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CH. 4 - WEATHERING
Chapter 2
Types of Weathering Processes
1) Mechanical Weathering
- rx break down into smaller pieces
- little or no chemical alteration
Types of Weathering Processes
2) Chemical Weathering
- alters crystalline structure and/or composition of minerals
- new minerals formed or minerals dissolve
Types of Mechanical Weathering
1) Frost wedging
- most effective in climates with freeze/thaw cycles
Ex: high altitudes, mid-latitudes
Talus slope
Types of Mechanical Weathering
2) Unloading
- caused by a decrease in pressure
- effective on large bodies of intrusive igneous rocks
Unloading
Rocks expand & crack parallel to surface (“sheeting”)
- produce rounded mountains called exfoliation domes
Ex: Yosemite National Park, CA
Stone Mountain, GA
Types of Mechanical Weathering
3) Abrasion
- grinding by friction
- usually done by water: rivers, waves, glaciers
- process by which rivers & glaciers carve their own valleys
Types of Mechanical Weathering
4) Organic Activity
Ex: root wedging
Mechanical Weathering
Where would it be most significant?
a) Amazon Rain Forest
b) Peoria, IL
c) North Pole
Chemical Weathering
New minerals formed or old ones dissolve
Agents:
1)Oxygen - abundant and reactive
Atomic Number = 8
Oxygen
Ex: Mafic mineral + oxygen = iron
oxide
Hornblende + Oxygen = Hematite
Agents of Chemical Weathering
2) Acid – destroys crystalline structure
Ex: How granite weathers (p. 89)
Feldspar + acid = clay mineral
Clay minerals = most common weathering product of silicates
Clay Minerals
Family of minerals
Microscopic sheet silicates (like mini-micas)
Ex: kaolinite
Sources of Acid:
a) Volcanic eruptions
Volcanic acids
Hot springs where water circulates near magma chambers
Ex: Yellowstone
National Park
Sources of Acid
b) Mine Drainage
- sulfur-bearing minerals react with water to form sulfuric acid
Ex: Pyrite = FeS2 (iron sulfide)
Galena = PbS (lead sulfide)
Sources of Acid
c) Air pollution
Coal-burning power plants release sulfur into atmosphere
- produce sulfuric acid (acid rain)
1990Clean Air Act Amendments
Requires power plants to reduce pollutants
1) Install “scrubbers” in smokestacks
2) Burn lower sulfur-content coal
Chemical Weathering
Environmental conditions that enhance chemical weathering:
1)
2)
Chemical Weathering
Where would it be most significant?
a) Amazon Rain Forest
b) Peoria, IL
c) North Pole
Working together:
Mechanical weathering creates smaller pieces w/ larger surface areas
Weathering Products
Original Mineral Product
Hornblende Clay minerals
Feldspar Clay minerals
Biotite/muscovite Clay minerals
Quartz Quartz
Stable minerals at surface
1) _______________________
2) _______________________
Factors controlling rate of weathering
1) Particle size
_________ particles = faster chemical weathering
_________ particles = faster mechanical weathering
Factors controlling rate of weathering
2) Composition
Which weathers faster in warm humid climate: basalt or granite?
Basalt = _____; Granite = _____
Factors controlling rate of weathering
3) ClimateEx: Limestone in humid climate- chemical weathering dominates- rolling hills- lots of caves & sinkholes
Climate
Ex: Limestone in arid climate
- mechanical weathering dominates
- steep cliffs with talus slopes