Ch 5 Weathering

1
Ch 5 Weathering Erosion

• Objectives
• Briefly contrast weathering and erosion.
• Contrast chemical and mechanical weathering.
• List and describe the types of mechanical
  weathering.
• List and describe the types of chemical
  weathering.
• List the products resulting from the chemical
  weathering of Igneous rocks.
• List and discuss the factors that influence the
  type and rate of rock weathering.

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The Rock Cycle
This Unit Ch 5 6
Last Unit Ch 3 4
What kinds of rock can be melted? Can be
weathered eroded? Heated Pressurized?
3
Weathering

• Breakdown of rock due to surface processes
• 2 types of weathering
• Physical (Mechanical)
• Chemical

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Physical Weathering

• Abrasion - wind water
• Frost wedging - water expands when it freezes
• 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.
• Plant Roots break rocks apart as they grow
• Salt Crystal Growth evaporation, crystals grow
  between rocks

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Type of physical weathering?
6
Type of physical weathering?
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Type of physical weathering?
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Type of physical weathering?
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Type of physical weathering?
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Type of physical weathering?
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Type of physical weathering?
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Type of physical weathering?
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Chemical Weathering

• Rock reacts with water, gases and solutions (may
  be acidic) will add or remove elements from
  minerals.
• 1. Dissolution (or solution)
• - also includes leaching
• 2. Oxidation
• 3. Hydrolysis
• 4. Biological Action
• 5. Spheroidal

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Acid Rain
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Dissolution

• 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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More Dissolution

• Caves and caverns typically form in limestone
• speleothems - cave formations made of calcite
• form a rock called travertine
• stalactites - from ceiling
• stalagmites - on ground

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More Dissolution

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

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Acid Leaching
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Oxidation

• Oxygen combines with iron-bearing silicate
  minerals causing "rusting"
• biotite
• Iron oxides are red, orange, or brown in color

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Hydrolysis affected by H20

• Feldspar alters to clay
• Feldspars stable at high temperatures and
  pressures
• Clays are stable under conditions at the Earth's
  surface
• Quartz turns to sand

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Biological Action

• Lichens, fungi, and other micro-organisms
• Chemically and physically change rock

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Spheroidal Weathering

• chemical weathering of jointed rocks.
• weather to form spherical shapes

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Factors Affecting Weathering
Surface Area Texture Temperature Humidity Pla
nt growth Topography Time

• Mineral Content
• Least stable
• Olivine
• Ca plagioclase feldspar
• Pyroxene
• Amphibole
• Biotite
• Na plagioclase feldspar
• Potassium feldspar
• Muscovite
• Quartz
• Most stable

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Factors Affecting Weathering

• Surface Area Most important
• Smaller particles, more surface area
• Examples
• Crushed ice cools faster
• Granulated sugar dissolves faster
• More exposed rocks weather more

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Factors Affecting Weathering

• Type of material both are from 1780s one is
  slate, the other is marble. Which is which? Why
  is there a difference?

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Erosion

• Movement / transportation
• Wind, water, glacier
• Mast Wasting - Movement of large amounts of
  material downhill under gravity
• Creep
• Mudflows
• Slump
• Rockfalls
• Landfalls
• Avalanches

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What happens to Granite?

• Granite contains Na Plagioclase feldspar, K
  feldspar, Quartz
• Lesser amounts of biotite, amphibole, or
  muscovite

Weathering 1. The feldspars undergo hydrolysis
to form ________ and Na and K ions The Na and K
ions removed by leaching. 2. The biotite
amphibole will undergo hydrolysis to form clay,
and oxidation to form _________. 3. The quartz
(and muscovite, if present) will remain as
residual minerals because they are very resistant
to weathering. They get smaller to make ________.
Weathered rock fragments are one of the
constituents of soil our next topic.