Understanding Weathering and Soils in Environmental Geology Lecture

1. Limestone dissolved by naturally acidic rainwater Weathering and Soils Lecture 5

2. Weathering and Soils • 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 – breaking of rocks into smaller pieces • Four types of mechanical weathering • Frost wedging – freezing and thawing of water in cracks disintegrates rocks Salt wedging is similar

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

6. Rockfall caused by frost wedging ends up in a stream Colluvium

7. Salt Weathering is similar, growing salt crystals in cracks split the rock apart. At the shore and desert playas.

8. 2. Unloading = Mechanical Exfoliation Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure Granite Batholith above subduction zone

9. Unloading = Exfoliation of a Pluton Batholith

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

11. 3. Thermal expansion and contraction Alternate expansion and contraction due to heating and cooling Minerals expand and contract at different rates Slide past one another, breaking any intergrowths Rock crumbles to individual mineral grains - Important in deserts

12. Tree Roots Growing in Rock FracturesAnimal Burrows Biological Activity Source:Runk/Schoenberger/Grant Heilman

13. Mechanical weathering causes increase in surface area Same volume of rock, but many pieces have much more surface area than one piece

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

15. 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 • These ions make the ocean salty Important for metals concentration

16. Three types of Chemical Weathering

17. DissolutionWeathered and Unweathered Limestone Boulders Source:Ramesh Venkatakrishnan

18. Dissolution of Calcite • Dissolution • By carbonic acid, CO2 in water Soluble ions contained in underground water H2O + CO2 <=> H2CO3 <=> H+ + HCO3- CaCO3 +2H+ <=> H2O (l) + CO2 (g) + Ca++ (aq)

19. Oxidation • Chemical reaction where compound loses electrons • Usually while bonding with oxygen • Important in breaking down mafic minerals (contain Fe+2 or +3) • Rust- colored mineral (Fe2O3 ) from weathering of Basalt Which contains Pyroxene (Fe, Mg)2 Si2O6 http://www.geol.umd.edu/~jmerck/gal04/events/htmls/dauttl.html

20. Oxidation of Basaltic Lava Flows Rust-colored Iron Oxide forms

21. 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

22. Hydrolysis – Feldspar to Clay expands in water Feldspars become Mechanical fracture due to chemical weathering

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

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

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

27. Bowens Reaction Series and Weathering

28. Soils Also very important in recognizing past climates

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

30. Components in topsoil that support plant growth

31. Factors related to bedrock composition (parent material) 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 Climate also very important, see below

32. Variations in soil development due to topography: slope and time

33. 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 Deep Regolith, shallow Soil Soil Deeply weathered bedrock (~40 - 50 meters deep) Bedrock Increasing depth of weathering Factors related to Climate

34. Soil Profile • Soil forming processes operate from the surface downward as water descends • Vertical differences are called horizons – zones or layers of soil

36. Soil Horizons • 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 – soluble minerals removed • B horizon – zone of re-precipitation • C horizon – partly altered parent material

37. ONLY ACTIVE EDUCATORS BECOME CHAMPIONS An idealized soil profile

38. Remember the different horizons O A E B C Organic Animal Activity Soluble minerals Exited Soluble Minerals Back Crushed Rock ONLY ACTIVE EDUCATORS BECOME CHAMPIONS

39. 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

40. E and B: Eluviation & Illuviation exited back

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

42. 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

43. pedalfer pedocal laterite Evergreen forests Shortgrass tropics

44. Pedalfer • Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon • Best developed under temperate forest landscapes

45. Pedocal • White calcium carbonate (caliche) in B horizon • Associated with dry grasslands and brush vegetation

46. Pedocal with Caliche in the B horizon O A E B C Organic Activity Leached Accumulation Crushed Rock

47. Laterite Hot and wet tropical climates Monsoonal Climate • Intense chemical weathering • Red Iron oxide - Topsoil not distinct from B horizon –bacterium responsible for dissolving soil iron is not present • Deep soil but usable nutrients shallow

48. Laterite in Sarawak, Borneo The percolating rain water causes dissolution of primary rock minerals and decrease of easily soluble elements as sodium, potassium, calcium, magnesium and silicon. This gives rise to a residual concentration of more insoluble elements predominantly iron and aluminum. Source:Fletcher & Baylis/Photo Researchers, Inc.

49. Earth’s surface processes • Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity

50. Natural rates of soil erosion depend on: • Soil characteristics • Climate • Slope • Type of vegetation Headed for the Sea Source:Ramesh Venkatakrishnan