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Weathering of Rocks

Weathering of Rocks. 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. Weathering in the Rock Cycle. Erosion + Deposition. Igneous. Sedimentary. Melting. Erosion + Deposition.

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Weathering of Rocks

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  1. Weathering of Rocks

  2. 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

  3. Weathering in the Rock Cycle Erosion + Deposition Igneous Sedimentary Melting Erosion + Deposition Burial + Heating Burial + Heating Melting Metamorphic

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

  5. Types of Weathering • Mechanical – physical breakdown of rocks. • Chemical – decomposition of rocks by chemical reactions.

  6. 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\

  7. Mechanical Weathering - making smaller pieces

  8. Mechanical Weathering • Frost Wedging • Salt Wedging • Biological Wedging • Unloading • Thermal Expansion

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

  10. Frost Wedging

  11. Types of Mechanical Weathering • Salt wedging – growth of minerals in cracks • Desert environments • Water evaporates, ions in solution combine to form minerals

  12. Salt Wedging

  13. Types of Mechanical Weathering • Biological wedging – plant roots penetrate into cracks causing cracks to widen. • Must have: • Climate hospitable for plants. • Adequate moisture and temperature.

  14. Biological Wedging

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

  16. Unloading • Sheeting Sheeting

  17. Thermal Expansion • repeated daily heating and cooling of rock; ex. Desert environments • heat causes expansion; cooling causes contraction.

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

  19. Types of Chemical Weathering • Hydrolysis - any reaction in which water participates. • Ion exchange – H+ replaces other cations. • Dissolution - mineral completely dissolves, leaving only ions in solution. • Oxidation - reaction in which elements gain or lose electrons (example: rust).

  20. 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

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

  22. Dissolution • Minerals dissolve in water • For example: salts

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

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

  25. Products of Weathering

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

  27. Climate and Weathering

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

  29. Soils

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

  31. 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:

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

  33. Soil Horizons

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

  35. 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

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

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

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