Weathering and Rock Breakdown

1. Weathering and Rock Breakdown GLY 2010- Summer 2012 - Lecture 9

2. Weathering • Two types - • Physical, also known as mechanical • Chemical, also known as disintegration

3. Physical Weathering • Faster when water is present, but proceeds at slower rates in dry climates • Many different processes are possible - a few on shown on the following slides • Breaks a rock into smaller pieces without affecting it chemically • Increases the surface area of rocks

4. Surface Area Increase

5. Weathering by Expansion

6. Boulder Split by Frost Wedging • Boulder split by the expansion of ice

7. Adsorption • Adsorption of fluid, usually water, can increase the volume, and lead to cracking

8. Expansive Soils • A: A sample of an expansive soil with moderate swell potential • B: Same soil sample after a small amount of water - Notice the sample has expanded considerably • C: Same sample 48 hours later, after the sample has had time to shrink to a smaller volume

9. Damage Due to Swelling Clays • Damage that might result from uneven expansion and contraction of soil containing swelling clays

10. Building Damage • Building damaged by expansion and contraction of clay minerals in the soil

11. Plant Roots • (Left) Roots grow into cracks in rock, causing expansion, and enlarging the crack • (Right) Roots exposed in rock

12. Thermal Expansion • Rocks expand when heated, contract when cooled • This process has been simulated in the laboratory • Despite many rapid heat/cool cycles, little effect was observed; likely not a very important process in nature

13. Development of Exfoliation

14. Granite Exfoliation • Enchanted Rock State Natural Area, south of Llano, Texas

15. Stone Mountain, Georgia Photo: Jon Cook FAU Junior Field Camp, March, 2004 • Exfoliation at Stone Mountain, Georgia, about twenty miles from Atlanta

16. Stone Mountain, Georgia • Exfoliation Domes Photo: Dr. Anton Oleinik, FAU Junior Field Camp, March, 2004

17. Martian Aeolian (Wind) Abrasion

18. Chemical Weathering • Minerals, formed under pressure and/or thermal conditions different than those at the surface, may be unstable • Chemical weathering changes the chemical composition of minerals that are unstable at the earth’s surface to minerals which are stable

19. Chemical Weathering at Surface

20. Weathering Rock writing on chemically weathered rock

21. Hydration • Water is added to a mineral, creating a new mineral • Anhydrite, CaSO4, may add two water molecules to create gypsum, CaSO4 2H2O • Addition of water leads to volume expansion (70% for the above case), which may lead to cracking of the rock

22. Hydration of Obsidian

23. Dissolution • Ions or ionic groups are removed and carried away by water • Ionic bonding is necessary • Over millions to billions of years, this process is responsible for saltiness of the oceans

24. Sunland Park Mall El Paso, Texas

25. Sunland Mall Marble Closeup

26. Sunland Mall Marble Closeup

27. Oxidation • Addition of oxygen, often with a change of oxidation state • Rusting is slow oxidation, while burning is rapid oxidation

28. Hydrolysis • Water, H2O, breaks down to yield ions • H2O  H+ + OH- • Responsible for the conversion of feldspars, the most common minerals in the earth’s crust, to clay, an important part of soil

29. Ion Exchange • Ions in solution exchange with ions held on the surface of minerals • K+ (in solution)  H+ (on clay) • The reaction may later be reversed, releasing potassium (K), an important nutrient for plants • Important for the retention and later release of water and nutrients

30. Factors Influencing Chemical Weathering • Heat • Water • Time • Stability of minerals • Minerals weather in the reverse order of Bowen’s Reaction series

31. Biological Influences on Weathering • Animals, plants and bacteria can influence weathering • 1) Burrowing animals increase air and water to surfaces • 2) Decay of organic matter produces acids • 3) Yellowboy is the product of bacterially controlled chemical weathering

32. Regolith • When physical weathering dominates, rocks are broken into smaller and smaller pieces called regolith • Regolith has few of the properties of soil - it is just broken rock

33. Soil • Soil is regolith which has been altered by weathering, and which may have had organic matter added to it • Soil is essential to life as we know it, since plants need soil to grow, and animals ultimately depend on plants for food

34. Parent Rock • The parent rock is the rock from which regolith is derived • Depending on their resistance to physical and chemical weathering, different parent rocks may form soil quickly or slowly • The parent rock also determines how rich in nutrients the soil is

35. Factors Influencing Soil Formation • Climate • Vegetation • Topography • Time

36. Topography

37. Soil Profile • Cross-section of soil layering • The layers represent different weathering zones, and are designated by letters • Each layer is called a “horizon”

38. O-horizon • Upper layer, rich in organic matter • Dead (leaf litter, etc.) • Living (bacteria, algae, fungi, insects, worms) • Poorly developed in most areas of South Florida

39. A-Horizon • Inorganic mineral matter • Humus, dark colored decomposed organic matter

40. E-Horizon • Light-colored mineral particles • Zone of eluviation and leaching • Eluviation is the washing out of fine soil components from the A-horizon by downward-percolating water

41. B-Horizon • Enriched by precipitation of minerals dissolved from O and A layers • In arid regions, this may include a caliche layer • Carbonate minerals deposited in quantity because of high rate of evaporation • Brief, heavy rains bring the carbonate downward, and deposit it in B horizon • Often impermeable

42. C-Horizon • Parent material which has been slightly weathered • Retains most of its original appearance

43. Example Soil Profile • A horizon extends to bottom of third mark from top • B horizon (with various subunits) below the third mark

44. Lateritic Soil, Brazil

45. Tropical Rainforest Soils • Very poor soils; nutrients are tied up in the plants • When a plant dies naturally, its nutrients support new replacement growth • If the forest is cleared and the plant material removed, the nutrients are also removed

46. Soil Taxonomy • Naming of soils based on their characteristics • Physical characteristics - named by obvious physical characteristics (color, thick/thin, shrinkage/expansion, oxidation, etc) • Texture - description of texture

47. Nutrient Depletion – Borneo Vs. Java • The soil on Java is formed from fresh, nutrient-rich volcanic rock - potassium, calcium, and magnesium rich • Java has a population density of 460 people per square kilometer

48. Java Pictures • Jakarta at left • Cirebon above • Large cities on Java

49. Borneo • The soils on Borneo are from the weathering of intrusive granitic rock, gabbro intrusions, and andesitic lavas, and are poor in nutrients • The population density is 2 people per square kilometer

50. Borneo Countryside • A jungle trail and a road leading to a small village in Borneo