This document discusses the two main types of weathering: mechanical and chemical. Mechanical weathering breaks rock into smaller pieces with little chemical change, such as through frost wedging or abrasion by water or glaciers. Chemical weathering alters the composition of minerals through reactions with oxygen and acid, forming new minerals or dissolving old ones. Factors like climate, rock composition, and particle size influence the rate of weathering.

1. CH. 4 - WEATHERING
Chapter 2

2. Types of Weathering
Processes
1) Mechanical Weathering
- rx break down into smaller
pieces
- little or no chemical alteration

3. Types of Weathering
Processes
2) Chemical Weathering
- alters crystalline structure and/or
composition of minerals
- new minerals formed or minerals
dissolve

4. Types of Mechanical
Weathering
1) Frost wedging
- most effective in climates with
freeze/thaw cycles
Ex: high altitudes, mid-latitudes

5. Granite boulder

6. Talus slope

7. Types of Mechanical
Weathering
2) Unloading
- caused by a decrease in
pressure
- effective on large bodies of
intrusive igneous rocks

8. Unloading
Rocks expand & crack parallel to
surface (“sheeting”)
- produce rounded mountains
called exfoliation domes
Ex: Yosemite National Park, CA
Stone Mountain, GA

9. Unloading

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

11. 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

12. Types of Mechanical
Weathering
4) Organic Activity
Ex: root wedging

13. Mechanical Weathering
Where would it be most
significant?
a) Amazon Rain Forest
b) Peoria, IL
c) North Pole

14. Chemical Weathering
New minerals formed or old ones
dissolve
Agents:
1)Oxygen - abundant and reactive
Atomic Number = 8

15. Oxygen
Ex: Mafic mineral + oxygen = iron
oxide
Hornblende + Oxygen = Hematite

16. 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

17. Clay Minerals
Family of minerals
Microscopic sheet silicates (like
mini-micas)
Ex: kaolinite

18. Sources of Acid:
a) Volcanic eruptions

19. Volcanic acids
Hot springs where water circulates
near magma chambers
Ex: Yellowstone
National Park

20. 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)

21. Open Pit Mine

22. Sources of Acid
c) Air pollution
Coal-burning power plants release
sulfur into atmosphere
- produce sulfuric acid (acid rain)

23. 1990
Clean Air Act Amendments
Requires power plants to reduce
pollutants
1) Install “scrubbers” in
smokestacks
2) Burn lower sulfur-content coal

24. Map of U.S. coal deposits

25. Chemical Weathering
Environmental conditions that
enhance chemical weathering:
1)
2)

26. Chemical Weathering
Where would it be most
significant?
a) Amazon Rain Forest
b) Peoria, IL
c) North Pole

27. Working together:
Mechanical weathering creates
smaller pieces w/ larger surface
areas

28. Weathering Products
Original Mineral Product
Hornblende Clay minerals
Feldspar Clay minerals
Biotite/muscovite Clay minerals
Quartz Quartz

29. Stable minerals at surface
1) _______________________
2) _______________________

30. Factors controlling rate of
weathering
1) Particle size
_________ particles = faster
chemical weathering
_________ particles = faster
mechanical weathering

31. Factors controlling rate of
weathering
2) Composition
Which weathers faster in warm
humid climate: basalt or
granite?
Basalt = _____; Granite = _____

33. Factors controlling rate of
weathering
3) Climate
Ex: Limestone in humid climate
- chemical weathering dominates
- rolling hills
- lots of caves & sinkholes

34. Climate
Ex: Limestone in arid climate
- mechanical weathering
dominates
- steep cliffs with talus slopes