Weathering and Soils

1. Weathering and Soils

2. Earth’s surface processes First some definitions: • Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface • Erosion – The transport of rock fragments

3. Preview of Mechanical Weathering Source:Tom Bean/DRK Photo

4. Mechanical Weathering • Mechanical weathering – breaking of rocks into smaller pieces • Four types of mechanical weathering 1. Frost wedging – freezing and thawing of water in cracks disintegrates rocks

5. Frost Wedging

6. Evidence of Frost Wedging in Wheeler Park, Nevada Source: Tom Bean/DRK Photo

7. Rockfall caused by frost wedging

8. Thermal Expansion and Contraction and Salt Weathering

9. Weathering • Mechanical Weathering (continued) 2. Unloading or Mechanical Exfoliation Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure 3. Thermal expansion and contraction alternate expansion and contraction due to heating and cooling - important in deserts 4. Biological activity – disintegration resulting from plants and animals - root wedging, animal burrows

10. Unloading - Exfoliation of a Pluton

11. Mechanical Exfoliation in Yosemite National Park Source:Phil Degginger/Earth Scenes

12. Unloading- Exfoliation of granite

13. Thermal Expansion andContraction Source:Tom Bean

14. Tree Roots Growing in Rock FracturesAnimal Burrows Source:Runk/Schoenberger/Grant Heilman

15. Increase in surface area by mechanical weathering

16. Joint-controlled weathering in igneous rocks Definition: Joints vs Faults Increases surface area for chemical weathering

17. Chemical Weathering • Breaks down rock and minerals • Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions) • The ions form the cements in Sedimentary Rocks • The salt in the ocean

19. Dissolution of Calcite • Dissolution • By carbonic acid, CO2 in water CO2 + H2O => H2CO3 H2CO3 => H+ + HCO3- • Soluble ions contained in underground water

20. DissolutionWeathered and Unweathered Limestone Boulders Source:Ramesh Venkatakrishnan

21. Oxidation • Chemical reaction where compound loses electrons • Important in breaking down mafic minerals (contain Fe) • Rust- colored mineral (Fe2O3 ) from weathering of Basalt [which contains Olivine (Fe, Mg) SiO4 ]

22. Oxidation of Basalt Rust (Iron Oxide) forms

23. Hydrolysis • Water makes H+and OH- ions • Hydrolysis is the reaction of any substance with water • Water’s ions replace different ions in a mineral • Feldspars, most abundant crust minerals, become fine clay particles. • clays are light weight, flat plates, easily transported by streams

24. Hydrolysis – Feldspar to Clay Feldspars become Mechanical fracture due to chemical weathering

25. Hydrolysis: Angular Boulder Decomposes and Rounds

26. Hydrolysis: Angular Boulder Decomposes and Rounds Source: Paul McKelvey/Tony Stone Images

27. HydrolysisCleopatra’s Needle, (Egypt) Granite in a Dry Climate Source:New York Public Library, Locan History and Genealogy Division

28. HydrolysisCleopatra’s Needle, (Central Park, NYC) Granite in a Wet Climate Source:Runk/Schoenberger/Grant Heilman

29. Vegetation and Soil Development Plants use Hydrolysis to get nutrient metals out of minerals

30. Calcite (Limestone) No solids Ca ++ CO3--

31. Weathering • Rates of weathering • Mechanical weathering aids chemical weathering by increasing surface area • Others factors affecting weathering • Rock characteristics e.g. minerals • Marble and limestone easily dissolve in weak acidic solutions -Dissolution

32. Factors related to bedrock composition Weathering-resistant sandstone (mostly quartz) yields little soil Soil Soil Soil Chemical weathering by hydrolysis Iron-rich basalt Chemical weathering by oxidation Feldspar-rich granite Chemical weathering by dissolution Limestone

33. Weathering • Other factors affecting weathering • Rock characteristics continued • Silicate minerals weather in the reverse order as their order of crystallization (Bowens Reaction Series) • Olivine least stable, conditions of formation least like surface. • Quartz (sandstone) most stable • Climate • Temperature and moisture most crucial factors • Chemical weathering most effective in warm, moist climates

34. Bowens Reaction Series and Weathering

35. Soils Also very important in recognizing past climates

36. Soil • Soil - combination of mineral and organic mater, water, and air • It is that portion of the regolith (weathered rock and mineral) that supports the growth of plants

37. Components in soil that support plant growth

38. Soil • Factors controlling soil formation • Parent material • parent material is the underlying bedrock - composition affects soil types

39. Soil • Factors controlling soil formation • Time • Soils get better developed (Thicker, with greater differences between layers, with more time • Climate • Biggest control on soil formation • Key factors are temperature and precipitation

40. Soil • Factors controlling soil formation • Plants and animals • Organisms influence soil properties • Also furnish organic matter to the soil (especially plants) • Slope • Steep slopes have poorly developed soils (due to faster erosion and downslope transport • Flatter terrain accumulates soil faster

41. Variations in soil development due to topography Note location of agriculture

42. Soil Profile • The soil profile • Soil forming processes operate from the surface downward • Vertical differences are called horizons – zones or layers of soil

43. Soil • The soil profile • O horizon – organic matter • A horizon – organic and mineral matter • High Biological Activity (animals live here) • Together the O and A horizons make up topsoil • E horizon – little organic matter • Zone of leaching • B horizon – zone of accumulation • C horizon – partly altered parent material

44. ONLY ACTIVE EDUCATORS BECOME CHAMPIONS An idealized soil profile

45. Remember the different horizons O A E B C Organic Activity Exited Back Crushed Rock ONLY ACTIVE EDUCATORS BECOME CHAMPIONS

46. Animal Activities in “A” horizon Worms ingest mineral grains because they are covered with living organisms –their food. Their burrows, not their feeding, increase chemical weathering by exposing the minerals to water and air Source:Runk/Schoenberger/Grant Heilman

47. Eluviation & Illuviation exited back

48. Illuviation and EluviationLeaching and Precipitation of Iron E B Source: Jens/Gutzmer/Rand Afrikaans University/Geology

49. Soil Types • Soil types • The characteristics of each soil type primarily depend on the prevailing climatic conditions • Three very generic soil types • Pedalfer • Pedocal • Laterite

50. Equator Equator to Poles Factors Arctic and tundra regions Temperature Annual precipitation Equatorial and tropical rain forests Low-latitude deserts and semi-deserts Temperate regions and mixed boreal forests Grasslands (steppes) Savannahs 30 degrees Latitude 40ºC Precipitation 1800 mm Temperature 30ºC Evaporation 20ºC 10ºC 600 mm Rainforest US & Europe Bedrock at or very near surface Desert Shallow nutrients Soil Soil Deeply weathered bedrock (~40 - 50 meters deep) Bedrock Increasing depth of weathering