Rocks and Weathering

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Rocks and Weathering

• S E C T I O N 2 - 1

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Objective

• What causes mechanical weathering?
• What causes chemical weathering?
• What determines how fast weathering occurs?

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• Weathering is the process that breaks down rock
  and other substances at Earths surface.

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• The forces of weathering break rocks into smaller
  and smaller pieces.

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• The forces of erosion carry the pieces away.
• Erosion is the movement of rock particles by
  wind, water, ice, or gravity.

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• Weathering and erosion work together continuously
  to wear down and carry away the rocks at Earths
  surface.

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• The type of weathering in which rock is
  physically broken into smaller pieces is called
  mechanical weathering.
• These smaller pieces have the same composition
  as the rock they came from.

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• Mechanical weathering breaks rock into pieces by
  freezing and thawing, heating and cooling,
• growth of plants, actions of animals, and
  abrasion.

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• In cool climates, the most important force of
  mechanical weathering is freezing and thawing.
• Water expands when it freezes, and acts as a
  wedge.

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• This process is called ice wedging.
• Heating and cooling can cause rocks to crack.
• Plant roots can pry apart cracked rocks.

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• Rock particles carried by wind, water, and ice
  can wear away rocks in the process called
  abrasion.

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• Animal actions, such as burrowing, can break
  apart rocks in the soil.

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• Chemical weathering is the process that breaks
  down rock through chemical changes.

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• The agents of chemical weathering include water,
• oxygen, carbon dioxide, living organisms, and
  acid rain.

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• Chemical weathering produces rock particles that
  have a different mineral makeup from the original
  rock.
• Chemical and mechanical weathering often work
  together.

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• As mechanical weathering breaks rock into pieces,
  more surface area becomes exposed to chemical
  weathering.

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• Water is the most important agent of chemical
  weathering.
• Water weathers rock by dissolving it.

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• The oxygen in air is an important cause of
  chemical weathering.
• Iron combines with oxygen in the presence of
  water in a process called oxidation.
• Rock that contains iron oxidizes, or rusts.

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• Another gas found in air, carbon dioxide,
  combines with rainwater to form carbonic acid.
• Carbonic acid easily weathers marble and
  limestone.

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• Plant roots produce weak acids that slowly
  dissolve any rock around the roots.

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• Lichens also produce a weak acid that weathers
  rock.
• Acid rain causes very rapid chemical weathering.

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• The most important factors that determine the
  rate at which weathering occurs are type of rock
  and climate.

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• Some types of rock weather more rapidly than
  others.
• For example, some rock weathers easily because it
  is permeable, which means that it is full of air
  spaces that allow water to seep through it.

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• Both chemical and mechanical weathering occur
  faster in wet climates.
• Chemical weathering occurs more quickly where the
  climate is both hot and wet.

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End