1. Weathering vs. Erosion What is weathering? What is erosion?
Breakdown of earth materials
What is erosion?
Removal of earth material
Glaciers
Wind
Water

2. Types of Weathering There are two primary types of weathering:
Physical – Breaking a material into smaller particles
Chemical – Changing the materials composition

3. Physical Weathering Physical or mechanical weathering
Breaking a rock or mineral into smaller pieces
No change in composition

4. Physical Weathering There are six types of physical weathering
Abrasion
Frost wedging
Salt cracking
Organic activity
Pressure relief
Thermal expansion

5. Physical Weathering 1) Abrasion –
A rock is pelted by a stream of small particles
Particles carried by wind or water
Think of sandblasting

6. Physical Weathering 2) Frost wedging – Water fills a crack
Freezing water expands cracks
Destroys rock

7. Physical Weathering 3) Salt cracking – Similar to frost wedging
Most common in coastal regions

8. Physical Weathering 4) Organic activity – Animals burrowing
Plants grow and the roots spread

10. Physical Weathering 5)Pressure relief cracking –
Depth equals higher pressure
Pressure makes a rock smaller
Pressure is released, and the rock expands and cracks

11. Physical Weathering Pressure relief (cont’d) Cracks are called joints
Joints - rocks separate but don’t move

12. Physical Weathering Pressure relief (cont’d)
Within igneous bodies, the joints tend to form parallel to the surface
This is called exfoliation

13. Physical Weathering 6) Thermal expansion –
Heating and cooling = expansion and contraction
Different minerals expand and contract differently
Weakens a rock

14. Chemical Weathering Chemical weathering –
A rock is broken down and it’s composition changes
Exposure to air or water initiate chemical changes

16. Chemical Weathering
Chemical weathering can be broken into a number of categories
Oxidation
Hydrolysis
Weathering by solution

17. Chemical Weathering 1) Oxidation – Elements reacting with free oxygen
Oxygen combines with elements, removing them from the mineral
Rusting
4 Fe (+3) + 3 O2 (-4) = 2 Fe2O3 (Hematite)

19. Chemical Weathering Oxidation (cont’d) –
Pyrite (FeS2) is commonly oxidized when exposed to oxygen at the surface

21. Chemical Weathering 2) Hydrolysis –
Hydrogen ions (H+) replaces other positively charged ions
Happens within slightly acidic water

22. Chemical Weathering Hydrolysis (cont’d) –
Acidic water travels across a rock, replacing cations with hydrogen
Common reaction:
Potassium feldspar + hydrogen ions + water = Muscovite mica + potassium ions + dissolved silica
12H2O + 2H+ + 3KAlSi3O8 = KAl3Si3O10(OH)2 + 2K+ + 6H4SiO4

24. Chemical Weathering 3) Weathering by solution –
Also called dissolutioning
Ions pulled into solution
Halite (salt) and calcite

25. Chemical Weathering Weathering by solution (cont’d) – Water is dipolar
Pure, neutral water can weather by solution

26. Chemical Weathering Weathering by solution (cont’d) –
Water attaches to the ions
The salt is dissolved

27. Chemical Weathering Weathering by solution (cont’d) –
Water isn’t normally neutral
Natural water is usually slightly acidic
H2O + CO2 = H2CO3 = H+ ion + HCO3)
Excess H+

28. Chemical Weathering Weathering by solution (cont’d) –
Carbonic acid reacts with calcite (CaCO3) in limestones
Calcite dissolves into solution
CaCO3 + H2CO3 = Ca + 2HCO3

30. Chemical Weathering Acid Rain –
Defined – Precipitation with a pH < 5.0
Excess acid rain can:
Rapidly weathers certain rock types
Effects groundwater quality

31. Chemical Weathering Acid rain (cont’d) –
Result of water and industrial wastes mixing
Wastes from burning fossil fuels

32. Chemical Weathering Acid rain (cont’d) - Waste products include:
SO2
NO2
These wastes create:
Nitric acid - NO2 + O2 + H2O = HNO3
Sulfuric acid - SO2 + O2 + H2O = H2SO4

33. Chemical Weathering Acid rain (cont’d) –
Acid rain dissolves marble or limestone (weathering by solution)
Excess acid rain pollutes ponds, lakes or reservoirs
Acid rain can leach into the groundwater

35. Rate of Weathering
Variables influencing the rate of weathering include
Climate
Rock type
Particle size
Particle shape

36. Rate of Weathering 1) Climate –
Very important consideration re: weathering
Moisture

38. Rate of Weathering 2) Rock type –
Different rock weather at different rates
Granite weathers slowly
Mineral strength
Limestone weathers quickly
Calcite

39. Rate of Weathering 3) Particle size –
Rock or particle size influences rate of weathering
More surface area exposed = quicker weathering

40. Rate of Weathering Particle size (cont’d) –
Small particles have more surface area exposed
More surface area = faster weathering

41. Rate of Weathering 4) Particle shape –
A particle with numerous edges or corners will weather quickly
A particle with few edges or corners will weather slowly

42. Rate of Weathering Particle shape (cont’d) A corner weathers quickest
An edge weathers slower
A face weathers slowest