1. Prentice Hall EARTH SCIENCE
Tarbuck Lutgens 

2. Weathering, Soil, and Mass Movements
Chapter 5
Weathering, Soil, and Mass Movements

3. 5.1 Weathering Mechanical Weathering
 Mechanical weathering occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s composition.
 Each piece has the same characteristics as the original rock, but breaking it apart, increases the surface area exposed to chemical weathering.

4. Increase in Surface Area by Mechanical Weathering

5. 5.1 Weathering Mechanical Weathering 1. Frost wedging
Caused by the expansion of freezing water in cracks and crevices after many freeze-thaw cycles.
Sections of rock that are wedged loose may tumble into large piles called talus, which typically form at the base of steep, rocky cliffs.
Frost wedging is most common in mountainous regions in the middle latitudes.

6. Frost Wedging

7. 5.1 Weathering Mechanical Weathering 2. Unloading
Reduced pressure on igneous rock due to uplift and erosion of overlying rocks causes it to expand and break off in layers in a process called exfoliation.
3. Biological activity
The activity of organisms, including plants, burrowing animals, and humans, can also cause mechanical weathering.

8. Unloading and Exfoliation of Igneous Rocks

9. Weathering and Biological Activity

10. 5.1 Weathering Chemical Weathering
 Chemical weathering is the transformation of rock into one or more new compounds.
 Water
• The most important agent of chemical weathering.
It promotes weathering by absorbing gases from the atmosphere and the ground.
The dissolved substances react with various minerals to produce rust and/or acid rain.

11. 5.1 Weathering Chemical Weathering  Chemical Weathering of Granite
• Weathering of potassium feldspar produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution.
• Quartz remains substantially unaltered.

12. 5.1 Weathering Chemical Weathering  Weathering of Silicate Minerals
• Produces insoluble iron oxides and clay minerals
 Spheroidal Weathering
• Causes the corners and edges of rock to be more rounded

13. Spheroidal Weathering

14. 5.1 Weathering Rate of Weathering
 Two other factors affecting the rate of weathering are rock characteristics and climate.
1. Rock characteristics
Mineral composition and solubility
Ex: Granite is more resistant to weathering
than marble
Physical features such as cracks

15. 5.1 Weathering Rate of Weathering 2. Climate
• Temperature and moisture are the most crucial factors.
• Chemical weathering is most effective in areas with high temperatures and abundant moisture.

16. 5.1 Weathering Rate of Weathering  Differential Weathering
• Caused by variations in composition as well as
Variations in the number and spacing of cracks in a
Rock mass.
• Creates unusual and spectacular rock formations and landforms

17. 5.1 Weathering • The Claron Formation in Bryce Canyon (UTAH) is
Limestone with a harder dolestone cap shaped into
spires formed by frost wedging and freeze-thaw cycles.

18. 5.2 Soil Characteristics of Soil
 Soil is part of the regolith that supports the growth of plants.
• Regolith is the layer of rock and mineral fragments that covers most of Earth’s land surface.

19. 5.2 Soil Characteristics of Soil  Soil Composition
• 4 major components:
1. mineral matter, or broken-down rock;
2. organic matter or humus, which is the decayed remains of organisms;
3. water;
4. air.

20. Composition by Volume of Good-Quality Soil

21. 5.2 Soil Characteristics of Soil  Soil Texture
• Texture refers to the proportions of different particle sizes.
- Sand (large size)
- Silt
- Clay (small size)
• Loam (a mixture of all three sizes) is best suited for plant life because they retain water better and store more nutrients than do soils with lots of clay.

22. Soil Texture

23. 5.2 Soil Characteristics of Soil  Soil Structure
• Soil particles clump together to give a soil its structure.
It determines how easily a soil can be cultivated and how susceptible it is to erosion.
It also determines the ease at which water can penetrate the soil and influences the movement of nutrients to plant roots.

24. 5.2 Soil Soil Formation 1. Parent material
• Residual soil—parent material is the bedrock
(like a river valley)
• Transported soil—parent material has been carried from elsewhere and deposited by gravity, water, wind, or ice.
The parent material influences the rate of weathering and the rate of soil formation.

25. Parent Material and Soils

26. 5.2 Soil Soil Formation 2. Time 3. Climate
• The longer a soil has been forming, the thicker it becomes.
3. Climate
• Greatest effect on soil formation.
Variations in temperature and precipitation influence the rate, depth, and type of weathering.
A hot, wet climate = thick layer of chemically weathered soil

27. 5.2 Soil Soil Formation 4. Organisms 5. Slope
• Microorganisms can speed up the decay of plants and animals.
• Decaying organisms release nutrients into the soil and speed up weathering.
Burrowing animals mix the mineral and organic matter in the soil and help water and air penetrate.
5. Slope
• Steep slopes = accelerated erosion and soil doesn’t
hold moisture well for plant growth
Flat areas = little erosion and soil might hold too
much moisture with little drainage and dark soil

28. 5.2 Soil Soil Formation 5. Slope
• Orientation of a slope affects soil formation.
In the Northern Hemisphere, south-facing slopes receive more sunlight than the north facing slopes.
South-facing slopes = warmer and drier than north facing slopes as a result

29. Soil Types
5.2 Soil
 Three common types of soil are pedalfer, pedocal, and laterite.
1. Pedalfer
• Best developed under forest vegetation
• Common in eastern US
Accumulation of iron oxides and aluminum-rich clays in the B horizon
Red-brown color

30. 5.2 Soil Soil Types 2. Pedocal 3. Laterite
• Accumulates calcium carbonate
• Associated with drier grasslands
Common in western US
Light gray-brown color.
3. Laterite
• Hot, wet, tropical climates
• Intense chemical weathering with little organic matter due to quick organism decomposition
Iron oxide and aluminum oxide
Orange or red color

31. The Soil Profile
5.2 Soil
 Soil varies in composition, texture, structure, and color at different depths. Soil horizons are zones or layers of soil. A soil profile is a vertical section through all the soil horizons.
• A horizon = “topsoil” with organic matter and living
organisms.
• B horizon = “subsoil” with clay particles washed out of A horizon
• C horizon = partially weathered as well as even unaltered parent material

32. Soil Profile

33. A Soil Profile Showing Different Horizons

34. 5.2 Soil Soil Erosion  Water erodes soil when it rains or flows
 Rates of Erosion
• Wind generally erodes soil slower than water.
Human activities that remove natural vegetation, such as farming, logging, and construction, have greatly accelerated erosion.
 Sediment Deposition
• Reservoirs fill with sediment making them less useful for storing water, flood prevention, etc.
• Sediments are contaminated by pesticides and fertilizers.

35. 5.2 Soil Soil Erosion  Controlling Erosion
• Planting rows of trees called windbreaks
Terracing hillsides
Plowing along the contours of hills
Rotating crops

36. 5.3 Mass Movements Mass Movements
 The transfer of rock and soil downslope due to gravity is called mass movement.
The combined actions of weathering and mass movements produce most landforms.
Once weathering weakens and breaks rocks apart, mass movement moves the debris downslope.

37. 5.3 Mass Movements Triggers of Mass Movements
Water: Heavy rains or rapid melting of snow can saturate surface materials with water causing the soil particles to spread past each other more easily.
Oversteepened Slopes: Mass movements become more likely if the slope is above 40 degrees. Can be created by waves pounding against a cliff or excavating during road construction.

38. 5.3 Mass Movements Triggers of Mass Movements
Removal of vegetation: When plants are removed by forest fires or human activity, the root systems are no longer there to bind soil together.
Earthquakes: An earthquake and its aftershocks can dislodge enormous amounts of rock and cause a mass movement.

39. 5.3 Mass Movements Types of Mass Movements
 Geologists classify mass movements based on the kind of material that moves, how it moves, and the speed of movement.
 Rockfalls
• A rockfall occurs when rocks or rocks fragments fall freely through the air usually resulting from many freeze-thaw cycles.

40. 5.3 Mass Movements Types of Mass Movements  Slides  Slumps
• In a slide, a block of material moves suddenly along a flat, inclined surface.
• Slides that include segments of bedrock are called rockslides.
 Slumps
• A slump is the downward movement of a block of material along a curved surface.

41. Heavy Rains Can Trigger Slumps

42. 5.3 Mass Movements Types of Mass Movements  Flows
• Flows are mass movements of material containing a large amount of water.
• Mudflows move quickly and carry a mixture of soil, rock, and water that has a consistency of wet concrete.
• Earthflows move relatively slowly and carry clay-rich sediment.

43. 5.3 Mass Movements Types of Mass Movements  Creep
• Creep is the slow, downhill movement of soil and regolith.

44. Creep