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Ch 14
Weathering
Two Types:
1. Physical or Mechanical
2. Chemical
Mechanical Weathering
• Rock is broken down into smaller
pieces by physical means.
• Agents: wind, water, ice, gravity, plants
& animals.
EXFOLIATION
• Rocks break
into curved
sheets that peal
away
Ice Wedging
• Water gets into cracks
and freezes
• Freeze/Thaw cycle
splits rock
ABRASION
Wearing away of rocks by other
rock particles
WIND
WATER
ICE
ROOT WEDGING
Animals - Burrowing
Chemical Weathering
• Rock is broken down by chemical interaction
with the environment.
OXIDATION (rusting)
• Elements combine
with oxygen to form
rust
• Iron bearing rocks
HYDROLYSIS: Minerals dissolve in water.
HYDROLYSIS: Minerals
decompose in water
as hydrogen
replaces cations in
feldspar (turns into clay).
LEACHING: Water carries
dissolved minerals away,
forms beds of aluminum
(bauxite)
Carbon Dioxide and water
react to form carbonic
acid...Carbonic acid “eats”
limestone.
CARBONATION: Limestone caves
Acid Precipitation
• Acids formed from the burning of fossil
fuels combining with atmospheric
water; bad for plants & structures.
Factors that affect Rates of
Weathering
DIFFERENTIAL WEATHERING
Softer, less-resistant rock wears
away, leaving harder resistant
rock behind.
Bryce Canyon “Hoodoos”
Rock composition
• Sedimentary rocks containing calcites
weather rapidly (limestone).
• Quartz containing rocks are strong and
weather slowly
Surface area
The more surface area per volume,
the faster the rock’s weathering rate.
Fractures and Joints
• Increase surface area of rock
• Allow weathering to happen faster
Climates
• Hot/Cold climates with freeze/thaw cycles
have fastest weathering rates.
• Hot/Dry climates weather the least.
Topography
• Elevation or slope of the land.
• High Elevations – cold temp, ice wedging
• Steep Slopes – gravity pulls rock down
Human Activities
• Mining
• Construction
Plants and Animals
• Roots, Burrowing, Waste (Bat Guano)