Download Weathering - Laboratory for Integrated Learning and Technology

Weathering of Rocks
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
Weathering in the Rock Cycle
Erosion + Deposition
Igneous
Sedimentary
Melting
Metamorphic
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
Types of Weathering
Mechanical – physical breakdown of
rocks.
Chemical – decomposition of
rocks by chemical reactions.
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\
Mechanical Weathering - making smaller
pieces
Mechanical Weathering
1.
2.
3.
4.
5.
Frost Wedging
Salt Wedging
Biological Wedging
Unloading
Thermal Expansion
Types of Mechanical
Weathering
Frost wedging – water penetrates into
cracks, expands when it freezes.
Must have:
• Adequate moisture
• Cracks in rocks
• Freeze/thaw cycles
Frost Wedging
Types of Mechanical
Weathering
Salt wedging – growth of minerals in cracks
• Desert environments
• Water evaporates, ions in solution
combine to form minerals
Salt Wedging
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.
Biological
Wedging
Unloading
Exfoliation Domes
Removal of overlying material allows
rock to expands
Top layers expand more than deeper
layers
Causes sheeting or exfoliation
Unloading
Sheeting
Sheeting
Thermal Expansion
1. repeated daily heating and cooling of
rock; ex. Desert environments
2. heat causes expansion; cooling
causes contraction.
Chemical Weathering
• Chemical alteration of minerals.
•
Results in new minerals and ions in
solution.
• Water and acid are essential.
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).
Acid Hydrolysis - Dissolution
Carbon Dioxide + Rain
H2O + CO2 H2CO3
Becomes Acid
CaCO3 + H2CO3 Ca2+ + 2HCO3-
Dissolves Minerals (i.e.
carbonates such as calcite)
Carries away---Ions
Acid Hydrolysis &
Secondary Minerals
Carbon Dioxide + Rain
Becomes Acid
Dissolves Minerals (i.e. silicates)
Leaves ----Clay
Carries away---Ions
Silica
Dissolution
Minerals dissolve in
water
For example: salts
Oxidation
1. Iron silicate
dissolves
2. Iron oxidizes
3. Oxidized iron
combines with
water
4. Leaves iron oxide
Weathering Products
Regolith – a loose layer of broken rock
and mineral fragments.
Sediments
Dissolved Ions
Products of Weathering
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+
Factors influencing
Weathering Rates
1. Rock structures – chemical/mineral
composition, physical features
2. Topography
3. Climate
4. Vegetation
5. Time
Climate and Weathering
Benefits of Weathering
• Creates soil
• Produces clay, sand, and gravel
• Produces minerals
Soils
Terminology
Regolith – blanket of loose, weathered
rock debris covering unweathered
bedrock.
Soil – uppermost part of regolith.
Rock particles, new minerals, organics
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.
More Terminology
Soil profile – sequence of soil horizons.
Soil horizon – layer within a soil profile
that has distinct characteristics.
Composition, color, texture
Soil Horizons
A horizon
• Zone of leaching (lots of chemical
weathering)
• Organic rich, often dark in color
• Decaying organic matter releases
nutrients
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
C horizon
Parent material – grading from
weathered to unweathered.
Parent material can be:
1. Bedrock
2. Stream sediments
3. Volcanic ash
Controls of Soil Formation
1. Parent Material
2. Time
3. Climate
4. Plants and Animals
5. Slope