1. Ch 12:
Weathering
and
Erosion

2. Rocks on the Earth’s surface undergo changes in appearance and composition

3. Weathering: Physical or chemical changes in rock material exposed at surface
Mechanical-
physically change rock into smaller pieces
Chemically-
breakdown of rock by changing its chemical composition

4. Types of Mechanical weathering

5. Types of Mechanical Weathering
1. Exfoliation- process where sheets of rock peel or flake away

7. Example of exfoliation:

8. Types of Mechanical weathering
2. Ice wedge- occurs when water seeps into rocks and freezes

11. Ice wedging

12. Types of Mechanical weathering
3. Organic activity- roots of plants and burrowing animals weather away rock

13. Weathering by plants

15. Weathering by sandmartins nesting in cliff

16. Types of Mechanical weathering
4. Abrasion- collision of rock with one another resulting in breaking and wearing away.

17. Abrasion refers to the breaking and grinding away of solid rock by collisions with moving particles.
Abrasion takes place in many environments:
fast-moving streams
beaches subject to storm waves
desert environments with high winds
beneath glaciers that are loaded with fragments of rock.

18. Water-carried particles eating away at the rock

19. Storm waves have eaten away at the shoreline

20. The ocean waves have eroded this beach leaving behind a cliff as the soil was washed away

21. Types of Chemical Weathering

22. Types of Chemical Weathering
1. Hydrolysis:
Change in composition of minerals when they react with water

23. Feldspar combines with H2O to form a common clay called kaolin

24. Types of Chemical Weathering
2. Carbonation –
When some minerals come in contact with carbonic acid, they form a new product

25. Stalactites caused by carbonation
Limestone is eaten away by acid and deposits as it drips from ceiling of cavern

26. Types of Chemical Weathering
3. Oxidation –
When metallic elements combine with oxygen

27. Oxidation = rust!

28. Types of Chemical Weathering
4. Acid Precipitation
“acid rain”

29. Types of Chemical Weathering
5. Plant acids – plants secrete acids that erode away the rock

30. Lichens and mosses grow on rocks and secrete weak acids that dissolve the surface

31. Leaching
Leaching is the movement of minerals or nutrients in the soil typically being dissolved in water as they are eroded and then carried away into deeper layers of the soil (or rock) and carried off in bodies of water.

32. Rate of weathering depends on 4 things…

33. 1. Rock composition
Igneous and metamorphic rocks don’t weather easily
Sedimentary rocks do

34. 2. Amount of exposure
More exposure it receives, faster it will weather
Amount of time and amount of surface area exposed is also important

36. 3. Climate
Climate: is the MOST influential of the controls on soil formation
- Hot/humid climate = more chemical weathering and thicker horizons (layers)
- Cold climate = more mechanical weathering, ex. Freeze/thaw, thin layers

37. Cleopatra’s needle after only one century in New York City

38. 4. Topography
Elevation or slope of surface where rock is located affects rate of weathering

39. Slope: steeper slope = less soil

40. Thinner on slope because water erodes soil & deposits it down the slope

41. Tends to be
thicker here
(deposited)

43. Results of
Weathering

44. Factors that contribute to soil formation:
The result of these Erosive forces is soil
Soil: Complex mixture of minerals, water, gases, and remains of plants and animals, as well as living organisms (bacteria)
Parent Material: original bedrock or source rock that weathers into soil.
- Once it is weathered it is known as regolith.
- determines the chemical make up of the soil
Time: 200 years for 1cm of soil to form from parent material
- faster in areas with more precipitation or lots of chemical weathering

45. Composition of Soil:

47. Soil profile: A mature soil will separate out into layers.
A cross-section in which the layers of the soil and bedrock can be seen
Each layer is called a horizon.
In fully developed residual soil, there are three horizons (some can have more or less)

48. Soil Profile
O horizon - (leaf litter) organic matter accumulation and decomposition. Almost all organic.
A horizon (top soil) – zone of leaching. High organic with some inorganic rock.
The dark coloration in the “O” and “A” horizons is due to organic material, remains of animals and plants
B horizon – (sub soil) zone of accumulation. More just weathered rock less organic. Mostly clay and minerals.
C horizon – (regolith) weathered parent material, either rock or sediments
R horizon – (bedrock), unweathered parent material.

49. Humus:

50. Bedrock

51. Porosity, Permeability, & Grain Size of Soil
How do they all relate to each other?

52. Porosity:
How much space there is between the grains of sediment in soil.

53. Permeability:
How fast water will move through a soil type.

54. Grain Size:
How large the individual pieces of Sediment are in the soil.

55. CLAY

56. SILT

57. SAND

58. The portions of clay, silt, and sands depends on the parent material
Parent material or “parent rock” is the rock from which the soil was weathered.

59.                                                                                                
                                                                                             
The porosity & permeability are related to the grain size, and how tightly packed the grains of sediment are.

60. Clay is very small disc shaped and tends to be tightly packed 
Topsoil is medium sized and tends to be loosely packed, it has organic material and a mix of grain sizes.
Sand is large and tends to be loosely packed.

61. SO what is the relationship between: POROSITY PERMEABILITY
& GRAIN SIZE
Do the lab and figure it OUT!