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 Mechanical Weathering

1) Frost wedging

- most effective in climates with freeze/thaw cycles

Ex: high altitudes, mid-latitudes

Granite boulder

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

Unloading

Sheeting

Frost Wedging Sheeting

Yosemite

Half Dome is an exfoliation dome cut in half by large glaciers during the Ice Age

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

Types of Weathering Processes

2) Chemical Weathering

- alters crystalline structure and/or composition of minerals

- new minerals formed or minerals dissolve

Chemical Weathering

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)

Open Pit Mine

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

Map of U.S. coal deposits

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