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