Weathering

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Weathering
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• Weathering is the physical and chemical breaking
  down of rock material into smaller fragments due
  to exposure to processes that occur at Earths
  surface
• Weathering occurs when rocks in the lithosphere
  are UPLIFTED and is exposed to the atmosphere,
  hydrosphere, and biosphere

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Weathering

• The most important agent involved in the
  weathering of material is WATER

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Two Types OF Weathering

• 1) Physical weathering is breaking rock by force
  into smaller pieces of the SAME material
• ex hitting, scratching, cracking

• 2) Chemical weathering is the chemical reaction
  of water and water vapor breaking down rock by
  changing its chemical composition
• The minerals that make up the rocks are changed
  into another substance

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4 Types of Physical Weathering

• 1) Frost action
• (ice wedging)- water seeps into cracks, freezes,
  expands and wedges the rock apart!
• Water and ice are more powerful than rock over
  time!
• Water seeps into small cracks in rocks.
• When the water freezes it expands creating great
  pressure.
• The crack widens and allows water to seep deeper
  into the rock.
• Alternating Freezing and Thawing

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Examples of Frost Wedging Potholes
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• 2) Plant action
• tree roots grow into small fissures or wedges
  and expand rock
• Tiny root hairs seek out small cracks and pits in
  rock.
• Once the root hairs find a place they grow and
  expand.
• The expansion causes great pressure and cracks
  the rock.

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• 3) Exfoliation
• the peeling of rock due to release of pressure
  experienced when rock was formed
• Rocks formed deep in the Earth are made under
  high pressure.
• When the pressure is released the rocks expand
  crack.
• May also be caused by alternate heating and
  cooling of rocks by weather conditions.
• This is basically the peeling away of rock

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Water (especially in streams

• 4) Abrasion-
• collision of rocks with one another
• Scraping, grinding, and wearing away other rocks
  during erosion (movement)

WIND
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Types of Chemical Weathering

• Oxidation
• Hydrolysis (water)
• Carbonation (acid rain)

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• Oxidation
• rocks with metallic elements combine with oxygen
  (in air and water) and rust away (rust stains the
  rocks REDDISH BROWN)
• ex. rusting

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• Water (hydrolysis)
• Minerals are dissolved in water when they react
  with it
• ex. Feldspar reacts with water to form kaolinite
• Much of the time these minerals will end up as
  clay

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• Acid-Carbonation
• carbon dioxide from air reacts with water to
  create carbonic acid which breaks down rocks
  (especially limestone, marble, etc.)
• Why? What mineral does these rocks have in them?

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• Acid Rain Sulfur Dioxide in the atmosphere
  dissolves in rain water forming a strong acid
  (sulfuric acid).

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NOTE chemical weathering typically results in a
rounded appearance of the landscapejust like
here in the Catskills!
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Carbonation Stalagtites and Stalagmites (karst
topography)
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Carbonation Karst Topography

• Karst Topography is the name given to a region
  where the bedrock is easily chemically weathered
• This leads to the rock weathering away and
  forming sinkholes, caverns, disappearing streams

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Karst Topography
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Differential Weathering

• Masses of rock do not weather uniformly due to
  regional and local factors
• Results in many unusual rock formations
• Example Bryce Canyon

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Bryce Canyon National Park
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Rates of weathering (how fast rocks break down
due to physical or chemical processes) will be
influenced by

• 1) Surface area exposed - weathering occurs on
  the surface. The more surface exposed, the faster
  the weathering will occur.

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A smashed piece has greatest surface area
exposed. The interior can now be attacked.
A full, solid block has the least surface area.
The interior is safe from exposure.
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Rates of Weathering will depend on

• 2) Rock and Mineral composition- some minerals
  are more resistant than others.
• Sedimentary rocks are generally soft and weather
  fast.
• Limestone is easily dissolved by acids.
• Igneous and metamorphic rocks are usually dense,
  hard, and more resistant to weathering.
• Any rock containing quartz (hardness of 7) will
  generally be resistant to weathering
  (conglomerate, quartzite, granite, etc.)

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Rate of Weathering will depend on

• Exposure to surface rocks exposed at surface
  weather much more quickly than buried rock.
• Rock Outcrops are places where rock layers
  actually stick out above groundwhere they are
  quickly exposed to lichens, moss, trees,
  rainfall, ice, and animals!

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Rate of weathering will depend on

• Topography higher elevations tend to be colder
  and experience more freeze-thaw cycles

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Where is the rock more resistant to weathering?
F
A
C
E
B
D
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Where is the rock least resistant to weathering?
F
A
C
E
B
D
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You might also see it like this
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You might also see it like this
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Rates of Weathering will depend on

• Climatic Conditions
• Cold and/or dry climates favor physical
  weathering.
• Warm and wet climates favor chemical weathering.
• Frost action works best in areas where the
  temperature fluctuates wildly.

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Soil - The product of weathering

• Soil-Soil is the result of weathering of rock,
  which produces smaller pieces called fragments,
  which combine with organic matter, air, and water
  to comprise soil. Soil rock fragments, humus
  (decayed plant and animal material), air and
  water
• Soil forms layers of different characteristics
  called horizons.

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Soil Horizons

• Soil separates over time into distinct layers as
  follows
• A horizon (Topsoil) the upper layer of dark
  brown soil containing humus and plenty of animal
  activity (bugs, worms, etc.)
• B horizon also called subsoil, containing many
  more minerals leached down by water infiltrating
  through the A horizon, so often stained red by
  oxidation of iron minerals
• C horizon made of weathered bedrock (parent
  material)
• Usually contains larger particles and rock
  fragments
• Bedrock the underlying rock that is still mostly
  un-weathered

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Soil Horizons