Rates of Weathering

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Rates of Weathering
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Key Points to Remember

• The processes of weathering generally work very
  slowly.
• Ex. carbonation dissolves limestone at a rate of
  1/20th of a cm every 100 years!
• (So, it would take 30 million yrs to dissolve
  150 m. thick layer)
• Rocks do not weather at the same rate!

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Factors that affect rate of weathering

• Rock composition
• Amount of exposure (surface area)
• Climate
• Topography

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1. Rock Composition

• For igneous and metamorphic rocks- the harder the
  minerals (on Mohs Scale), the slower the rate of
  weathering.
• Olivine Acid
  Clay

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Feldspar Acid ? Clay
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Quartz anything!! ? Quartz
(Quartz is 8 on Mohs Scale)
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Rock Composition- Sedimentary

• Rocks containing calcite (limestones) are rapidly
  weathered by carbonation.
• For other sed. rocks, the rate of weathering
  depends on the material cementing the fragments
  together!

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Honeycomb weathering often occurs in sandstones
along the coast of Puget Sound, Washington. This
weathering occurs from the evaporation of water
in brine-soaked rocks at high tide level. Sodium
left within the sandstone pores gradually
accumulates, displacing clasts.
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2. Amount of Exposure

• The MORE SURFACE AREA EXPOSED the FASTER the
  weathering!
• Fractures and joints (caused by mechanical
  weathering) increase the surface area of a rock
  and allow chemical weathering to take place more
  rapidly.

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Surface Area and Weathering
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• Joints in a rock are a pathway for water they
  can enhance mechanical and chemical weathering!

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3. Climate

• Freezing and thawing have the greatest effects
• In hot, dry climates, weathering takes place the
  most slowly.
• In very cold climates (no thaw)- weathering also
  slow
• In warm, humid climates, chemical weathering is
  fairly rapid.

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Alaska
Seattle
Amazon
Cairo
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Ex. Cleopatras Needle (granite)

• Egypt (30 centuries) NYC (1
  century)

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4. Topography

• Topography- elevation and slope of the land
• Elevation influences climate (ex. lots of
  freeze-thaw at higher elevations)

• On a slope, weathered particles are removed
  by gravity or water exposing new surfaces.

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Topography, cont.
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Advanced weathering in a mountainous area.
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Review

• Factors that affect weathering
• Rock composition
• -igneous/met., depends on minerals
• -sed., depends on whats holding it together.
• -calcite rapid weathering by acids

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• 2. Amount of exposure
• - more surface area faster weathering
• - mechanical weathering increases SA
• 3. Climate
• - water is bad for rocks (humid or freeze/thaw)
  and increases rate of weathering
• - very cold or very hot and dry areas have the
  slowest weathering rates.
• 4. Topography
• - mountainous areas have rapid weathering due to
  both climate and physical weathering by gravity
  and water

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Weathering as ART!