describe four kinds of downslope movement and the characteristics of each.
  9. Describe the ways in which people can disturb slopes and speed up mass wasting.

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