1. Ch 5:
Weathering, Soil,
& Mass Movements

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
Chemical-
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. Frost wedging- occurs when water seeps into rocks and freezes

11. Ice wedging

12. Types of Mechanical weathering
3. Biological 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. Observe the effects of mechanical weathering.

22. Types of Chemical Weathering

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

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

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

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

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

28. Oxidation = rust!

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

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

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

32. Rate of weathering depends on 4 things…

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

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

37. 3. Climate
Slow in very hot / very cold climates
Fairly rapid in warm, humid climates

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

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

41. Results of
Weathering

42. Results of Weathering:
Humus
Soil
Bedrock
Regolith

43. Humus
Dark, organic material, remains of animals and plants

44. Humus:

45. Soil
Complex mixture of minerals, water, gases, and remains of plants and animals

46. Regolith A layer of weathered rock fragments
(covers much of Earth’s surface)

47. Bedrock
Solid, unweathered rock that lies beneath regolith

48. Bedrock

49. Bedrock

50. Composition of Soil:

51. The proportion of different soil particle sizes
Soil Texture:
The proportion of different soil particle sizes
Influences the soil’s ability to support plants
Consists of 3 main types classified by particle size

52. Soil Composition
Clays: less than mm in diameter (feldspar)
Silts: between .0002mm - .06mm
Sand: between .06mm – 2 mm (granite)

53. 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.

54. CLAY

55. SILT

56. SAND

58. Soil Texture Diagram
Loamy soils are the best for growing plants

59. Soil profile:
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

60. Humus & Topsoil A B
Subsoil C R
Regolith
Parent Rock

61. A horizon (topsoil) B horizon (subsoil)
mixture of organic and small rock particles
B horizon (subsoil)
contains minerals and clay

62. partially weathered bedrock
C horizon (regolith)
partially weathered bedrock

69. Types of Soil
Climate is most important factor influencing soil formation

70. Tropical (wet & warm) climate:
Laterite soil (thick and infertile)
*rain always is washing away the A horizon but constant rotting vegetation covers the B Horizon

71. Laterite

72. Desert (dry) Climate
Forms soils from mechanical weathering
Soil is thin and mostly of regolith

73. Temperate climates
Temperatures range from cool to warm
Rain fall is not excessive
Two types of soil are found dependent on amount of rainfall:

74. Soil types in temperate climates:
2. Pedalfer – soil formed if an area receives 65 cm or more of rain per year (clay, quartz, iron)
-found in places east of the Mississippi
3. Pedocal – soil formed if rainfall is less then 65 cm a year (calcium carbonate)

75. Pedalfer

76. Pedocal

77. Soil and Topography
The configuration of the land (topography) plays a role in soil formation.

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

79. Tends to be
thicker here
(deposited)

80. Mass Movements

81. Mass Movements The transfer of rock and soil down slope due to gravity
The combined actions of weathering and mass movement produce most landforms
Streams and valleys are the most common landforms on earth
Most mass movements end up in a stream or valley

82. Triggers of Mass Movements
Water – heavy rain and snow saturate the ground
Oversteepened Slopes – angles 25 to 40 degrees will hold loose particles. If the slope is greater than 40 degrees slips become less likely.
Removal of Vegetation – plant roots stabilize
Earthquakes

83. talus: a pile of rock fragments that accumulates at the base of the slope in piles

84. Types of Mass Movements
Geologists classify mass movements based on 3 things:
Kind of material that moves
How it moves
Speed of movement

85. 1. Rockfall: rocks or rock fragments fall freely through the air
is the most rapid
Common on steep slopes & can trigger other mass movements

88. 2. Slide: a block of material moves suddenly along flat, inclined surface

89. Hinsdale County, Colorado 700 years old and still moving
Slumgullion landslide
Hinsdale County, Colorado
700 years old and still moving

90. Rockslide: slides including segments of bedrock

91. 3. Slump: downward movement of a block of material along a curved surface
Leaves a crescent-shaped cliff

92. A slump is a type of slope failure which involves the rotational movement of soil or rock

94. 4. Flows: mass movements of materials containing large amount of water

95. Earthflow – moves slowly
Mudflow – moves quickly

96. 5. Creep: slowest type of mass movement. (freezing & thawing)