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Weathering and Erosion

Weathering and Erosion. Painted Desert, Arizona. Weathering Vs. Erosion. Weathering The general process in which rocks are broken down at the Earth’s surface Mechanical – rocks broken w/o changing composition Chemical – minerals are altered or dissolved Erosion

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Weathering and Erosion

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  1. Weathering and Erosion Painted Desert, Arizona

  2. Weathering Vs. Erosion • Weathering • The general process in which rocks are broken down at the Earth’s surface • Mechanical – rocks broken w/o changing composition • Chemical – minerals are altered or dissolved • Erosion • Move weathered materials under influence of gravity on grain-by-grain basis

  3. Intertwined Processes • Weathering breaks rocks down • Erosion carries away loose material exposing new, fresh surface to weathering

  4. Mechanical Weathering: Pressure-Release Fracturing • Igneous and meta rx generally form deep in the crust • Tectonic forces may cause rocks to rise forming a mtn • As mtn erodes, less pressure on stuff beneath it • Rx expand due to lower pressure--fractures

  5. Mechanical Weathering: Wedging • Frost wedging • Can also occur when • Salt water seeps into cracks and evaporates—-haloclasty • Or when • Tree roots=wedge—organic activity

  6. Rocks wedged apart by growing Tree roots Honeycomb texture Characteristic of a rock that Has undergone haloclasty

  7. Mechanical Weathering: Abrasion • Weathering of rocks by friction and impact • Actions make jagged edges round • Natural sandblasting in desert environments—weird shaped rocks

  8. Mechanical Weathering: Thermal Expansion • Occurs only in areas with dramatic diurnal temperature variation • i.e. deserts • Outside of rock cools/heats more quickly than interior—may cause fractures to form

  9. Chemical Weathering • Dissolution: dissolving stuff in water • Ex: Halite • Polar water molecule • Water can pull apart ionic bonds

  10. Acids and Bases • Acidic solution • High concentration of H+ • Basic solution • High concentration of OH- • Acids and bases dissolve minerals faster than water because they have more H+ and OH- ions to pull atoms away from minerals • Acid rain

  11. What Governs Weathering?

  12. Properties of the Parent Rock • Different minerals weather at different rates • Different solubilities • The extent to which their minerals will dissolve in water

  13. Properties of the Parent Rock • Type of rock and structure • Determines rate of P and C. weathering Ex. Niagara Falls

  14. Climate • Amount of rainfall and temperature • Water is the best agent for dissolving • Can hold lots of dissolved ions • Chemical weathering • Fastest where water abundant, hot (humid) • Slower in dry, hot climates • Slowest in cold climates because water frozen, inert

  15. Soil • Fragments of rock, clay formed by altering minerals, and organic matter • Can trap rainwater, host vegetation, bacteria, and organisms • Create an acidic environment which speeds weathering • Roots and organisms create fractures • A positive-feedback process

  16. Length of Exposure • Obviously, the longer the exposure time, the more the weathering • But dependent on other factors

  17. Colorado Rockies: Short Exposure

  18. Appalachians: Long Exposure

  19. Surface Area vs. Volume • The more surface area compared to volume, the faster that part will weather

  20. Works Much Like Coffee • Add hot water to coffee beans • Water can only affect the surface of the beans • Smaller beans means more reactions (Hooray!)

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