Weathering

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Weathering

• Objectives
• Describe the Rock Cycle.
• Briefly contrast weathering and erosion.
• Contrast physical, chemical and biological
  weathering.
• List and describe the types of mechanical
  weathering.
• List and describe the types of chemical
  weathering.

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The Rock Cycle

• The three major types of rocks igneous,
  sedimentary and metamorphic.
• Rocks are interrelated by a series of natural
  processes.
• Igneous rocks (e.g. granite, basalt) form from
  the cooling and crystallization of hot molten
  lava and magma. Igneous rocks undergo weathering
  and erosion to form sediments. When Sediments are
  deposited they form Sedimentary rocks (e.g.
  sandstone, coal and chalk).

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• Sedimentary rock becomes buried by additional
  sedimentary deposition, and when they are deep
  within the Earth, they are subjected to intense
  heat and pressure which causes them to become
  Metamorphic rocks (e.g. marble and slate).

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• With further burial and heating, the metamorphic
  rocks begin to melt. Partially molten metamorphic
  rocks are known as migmatite.
• As melting proceeds with increasing temperatures
  and depths of burial, eventually the rock becomes
  molten (magma), which can be erupted onto the
  Earth's surface as lava, and cools and
  crystallizes to form volcanic igneous rock.

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• Complications within the rock cycle include
• Weathering of sedimentary and metamorphic rocks
  (in addition to igneous rocks)
• Metamorphism of igneous rocks and repeated
  metamorphism of metamorphic rocks.

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Definition of weathering

• Weathering is the disintegration (break down) and
  decomposition (decay) of rocks in situ (in their
  place of origin) to form sediment.
• Weathering (unlike erosion) need not involve the
  movement (transport) of material.

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A) Physical or Mechanical weathering

• Freeze / Thaw water expands when it freezes

Talus slope, Lost River, West Virginia
Shale chips, West Virginia
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• Exfoliation or unloading -
• rock breaks off into leaves or sheets along
  joints which parallel the ground surface
• caused by expansion of rock due to uplift and
  erosion removal of pressure of deep burial.
• Thermal expansion -
• repeated daily heating and cooling of rock
• heat causes expansion cooling causes
  contraction.
• different minerals expand and contract at
  different rates causing stresses along mineral
  boundaries.

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B) Chemical weathering

• Rock reacts with water, gases and solutions
  (maybe acidic). Thus chemical processes can add
  or remove elements from minerals.
• Dissolution (or Solution, Carbonation) -
• dissolving of calcium carbonate (limestone) in
  acidic rain or ground water.
• Several common minerals dissolve in water
• (halite, calcite)
• Limestone and marble contain calcite and are
  soluble in acidic water
• Marble tombstones and carvings are particularly
  susceptible to chemical weathering by
  dissolution.

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• Photo taken in an above-ground cemetery in New
  Orleans

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• Caves and caverns typically form in limestone
  (karst environments)
• speleothems are cave formations
• speleothems are made of calcite
• Stalactites hang from ceiling
• Stalagmites on the ground

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• Karst topography forms on limestone terrain and
  is characterized by
• caves/caverns, sinkholes (dolines), disappearing
  streams, springs.

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Sink-hole (doline) formation
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• Oxidation
• Oxygen combines with iron-bearing silicate
  minerals causing "rusting" (olivine, pyroxene,
  amphibole,biotite).
• Iron oxides are produced (limonite, hematite,
  goethite)
• Iron oxides are red, orange, or brown in color.
• Mafic rocks such as basalt (which may contain
  olivine, pyroxene, or amphibole) weather by
  oxidation to an orange color.
• "Georgia Red Clay" derives its color from the
  oxidation of iron bearing minerals.

Weathering Rind, Wilhite Formation, eastern
Tennessee
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• Hydrolysis
• Silicate minerals weather by hydrolysis to form
  CLAY.
• Feldspar alters to clay (kaolinite) plus
  dissolved materials (ions)
• Feldspars are stable at high temperatures and
  pressures (but not at the temperatures and
  pressures of the Earth's surface)
• Clays are stable under conditions at the Earth's
  surface
• Feldspars and clays are similar in composition.
• Feldspar readily alters to clay when in contact
  with acid and water.

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• Iron-bearing silicate minerals weather to form
  clays by hydrolysis (in addition to iron oxides)
• Spheroidal weathering is caused by chemical
  weathering of jointed rocks. The jointed rocks
  weather to form roughly spherical shapes.

Spheroidal weathering in jointed basalt, Culpeper
Basin, Virginia
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C) Biological weathering

• Organisms can assist in breaking down rock into
  sediment or soil.
• Roots of trees and other plants
• Lichens, fungi, and other micro-organisms
• Animals (including humans)

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Roots of trees and plants
Lichens