Topic IV: Weathering

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Topic IV Weathering Erosion

• The Destructive Geologic Forces

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When You Have Finished This Unit, You should be
able to answer the following questions...

• How is the Earths crust affected by its
  environment ?
• What are the products of weathering ?
• How are the products of weathering transported ?
• How are eroded materials deposited ?
• How is the Earths surface shaped by weathering,
  erosion, and deposition ?

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Definitions..

• Weathering - the slow continuous process by which
  nature breaks down rock material into smaller
  particles. The two major types of weathering are
  mechanical chemical

• Erosion - the breakdown and transport of
  weathered material by the agents of erosion -
  running water, wave and current action, glaciers,
  wind, and gravity.

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Mechanical Weathering

• The breakdown of rock material into smaller
  particles of the same material.
• Example - Granite bedrock breaks down into
  smaller particles of granite

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Agents of Mechanical (aka Physical) weathering
include...

• Ice wedging
• Frost action
• Unloading
• Plant Action
• Animal Action

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Ice Wedging

• As water seeps into the pre-existing cracks in
  rocks, it may freeze.
• As the water freezes, it expands.
• The expanding freezing water wedges the crack in
  the rock further open.

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Frost Action

• As rock material is heated, it expands.
• The exterior of the rock may be heated while the
  interior is still cold and contracted from the
  cold night temps.
• This pulls the outer layer of rock away in a
  process called exfoliation.

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Unloading

• When rock which has been buried by other
  sediment, rock, or ice is exposed, it may expand
  under the released pressure.
• This expansion causes the outer layers of rock to
  exfoliate

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Animal Action

• As animals burrow into rock and soil, they break
  the material down into smaller particles of the
  same material.
• Examples include worms, ground hogs, chipmunks,
  etc.

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Plant Action

• The roots of growing plants will grow into
  existing cracks of rock and enlarge the openings.
• Smaller plants (lichens, e.g.) dig their tiny
  roots into small cracks with the same result.

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Chemical Weathering...

• involves the breakdown of rock material into new
  material with a new chemical make-up. In other
  words, the rock has turned into new rock
  material. (decomposition)
• Agents of chemical weathering include oxidation,
  carbonation, hydration, and dissolution.

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Oxidation

• Oxidation is the chemical union of oxygen with a
  molecule within the rock. Rust is a common
  form of oxidation and is seen on many iron rich
  rocks around the ONeill campus.

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Carbonation

• As water falls through the atmosphere, a mild
  acid forms which can dissolve away limestone to
  form caves.

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Hydration

• The chemical union of water molecules with
  mineral molecules is known as hydration.
• Iron and magnesium minerals will alter to clay
  minerals due to hydration. Feldspar also
  hydrates to form clay minerals.

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Dissolution...

• Certain minerals will actually dissolve when
  exposed to water or mild acids. The sample shown
  here is halite, a highly soluble mineral more
  commonly known as rock salt

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In order for erosion to occur...

• Material must first be broken down by weathering
  so that it can be transported by the agents of
  erosion.
• Remember that chemical weathering can only occur
  on exposed surfaces.