weathering of rocks. why we see weathering 1.most minerals are not stable at the earth’s surface...
TRANSCRIPT
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Weathering of Rocks
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Why we see weathering
1. Most minerals are not stable at the Earth’s surface
2. The Earth wants to be flat – lowest energy state
• Topography provides energy
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Igneous Sedimentary
Metamorphic
Melting
Erosion + Deposition
Melting
Ero
sion
+ D
epos
itionB
urial + Heating
Buri
al +
Hea
ting
Weathering in the Rock Cycle
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Three Dynamic Processes of Breaking and Removing rock
1. Weathering - the disintegration
and decomposition of rock at or
near the surface
2. Erosion
3. Mass wasting
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Types of Weathering
Mechanical – physical breakdown of rocks.
Chemical – decomposition of rocks by chemical reactions.
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Mechanical Weathering
Mechanical weathering breaks down rocks/minerals
Smaller particles increases the amount of exposed surface area
Does not change the chemical composition of the minerals\
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Mechanical Weathering - making smaller pieces
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Mechanical Weathering
1. Frost Wedging
2. Salt Wedging
3. Biological Wedging
4. Unloading
5. Thermal Expansion
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Types of Mechanical Weathering
Frost wedging – water penetrates into cracks, expands when it freezes.
Must have:• Adequate moisture• Cracks in rocks• Freeze/thaw cycles
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Types of Mechanical Weathering
Salt wedging – growth of minerals in cracks
• Desert environments• Water evaporates, ions in solution
combine to form minerals
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Salt Wedging
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Types of Mechanical Weathering
Biological wedging – plant roots penetrate into cracks causing cracks to widen.
Must have:• Climate hospitable for plants.• Adequate moisture and temperature.
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Biological Wedging
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Removal of overlying material allows rock to expands
Top layers expand more than deeper layers
Causes sheeting or exfoliation
Exfoliation Domes
Unloading
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Sheeting Sheeting
Unloading
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Thermal Expansion
1. repeated daily heating and cooling of rock; ex. Desert environments
2. heat causes expansion; cooling causes contraction.
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Chemical Weathering
• Chemical alteration of minerals.
• Results in new minerals and ions in solution.
• Water and acid are essential.
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Types of Chemical Weathering
1. Hydrolysis - any reaction in which water participates.
a) Ion exchange – H+ replaces other cations.
b) Dissolution - mineral completely dissolves,
leaving only ions in solution.
c) Oxidation - reaction in which elements gain
or lose electrons (example: rust).
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Carbon Dioxide + Rain
Becomes Acid
Dissolves Minerals (i.e. carbonates such as calcite)
Carries away---Ions
Acid Hydrolysis - Dissolution
H2O + CO2 H2CO3
CaCO3 + H2CO3 Ca2+ + 2HCO3-
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Carbon Dioxide + Rain
Becomes Acid
Dissolves Minerals (i.e. silicates)
Leaves ----Clay
Carries away---Ions
Silica
Acid Hydrolysis & Secondary Minerals
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Dissolution
Minerals dissolve in water
For example: salts
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Oxidation1. Iron silicate
dissolves
2. Iron oxidizes
3. Oxidized iron combines with water
4. Leaves iron oxide
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Weathering Products
Regolith – a loose layer of broken rock and mineral fragments.
Sediments
Dissolved Ions
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Mineral Residual Products Material in Solution
Quartz quartz grains silica
Feldspar clay minerals silica, K+, Na+, Ca2+
Amphibole (hornblende)clay minerals, limonite,
hematite silica, Mg2+, Ca2+
Olivine limonite, hematite silica, Mg2+
Products of Weathering
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Factors influencing Weathering Rates
1. Rock structures – chemical/mineral composition, physical features
2. Topography
3. Climate
4. Vegetation5. Time
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Climate and Weathering
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Benefits of Weathering
• Creates soil
• Produces clay, sand, and gravel
• Produces minerals
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Soils
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Terminology
Regolith – blanket of loose, weathered rock debris covering unweathered bedrock.
Soil – uppermost part of regolith.
Rock particles, new minerals, organics
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How soil differs from regolith:
1. More chemical weathering in soil.
2. Soil has structure (layers called
soil horizons).
3. Soil retains nutrients and
moisture, essential for abundant
plant growth.
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More Terminology
Soil profile – sequence of soil horizons.
Soil horizon – layer within a soil profile that has distinct characteristics.
Composition, color, texture
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Soil Horizons
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A horizon
• Zone of leaching (lots of chemical
weathering)
• Organic rich, often dark in color
• Decaying organic matter releases
nutrients
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B horizon
• Zone of accumulation – minerals (clay and iron oxide) are washed down from above.
• High clay content, reddish color from iron.
• Able to retain moisture because of clay content
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C horizon
Parent material – grading from weathered to unweathered.
Parent material can be:
1. Bedrock
2. Stream sediments
3. Volcanic ash
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Controls of Soil Formation
1. Parent Material
2. Time
3. Climate
4. Plants and Animals
5. Slope