Transcript Weathering - Illinois State University

Weathering of Rocks
Exam 1
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
Bowen’s Reaction Series
and Weathering
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
Physical (Mechanical) –breakdown of
rocks into smaller pieces.
Chemical – decomposition of
rocks by chemical reactions.
Page 250
Daily Questions
Examine the five World Heritage Sites
in Figure 9.1 (pg. 244)
1) Which site is at the greatest risk
from physical weathering? Why?
2) Which site is at the greatest risk
from chemical weathering? Why?
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
Types of Mechanical
Weathering
Frost wedging – water
penetrates into cracks,
expands when it
freezes.
Must have:
• Adequate moisture
• Cracks in rocks
• Freeze/thaw cycles
Salt Wedging
Biological
Wedging
Unloading
Removal of overlying material allows
rock to expands
Top layers expand more than deeper
layers
Causes sheeting or exfoliation
Unloading
Sheeting
Sheeting
Chemical Weathering
• Chemical alteration of minerals.
•
Results in new minerals and ions in
solution.
• Water and acid are essential.
Dissolution
Minerals dissolve in
water
For example: salts
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
Oxidation
1. Iron silicate
dissolves
2. Iron oxidizes
3. Oxidized iron
combines with
water
4. Leaves iron oxide
Results of
Weathering
1. Water moves
through joints
a. Frost wedging
b. Chemical dissolution
– get clays
2. Joints enlarge
3. Corners are
attacked more
aggressively – get
rounding of the
material
4. Fines (clays and
silts) wash out of
rock
Weathering Products
Regolith – a loose layer of broken rock
and mineral fragments.
Sediments
Dissolved Ions
Relative susceptibility
To weathering
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