1. Weathering and Soil Earth - Chapter 6

2. Earth’s external processes
Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface
Mass wasting – the transfer of rock and soil downslope under the influence of gravity

3. Weathering (Disintegration & Decomposition) Accentuates Differences in Rock Types

4. Earth’s external processes
Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

5. Weathering Two types of weathering
Mechanical weathering – breaking of rocks into smaller pieces
Chemical weathering – decomposition into new compounds

6. Mechanical Weathering (Physical Breakdown of Rocks) Produces Smaller & Smaller Pieces that Increase the Surface Area Available for Chemical Attack

7. Weathering Four types of mechanical weathering
Frost wedging – alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks
Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure
Thermal expansion – alternate expansion and contraction due to heating and cooling
Biological activity – disintegration resulting from plants and animals

8. Frost wedging alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks

9. Unloading – exfoliation of igneous & metamorphic rocks at the earth’s surface due to a reduction in confining pressure

10. Thermal expansion alternate expansion and contraction due to heating and cooling

11. Not all fractures are created by thermal processes
Not all fractures are created by thermal processes. These “joints” near Moab, Utah, are cracks produced by tectonic forces generated during mountain building.

12. Biological activity disintegration resulting from plants and animals

13. Weathering Chemical Weathering
Breaks down rock components and internal structures of minerals
Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

14. Weathering Major processes of chemical weathering Dissolution
Aided by small amounts of acid in the water
Soluble ions are retained in the underground water supply

15. Dissolution
Halite (Rock Salt) Dissolving in Water. Sodium & Chloride Ions Are “Attacked” by Polar Water Molecules. Once Removed, the Na & Cl ions are Surrounded & Held by Numerous Water Molecules

16. The Effects of “Acid Rain” – Partially Dissolved Limestone Figure, Damaged Trees in Great Smoky Mtns.

17. Weathering Major processes of chemical weathering Oxidation
Any chemical reaction in which a compound or radical loses electrons
Important in decomposing ferromagnesian minerals

18. A very common example of oxidation is the reaction of iron with oxygen to create iron oxide (rust). Similar oxidation of iron in rock-forming minerals produces a red color

19. Similar oxidation of iron in rock-forming minerals produces a red color on outcrop

20. Acid mine drainage results when pyrite (FeS2) is oxidized & releases sulfur that combines with H2O to form sulfuric acid

21. Weathering Major processes of chemical weathering Hydrolysis
The reaction of any substance with water
Hydrogen ion attacks and replaces other positive ions

22. Weathering Alterations caused by chemical weathering
Decomposition of unstable minerals
Generation or retention of materials that are stable
Physical changes such as the rounding of corners or edges

23. Generation of more stable minerals from weathering of igneous rocks

24. Rates of Weathering Factors affecting weathering Surface area
The smaller the particles to be weathered, the more surface area is exposed to be weathered and the more rapidly the weathering proceeds

25. Mechanical Weathering (Physical Breakdown of Rocks) Produces Smaller & Smaller Pieces that Increase the Surface Area Available for Chemical Attack

26. Rates of Weathering Factors affecting weathering Rock characteristics
Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions
Silicate minerals weather in the same order as their order of crystallization

27. These 2 tombstones were erected at roughly the same time, but the marble stone on the right weathered much more rapidly

28. Silicate Minerals Weather in the Same Order in Which They Form: i. e
Silicate Minerals Weather in the Same Order in Which They Form: i.e. the Highest Temperature Silicates Are the Least Resistant to Weathering

29. Weathering Others factors affecting weathering Climate
Temperature and moisture are the most crucial factors
Chemical weathering is most effective in areas of warm, moist climates

30. Weathering Differential weathering
Masses of rock do not weather uniformly due to regional and local factors
Results in many unusual and spectacular rock formations and landforms

31. Differential weathering controlled by jointing patterns

32. Joint-controlled weathering in igneous rocks

33. Successive shells are loosened as the weathering process continues to penetrate deeper into the rock

34. Soil Soil = combination of mineral and organic mater, water, and air
Rock and mineral fragments produced by weathering (regolith) that supports the growth of plants
Humus (decayed animal and plant remains) is a small, but essential, component

35. Soil is an ESSENTIAL resource that supports plant life, which in turn supports animal life

36. Composition of a “good” soil

37. Soil Factors controlling soil formation Parent material
Residual soil – parent material is the underlying bedrock
Transported soil – forms in place on parent material that has been carried from elsewhere and deposited

38. Soil Factors controlling soil formation Time Climate
Amount of time for soil formation varies for different soils depending on geologic and climatic conditions
Climate
Most influential control of soil formation
Key factors are temperature and precipitation

39. Soil Factors controlling soil formation Plants and animals
Organisms influence the soil’s physical and chemical properties
Also furnish organic matter to the soil
Slope (Topography)
Steep slopes often have poorly developed soils
Optimum terrain is a flat-to-undulating upland surface

40. Variations in soil development

41. Soils in Alaska’s Denali Nat
Soils in Alaska’s Denali Nat. Park are produced by intense acid leaching of conifer humus shed from the overlying trees

42. Soil
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
Together the O and A horizons make up the topsoil
E horizon – little organic matter
Zone of eluviation and leaching

44. Soil
The soil profile
B horizon – zone of accumulation
C horizon – partially altered parent material
The O, A, E, and B horizons together are called the solum, or “true soil”

45. An idealized soil profile

46. Soil Profiles: A = well developed soil in South Dakota; B = indistinct soil horizons in soil from Puerto Rico A B

47. Soil Classifying soils
Variations in soil formation over time and distances has led to a great variety of recognized soil types
Groups have been established using common characteristics
In the United States a system is used called the soil taxonomy

48. Soil Classifying soils Soil taxonomy
Emphasis is placed on physical and chemical properties
Six hierarchical categories exist, ranging from order (12 broad catagories) to series (>19,000)
Descriptive names are derived from Latin and Greek
12 basic soil orders are recognized

50. Distribution of the 12 soil orders in the western hemisphere

51. Distribution of the 12 soil orders in the eastern hemisphere

52. An example of soil order Oxisol (rain forest iron & aluminum soils)
An example of soil order Oxisol (rain forest iron & aluminum soils). Although these provide an excellent foundation for trees, they are NOT good for farming

53. Angkor Wat, Cambodia, is built of bricks cut from oxisols in the region. When exposed to the sun (trees removed) this soil hardens like stone, thus it is not very good for growing crops.

54. Soil Soil erosion Part of the natural recycling of earth materials
Natural rates of soil erosion depend on
Soil characteristics
Climate
Slope
Type of vegetation

55. Soil
Soil erosion
In many regions the rate of soil erosion is significantly greater than the rate of soil formation
Sedimentation and chemical pollution
Related to excessive soil erosion
Occasionally soil particles are contaminated with pesticides

56. Exposed soil particles respond to rain drops
Soil dislodged by splash erosion is more easily moved by sheet erosion.

57. Evidence of soil erosion in Wisconsin
1 mm of soil eroded from a single acre of land amounts to about 5 tons of soil lost

58. Gully erosion is severe in this poorly protected oxisol soil in Columbia

59. Soil Erosion Due to Drought and Excessive Farming Was Common During the 1930’s