WEATHERING

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WEATHERING

• Big Rocks Into Small Rocks

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2 Types Physical/Mechanical Chemical

• Physical/Mechanical Weathering The physical
  breakdown of rocks into smaller pieces
  (sediments) by the actions of weather.
• Chemical Weathering The breakdown of rock
  through a change in mineral or chemical
  composition.

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Frost Wedging

• Occurs in climates with freeze/thaw cycles.
• Water occupies about 10 more volume when it
  freezes.
• When water that has collected in cracks in rocks
  freezes, it wedges the rock apart.

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Ouch! Roads are at the mercy of frost wedging.

1. Water seeps under pavement.
2. It freezes and expands.
3. With warming the ice melts and a cavity is left.
4. An autos weight crushes the road into the
   cavity.

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Abrasion

• Water, wind and ice are capable of moving rock
  material.

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Water

• Water tumbles sand, pebbles and even boulders
  along streambeds.

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Wind

• Wind blows sand and pebbles across beaches and
  deserts.
• It can wear away boulders.

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Ice

• Glaciers, which are moving masses of ice, move
  rock over long distances.

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As water, wind and ice move sand, pebbles and
larger rocks they grind and scrape against each
other. These rock materials are worn
down. This is called abrasion.
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Biological Plants Animals

• Plants
• Trees often take root in cracks in rocks.
• With growth these roots widen the crack.

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Root Wedging

• Even small plants such as lichens and mosses
  break down rocks by rooting on rocks.
• This is called root wedging.

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Animals

• Burrowing insects and animals dig holes and bring
  rock fragments to the surface exposing rock
  materials to weathering.
• How much wood could a woodchuck chuck if a
  woodchuck could chuck wood?

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More Burrowing Animals
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Exfoliation

• The scaling off or peeling of successive layers
  from the surface of rocks.
• Reduction of pressure and expansion of surface
  allows large curved sheets to peel away.

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Chemical Weathering

• Occurs when rock is broken down by
• chemical action resulting in a change in
  composition.
• Agents of chemical weathering
• Oxygen
• Rainwater
• Carbon dioxide
• Acids produced by decaying plants and animals

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Oxidation

• Occurs when oxygen unites chemically with
  minerals.

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Hydration

• Occurs when water unites chemically with minerals
  such as feldspar and hornblende (amphibole) and
  they crumble into clay particles.

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Carbonation

• Occurs when carbon dioxide unites chemically with
  minerals.
• Rainwater contains dissolved CO2 which forms
  carbonic acid.
• This dissolves limestone bedrock forming

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Sinkholes
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Caverns
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Factors Affecting the Rate of Weathering

• Climate is the single most important factor
  affecting rock weathering.
• Climate is the average condition of the
  atmosphere over a long period of time.

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Surface Exposure

• When a rock is broken into smaller pieces, more
  surface area is exposed to weathering and it
  weathers faster.

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Mineral Composition

• Greatest importance in chemical weathering.
• Rocks composed of minerals that react with acids
  or oxygen will weather most rapidly.
• Quartz is almost unchanged by chemical
  weathering. And because it is hard (7) and does
  not have cleavage it resists physical weathering.

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Acid Rain

• The burning of fossil fuels has accelerated
  chemical weathering.
• Nitric and sulfuric acid is produced in the
  atmosphere.

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Natural Producer of Acid Rain

• Volcanoes also add gasses to the atmosphere that
  produce acid rain.