Lecture 20 Weathering

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