Weathering vs. Erosion

1. Weathering vs. Erosion • What is weathering? • 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