1. Weathering
Chapter 5

2. 5.1 Weathering = breakdown of rock and minerals 2 Types of weathering
Mechanical
Chemical

3. Mechanical Weathering =
physical forces breakdown rocks into smaller pieces without changing the mineral composition

4. 3 Types
Frost Wedging
Unloading (exfoliation)
Biological Activity

5. Increases the surface area of the rock

6. FROST WEDGING
The mechanical breakup of rock caused by the freeze-thaw cycles of water in the cracks of rocks.
When water freezes, its volume expands

7. The freeze-thaw cycles break rock into pieces
Cracks are formed from water freezing, expanding, and breaking the asphalt or concrete

8. Sections of rock that are wedged loose may tumble into large piles called talus which typically form at the base of steep rocky cliffs
Most common in mountain regions of middle latitudes

9. UNLOADING
Exfoliation- reduced pressure on igneous rock causes it to uplift and allows slabs of outer rock to break off in layers

10. Biological Activity Roots Construction Animals digging into rock
Plants , burrowing animals and humans affect the weathering of rocks.
Roots
Construction
Animals digging into rock

12. Chemical Weathering
Occurs when the chemical and mineral composition of a rock have been changed
WATER is the most important agent of chemical weathering

13. Water absorbs gases from the atmosphere and chemically react with minerals :
EX: Water + CO2 + Calcite = Carbonic Acid
breaks down limestone

14. EX: H2O + O2 + iron + = rust
EX: H2O + sulfur gas OR nitrogen gas = Acid Rain
Evidence of chemical weathering

16. RATE OF WEATHERING
Mechanical weathering affects the rate of chemical weathering
Mechanical weathering breaks rocks into several pieces, which INCREASES the surface area of exposed rock, allowing faster chemical weathering to take place.

17. 3 things that affect the rate of weathering
Rock characteristics (chemical composition)
Climate
Surface Area (amount) of exposed rock

18. Chemical Composition Rock’s mineral composition affects weathering
Granite is resistant to chemical weathering
Marble chemically weathers very quickly
Marble is formed from calcite which easily dissolves in acid

19. CLIMATE Affected by temperature and moisture
Chemical weathering is most effective in areas with high temperatures and an abundance (lots) of moisture

20. CLIMATE
Tropical- high temperatures + high moisture = lots of chemical weathering
Polar- low temperatures + low moisture = little or no chemical weathering
Deserts- high temperatures + low moisture = very slow chemical weathering

21. Remember……
Climate with most favorable weathering conditions has high temperatures & lots of moisture

22. Exposed Surface Area Cracks allow water to seep into the rock
The more the crack & crevices, the greater the weathering

23. 5.2

24. Soil Composition 4 major components of soil
45% Mineral Matter (mineral broken down rock)
25% Water
25 % Air
5% Organic Matter or dead stuff
(decayed remains of organisms)

25. Soil texture is determined by particle size sand (large) silt
clay (small)
Loam - a mixture of all three sizes
Best suited for supporting plant life

27. Soil Formation
Parent Material
Time
Climate
Organisms
Slope

28. PARENT MATERIAL Source of mineral matter in the soil
TOPSOIL- PARENT MATERIAL IS THE MINERAL MATTER
SUBSOIL- PARENT MATERIAL HAS BEEN DEPOSITED FROM ELSWHERE AND WEATHERS

29. Time Important to ALL weathering
the longer the soil has been forming the thicker it becomes

30. CLIMATE Major effect on soil formation
Variations in temperature and precipitation influence rate, depth, and type of soil
*change weathering in your notes to “soil” )

31. Hot & wet climate may produce a thick layer of chemically weathered soil
Dry & cold climates may produce only a thin layer of chemically weathered soil

32. Influence the soil’s physical and chemical properties
ORGANISMS
Influence the soil’s physical and chemical properties
Plants are the main source of organic matter in the soil
They also aerate the soil (provide air to the soil)

33. SLOPE / Angle Steep slopes erode quickly and have little or no soils
Flat slope has little erosion and produces quality soils.

34. Orientation North slopes are cooler and wetter (poorer soil)
South slopes are warmer and drier (better soil)
*** soil forms best in hot/moist, tropical climates ***

35. Soil Profile
Soil profile shows all of the soil horizons in a cross section
Soil varies in composition, texture, structure, & color at different depths
Variations divide the soil into zones called horizons

36. Mature soils have three distinct soil horizons A Horizon B Horizon
C Horizon

37. Soil Horizons A Horizon = known as topsoil
consists of organic matter, insects, fungi & microorganisms
B Horizon = known as subsoil
Contains:
fine clay particles washed out of A horizon, by water that filters through pore spaces
Lower limit of most plant roots & burrowing animals

38. C Horizon = falls between the B horizon and unaltered parent material
Contains partially weathered parent material
Unaltered parent material
The bedrock that has NOT gone through any weathering

39. Soil Erosion Loss of fertile topsoil major problem in United States
We are eroding much more soil now than we have in the past

40. Rate of Erosion Cross out “water carries tiny pieces of _______
All the way to
Rate of Erosion

41. Rate of Erosion
Depends on soil characteristics, climate & slope, and the amount of vegetation
Accelerated by human activity
Removal of natural vegetation by:
Farming
Logging
Construction

42. Water erodes soil faster than wind
Exception with unprotected spoil. EX: 1930’s Dust Bowl

43. Sediment Deposition Reservoirs fill up with sediment
Sediments are contaminated by pesticides and fertilizers

44. Controlling Erosion Planting rows of trees called windbreaks
Prevents wind erosion
Terracing hillsides
Prevents water & wind erosion

45. Soil erosion is accelerated by removing protective natural vegetation
Human activities such as farming, logging, and construction increase erosion greatly = soil is being eroded faster than it can be created

46. EX: rivers are becoming clogged with eroded sediment from farmland = interfere with river transportation, increase pesticide levels in water, fill up reservoirs, and increase flooding