1 / 47

Lecture 20 Weathering

Learn about weathering, erosion, mass wasting, and the rock cycle in this lecture. Discover how rocks disintegrate and decompose near Earth's surface, factors affecting weathering, chemical processes like hydrolysis, oxidation, dissolution, and more.

dons
Download Presentation

Lecture 20 Weathering

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Lecture 20 Weathering

  2. About Weathering, etc. • Weathering produces all the soils, clays, sediments, and dissolved substances. • Erosion is the removal of sediments by natural processes such as wind and rivers. • Mass wasting is the downslope movement of masses of Earth materials.

  3. Weathering, Erosion, Mass Wasting, and the Rock Cycle ● Weathering is the process of converting rock into sediment and forming soils, a major process in the rock cycle. ● Erosion and mass wasting are the processes that loosen and transport soil and rock downhill.

  4. Weathering • Igneous, metamorphic and sedimentary rocks form at depth or at high temperature, in an environment where they are stable. If these rocks are brought to the Earth’s surface and/or cooled, they will become mechanically and chemically unstable Weathering The disintegration and decomposition of rocks at or near the surface of the Earth Disintegration Physical weathering; to de-integrate the constituent parts (minerals) of a rock Decomposition Chemical weathering; to de-compose the chemical structure of the minerals themselves

  5. Controls on Weathering ● Properties of the parent rock ● various minerals weather at different rates ● a rock’s structure affects its susceptibility to cracking and fragmentation

  6. Controls on Weathering ● Other important factors ● climate (rainfall and temperature) ● soil (presence or absence) ●time (length of exposure)

  7. Controls on Weathering

  8. Chemical Weathering ● Occurs when minerals react with air and water ● role of water (hydrolysis) ● carbon dioxide (carbonic acid) ●moist soils

  9. Decomposition • Mostly effected by water • As water falls through air and flows through soil, carbon dioxide is dissolved within it: • Hydrogen (hydronium) and bicarbonate ions are effective at attacking minerals

  10. Chemical Weathering: The Role of Increasing Surface Area 2 cm 2 cm

  11. 2 cm 1 cm 1 cm 2 cm

  12. 2 cm 1 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering …

  13. 2 cm 1 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering… … than small rocks do, so smaller rocks weather more quickly.

  14. Hydrolysis Hydrolysis Replacement of ions by or ions from water • What do we note in a rock’s color? • Hydrolysis can change the chemical composition of a rock: • Potassium feldspar Hydronium Water Ionic K Clay Silica

  15. Oxidation Oxidation Loss of an electron • Iron is perhaps the most readily oxidized of the common, rock-forming cations, i.e. • Magnetite Water Oxygen Goethite

  16. Dehydration Dehydration Loss of water • commonly results in formation of hematite from goethite • Goethite = yellowish, hematite = reddish • (or silvery, as in rock shops!)

  17. Dissolution Dissolution Breaking into constituent ions and holding in solution Limestone Carbonic acid Ionic Ca Bicarbonate • Typical reaction for limestone, which is common rock type in western New York • Can result in unusual topography called karst • There are some karstic features along Main Street, Amherst (near throughway)

  18. Leaching Leaching The removal of soluble matter by aqueous solutions • Not so much a type of chemical reaction, but important to chemical weathering • There is a loss of mass with leaching • e.g., in the rock in the preceding example, many of the ions released by hydrolysis have subsequently been taken into solution and removed by leaching

  19. Decomposition • Pit formation in Wheeler Crest granodiorite at June Lake, CA, caused by differential mineral decomposition

  20. Decomposition • Differential decomposition of syenite, Princess Sodalite Mine, Bancroft, ONT

  21. Decomposition • Differential decomposition of syenite, Princess Sodalite Mine, Bancroft, ONT. Close up view.

  22. Physical Weathering ●What determines how rock breaks? ● natural zones of weakness ● activity of organisms ●frost wedging ●exfoliation

  23. Physical Weathering: Joints in Rocks

  24. Physical Weathering ●Physical weathering and erosion ● duration of weathering ● bedrock type ●climate ● topography

  25. Physical Weathering

  26. Disintegration • Does not change the chemical composition of the rock • Caused by • freezing water • heating and cooling • unloading • abrasion • organic processes

  27. Freezing • Most effective where there are many freeze-thaw cycles (like here) • Water expands 9% in volume when it freezes • Called frost wedging

  28. Physical Weathering: Frost Wedging

  29. Heating and Cooling • Fire probably responsible for most of the mass removed by heating and cooling in many areas (particularly semiarid Western U.S.) • Expansion during the day from insolation and heating also may contribute • Contraction at night with cooling • Diurnal cycle

  30. Types • Each mineral has a different coefficient of thermal expansion • Granular disintegration • Heating on outside of the rock (differential heating) • Spalling or spallation

  31. Granular disintegration and decomposition • Wheeler Crest granodiorite deeply weathering in pits due to granular disintegration following differential decomposition.

  32. Spallation by fire • Fire spallation of a granitic boulder following the Old Fire, Oct, 2003

  33. Spallation by fire • Fire spallation of a granitic boulder following the Old Fire, Oct, 2003. Note chips on ground ready for erosion.

  34. Unloading Unloading Release of pressure as rocks near surface from depth • Expansion of rock in planes parallel to surface • Tensional stress perpendicular to surface • Brittle failure • Exfoliation or sheeting

  35. Physical Weathering: Exfoliation Face of Half Dome, Yosemite Valley, CA, showing granitic sheeting

  36. Exfoliation • Royal Arches, Yosemite Valley, showing sheeting of granite

  37. Abrasion • Running water, ice and wind carry particles • These particles strike surfaces breaking off other particles • Sandblasting or abrasion

  38. Glacial abrasion • Glacially abraded and striated granitic boulder near Bancroft, ONT

  39. Ventifactionweathered by aeolian abrasion • Ventifacted Wheeler Crest granodiorite, Sierra Nevada, CA

  40. Organic destruction • Roots pry rocks apart with growth Salt Weathering • Salt is carried in solution (salt spray) and crystallizes, expanding in cracks

  41. Disintegration and decomposition working together • Decomposition and disintegration work together in Bandelier National Momument, NM, to form distinct weathering features

  42. Rate of Weathering • Clearly, weathering procedes with time (as measurements suggest) • As it procedes, it is influenced by • Rock structure • degree of fracturing • Rock type • i.e., some minerals more unstable under surface conditions

  43. Rate of Weathering (cont.) • Climate • wet fast decomposition; variable precipitation fast disintegration • warm fast decomposition; variable temperature high diurnal variation fast disintegration • Topography • steep little retention of water; flat great retention of water

  44. Results • Broken down rock; rock available for erosion • Movement of ions • Soils and regolith

  45. Soil: The Result of Weathering ● Soils as geosystems ● input material ● transformations and translocations ●output material

  46. Soil: The Result of Weathering ● The basic soil-forming processes result in losses (transformations) and additions (translocations).

  47. Losses Additions Organic material Airborne dust Water erosion Chemicals and minerals from bedrock Wind Leaching BEDROCK

More Related