1. Weathering of Rocks

2. Exam 1

3. Why we see weathering
Most minerals are not stable at the Earth’s surface
The Earth wants to be flat – lowest energy state
Topography provides energy

4. Bowen’s Reaction Series and Weathering

5. Three Dynamic Processes of Breaking and Removing rock
Weathering - the disintegration and decomposition of rock at or near the surface
Erosion
Mass wasting

6. Types of Weathering
Physical (Mechanical) –breakdown of rocks into smaller pieces.
Chemical – decomposition of rocks by chemical reactions.

7. Page 250

8. Daily Questions
Examine the five World Heritage Sites in Figure 9.1 (pg. 244)
Which site is at the greatest risk from physical weathering? Why?
Which site is at the greatest risk from chemical weathering? Why?

9. Mechanical Weathering
Mechanical weathering breaks down rocks/minerals
Smaller particles increases the amount of exposed surface area
Does not change the chemical composition of the minerals\

10. Mechanical Weathering - making smaller pieces

11. Types of Mechanical Weathering
Frost wedging – water penetrates into cracks, expands when it freezes.
Must have:
Adequate moisture
Cracks in rocks
Freeze/thaw cycles

12. Salt Wedging

13. Biological Wedging

14. Unloading Removal of overlying material allows rock to expands
Top layers expand more than deeper layers
Causes sheeting or exfoliation

15. Unloading
Sheeting
Sheeting

16. Chemical Weathering Chemical alteration of minerals.
Results in new minerals and ions in solution.
Water and acid are essential.

17. Dissolution
Minerals dissolve in water
For example: salts

18. Acid Hydrolysis - Dissolution
Carbon Dioxide + Rain
H2O + CO2 H2CO3
CaCO3 + H2CO3 Ca2+ + 2HCO3-
Becomes Acid
Dissolves Minerals (i.e. carbonates such as calcite)
Carries away--- Ions

19. Acid Hydrolysis & Secondary Minerals
Carbon Dioxide + Rain
Becomes Acid
Dissolves Minerals (i.e. silicates)
Leaves ----Clay
Carries away--- Ions
Silica

20. Oxidation Iron silicate dissolves Iron oxidizes
Oxidized iron combines with water
Leaves iron oxide

21. Results of Weathering Water moves through joints Joints enlarge
Frost wedging
Chemical dissolution – get clays
Joints enlarge
Corners are attacked more aggressively – get rounding of the material
Fines (clays and silts) wash out of rock

23. Weathering Products
Regolith – a loose layer of broken rock and mineral fragments.
Sediments
Dissolved Ions

24. Relative susceptibility To weathering

25. Products of Weathering

26. Factors influencing Weathering Rates
Rock structures – chemical/mineral composition, physical features
Topography
Climate
Vegetation
Time

27. Climate and Weathering

29. Benefits of Weathering
Creates soil
Produces clay, sand, and gravel
Produces minerals

30. Soils

31. Terminology
Regolith – blanket of loose, weathered rock debris covering unweathered bedrock.
Soil – uppermost part of regolith.
Rock particles, new minerals, organics

32. How soil differs from regolith:
More chemical weathering in soil.
Soil has structure (layers called soil horizons).
Soil retains nutrients and moisture, essential for abundant plant growth.

33. Composition, color, texture
More Terminology
Soil profile – sequence of soil horizons.
Soil horizon – layer within a soil profile that has distinct characteristics.
Composition, color, texture

34. Soil Horizons

35. A horizon Zone of leaching (lots of chemical weathering)
Organic rich, often dark in color
Decaying organic matter releases nutrients

36. B horizon
Zone of accumulation – minerals (clay and iron oxide) are washed down from above.
High clay content, reddish color from iron.
Able to retain moisture because of clay content

37. C horizon Parent material – grading from weathered to unweathered.
Parent material can be:
Bedrock
Stream sediments
Volcanic ash

38. Controls of Soil Formation
Parent Material
Time
Climate
Plants and Animals
Slope