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Weathering of Rocks

Weathering of Rocks. Exam 1. 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. Bowen’s Reaction Series and Weathering. Three Dynamic Processes of Breaking and Removing rock.

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Weathering of Rocks

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  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. H2O + CO2H2CO3 CaCO3 + H2CO3 Ca2+ + 2HCO3- Acid Hydrolysis - Dissolution Carbon Dioxide + Rain 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 • 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

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

  23. Relative susceptibilityTo weathering

  24. Products of Weathering

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

  26. Climate and Weathering

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

  28. Soils

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

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

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

  32. Soil Horizons

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

  34. 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

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

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

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