1. Mechanical Weathering
Includes all processes that crack, abrade, crunch, and otherwise break rocks.

2. Unloading Exfoliation
Expansion of igneous rock at surface pressures causes fracturing and shedding of outer layers
Fig

3. Fig

4. Mechanical Weathering
Frost/Ice Wedging
Ice is less dense than water so water expands as it freezes.

5. Mechanical Weathering
Frost/Ice Wedging

6. Mechanical Weathering
Abrasion
Wearing away of rock by grit suspended in water or air

7. Mechanical Weathering
Abrasion

8. Mechanical weathering is an inefficient way to break down rocks
Mechanical weathering is an inefficient way to break down rocks. It does, however, increase the surface area on which chemical weathering can act.

9. Chemical Weathering Break down of rocks due to chemical reactions.
This is by far the most important of the three kinds of weathering.

10. Chemical Weathering Acid/Base Reactions
Changes in acid or basic conditions cause break down, e.g. limestone dissolved in acid rainwater

11. pH of natural and polluted rainwater
Chemical Weathering
Acid/Base Reactions
Box f2
pH of natural and polluted rainwater

12. slate
limestone
Differential weathering of slate and limestone headstones. The limestone dissolves much more readily with acidic rainwater.
Fig a

13. Oxidation/Reduction Reactions
Chemical Weathering
Oxidation/Reduction Reactions
Loss or gain of an electron from an element (e.g., C, Fe or Mn).
Oxidation usually results in the formation of an oxide, e.g., “rusting” of iron-bearing minerals by oxidation of iron.

14. Oxidation/Reduction Reactions
Chemical Weathering
Oxidation/Reduction Reactions
FeS + O2- + H2O  FeO.OH +S2-

15. Hydration/Dehydration
Chemical Weathering
Hydration/Dehydration
Addition and subtraction of H2O into the mineral to form a different mineral, e.g., anhydrite (CaSO4) + H2O  gypsum (CaSO4*2H2O)
+ H2O

16. Chemical Weathering Hydrolysis
Reactions between H+ and OH- and silicate minerals yielding soluble positively charged ions and silica in solution.
Hydrolysis is the main decay pathway for silicate minerals (remember, the most abundant minerals in the Earth’s crust are silicates!).
Further reactions result in the formation of clay minerals.

17. Chemical Weathering Hydrolysis Feldspar (microcline)
Feldspars chemically break down when exposed to surface conditions, and form clay minerals through the process of hydrolysis
Feldspar (microcline)
KAlSi3O8 + H+ + H2O  H4SiO4 + KAl3SiO10(OH)2
Illite (clay mineral)
Feldspar (albite)
NaAlSi3O8 + H2CO3 + H2O  Na + H4SiO4 + Al4Si4O10(OH)8
Kaolinite (clay mineral)
Other common clay minerals: schmectite, gibbsite, montmorillonite, etc.

18. Fig

19. Bowen’s Reaction Series and Weathering
felsic
Felsic minerals are most resistant to chemical weathering. Mafic minerals are least resistant to chemical weathering.

20. Biological Weathering
Weathering due to the activities of living things (plants, animals, fungi, bacteria, protists, etc.)
All organisms alter their local environments, causing both physical and chemical weathering. Plants are the most obvious destroyers of rocks, but bacteria are by far the most important biological weather-ers.

21. Biological Weathering
Lichen breaking down rock. Primarily (bio)chemical

22. Biological Weathering
Root Wedging
Tree roots breaking down rock. Both physical and chemical weathering

23. Weathering Mechanical Weathering Chemical Weathering
Biological Weathering

25. P Quartz Feldspar Mica Physical Weathering resistant Hardness = 7
no cleavage, brittle
Tenacity - strong
susceptible
resistant
Feldspar
Hardness = 6
2 directions of cleavage, brittle
Tenacity - intermediate
susceptible
resistant
Mica
Hardness = 2.5
1 perfect cleavage, elastic
Tenacity - weak
susceptible

26. P C Quartz Feldspar Mica Chemical Weathering resistant
Chemically resistant to most surface processes
susceptible
resistant
Feldspar
Breaks down quickly to form clay minerals
susceptible
resistant
Mica
Breaks down slowly to form clay minerals
susceptible

27. Quartz: physically breaks down to form smaller quartz grains
Weathering
Quartz: physically breaks down to form smaller quartz grains
physical break down
fracture
Feldspar: physically breaks down to form smaller feldspar grains, chemically breaks down to form clay minerals
physical break down
cleavage
chemical break down
hydrolysis
Mica: physically breaks down to form smaller mica grains, chemically breaks down to form clay minerals
physical break down
cleavage
chemical break down
hydrolysis

28. Sediment Produced from Granite
Given enough time, all that will remain will be sand-sized quartz grains and mud-sized clay minerals

29. Clast Sorting
The farther the clasts have traveled from the source, the more well sorted they tend to be.

30. Clast Size Comparisons
gravel
coarse sand
fine sand
Large clasts require more energy to transport than small clasts. Smaller clasts tend to be transported greater distances than larger clasts.
silt and clay

31. Clast Rounding
As clasts are rolled around during transportation, sharp corners tend to be knocked off, and the clasts become more rounded the farther they are transported.
Angular grains are usually found only near the source rock.