1. Weathering and erosion
Chapter 12
Weathering and erosion

2. Weathering Process
Weathering changes the physical form of chemical composition of exposed rock.
Mechanical weathering physically breaks down rock into smaller pieces
agents include ice, plants & animals, gravity, running water, wind
Physical changes in the rock can increase weathering
exfoliation

3. Mechanical weathering
Ice wedging occurs when water gets into cracks and then freezes, causing cracks to widen
Thawing allows more water in
Rocks eventually split apart
Organic activity of plants and animals:
Plant roots get into cracks and widen as they grow
Animals dig – expose rock to weathering
Abrasion occurs when rocks collide with each other
Gravity, water, wind

4. Chemical Weathering (Decomposition)
Occurs when chemical reactions take place between rock minerals and water, CO2, O, and acids
Change the chemical composition and physical appearance
Hydrolysis – change in mineral composition due to chemical reaction with water
Minerals are dissolved in water and carried to lower rock layers = leaching -> mineral ore deposits

5. Chemical Weathering
Carbon dioxide dissolves in water to form carbonic acid
H2O + CO2 -> H2CO3
Some minerals combine with H2CO3 to form new substance = carbonation
Calcite reacts with H2CO3 to form calcium bicarbonate which dissolves easily in water -> underground caverns in limestone form this way

6. Chemical Weathering
Oxidation occurs when metallic elements combine with oxygen
Iron combines with oxygen to form rust
4Fe + 3O2 -> 2Fe2O3
Red soil in SE US
Rain water is slightly acidic naturally but is a damaging weathering agent in industrial areas
Some plants such as lichens and mosses produce acids that can dissolve the surface of rock and seep into cracks to break rock.

7. Rates of Weathering Major factors: Composition Exposure Climate
Topography

8. Composition Sedimentary weathers fastest of 3 types
Limestone & others with calcite fastest
carbonation
Rate of mechanical weathering depends on what is holding sediments together
Igneous and metamorphic
Quartz is affected least (one of hardest minerals)

9. Exposure More exposure = faster weathering
Most rock at surface are broken by fractures an joints -> split into smaller pieces
Fractures & joints increase surface area to allow more rapid weathering

10. Climate Rainfall and freezing/thawing have biggest effect
Variable weather causes ice wedging fractures
Exposes new rock surface
Chemical weathering attacks fractured rock more quickly
Temperatures changing daily or seasonally speed up chemical reactions
Hot, dry climates slow weathering because of lack of water for reactions
Weathering is very slow in cold climates – cold slows rxns
Warm, humid climates are most damaging
Faster chemical rxn rate, moisture is damaging

11. Cleopatra’s Needle
Cleopatra's Needle, an obelisk with inscribed Egyptian hieroglyphs, remained virtually unweathered for more than 3,000 years in Egypt (A), but then began to weather rapidly when brought to New York City's Central Park in the late 1800s (B).  The difference in climate (arid versus humid) accounts for the change in weathering rates. 

12. Topography & Elevation
Ice wedging increases with elevation
Rock fragments on steep slopes are pulled down by gravity and washed along by heavy rain.

13. Weathering and Soil Weathering breaks and alters all exposed rock
Forms regolith (rock fragments) over solid bedrock
Upper regions of regolith weather faster and form a layer of fine particles = basic components of soil
Soil = mixture of minerals (from rock), water, gasses, and humus (organic matter from decaying organisms)

14. Soil Composition
Rock material in soil consists of sand, silt, & clay (in that order from largest to smallest)
Proportions of these particles depends on the “parent rock” = the rock it was weathered from
Rich in clay = rich in feldspar
Sandy soils = rich in quartz
Silt – gives soil a gritty feel. Often carried by river currents and deposited on soils near river banks
Minerals may be carried by wind or water away from the parent rock = transported soil
may have a different composition than underlying bedrock.

15. Soil Profile Shows the layers of soil and the underlying bedrock
3 main layers = horizons
A horizon = topsoil – contains humus, other organic matter, and living organisms
B horizon = subsoil – minerals leached from topsoil, clay, little humus
C horizon = bottom layer – partially weathered bedrock (will eventually weather into B then A horizon)
Transported soils do not have horizons

16. Soil Profile
A horizon
B horizon
C horizon

17. Climate Influences Soil Formation
Humid tropical climate – chemical weathering causes thick laterites to develop
Contain iron and aluminum – do not dissolve easily
Rain leaches mineral form A & B horizons and washes away topsoil (\ A is very thin)
Dense vegetation continuously adds organic material to topsoil to support plants
Desert climate – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith
Arctic – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith

18. Climate Influences Soil Formation
Temperate climate – 2 main soil types:
Pedalfer
Forms with more than 65 cm of rain annually
Contains clay, quartz & iron compounds
Gulf Coast States & States East of Mississippi
Pedocal
Forms with less than 65 cm of rain annually
Contains much calcium carbonate – combines with soil H to make soil less acidic and very fertile
SW States and States West of Mississippi

19. Topography Affects Soil Formation
Rainwater washes soil from steep slopes causing topsoil to be thicker at the bottom than on the slope
Topsoil left on slopes is often too thin and dry to support plant growth \ does not accumulate organic matter = poor for cultivation
Lowlands that hold water have thick, wet soils with much organic matter to form humus = good for cultivation

20. Erosion Soil erosion occurs about as slowly as residual soil forms
Unwise use of land and unusual climate conditions accelerate erosion
Unwise farming and ranching
Clearing forests & overgrazing
Expose soils for increased erosion
Gullying = water runs through furrows in plowed land washing away soil until furrows become deep gullies

21. Gullying

22. Erosion Sheet erosion strips away layers of soil to expose subsoil
Continuous rainfall evenly washes away topsoil which may end up clogging streams or changing their course
In dry periods wind carries away loose soil as clouds of dust and drifting sand

23. Erosion
Constant erosion reduces fertility by removing the A horizon and exposing the B horizon
B horizon resists plant growth \ has no protection
All soil layers could be removed within a few years

24. Rapid Erosion Can Be Prevented by Soil Conservation
Cover crops can protect soil
Some crop planting methods can reduce erosion
Contour plowing
Strip-cropping
Terracing
Crop rotation

25. Contour Plowing
Plowing follows the shape of the land and prevents water from running directly down slope

26. Strip-Cropping
Crops are planted in alternating bands to protect the soil by absorbing and holding water.
Ex. Corn bands alternating with alfalfa (cover crop)
When combined with contour plowing soil erosion can be reduced 75%

27. Terracing Cutting step-like ridges that follow the contour of the land
Slows the downslope movement of water

28. Crop Rotation
Farmers alternate crops every year to stop erosion early and repair the damage already caused by erosion

29. Urban Conservation
In urban areas clearing vegetation and removing soil to build houses, roads, etc contributes to erosion thereby making conservation measures necessary.

30. Mass Movement
Gravity causes rock fragments to move downhill = mass movement
Rocks break into fragments which accumulate at the base of slopes in piles called talus

31. Rapid Mass Movements
Rockfall – the fall of rock from a steep cliff is the most rapid mass movement
Landslide – masses of loose rock and soil move downslope
May be triggered by heavy rains, spring thaw, volcanic eruptions and earthquakes

32. Rapid Mass Movements
Mudflows - masses of mud flow downslope and spread out in a fan shape at the bottom
Occur in dry mountainous regions during heavy rains
Slump – large block of soil and rock becomes unstable and moves downhill in one piece

33. Slow Mass Movements
Solifluction – in regions where soil is permanently frozen, thawing of the topsoil makes it muddy and it slowly flows downslope.
Creep – extremely slow downhill movement of weathered material
Most effective mass movement
Usually undetected

34. Landforms Shaped by weathering and erosion
Result from two opposing forces which bend, break and lift the crust then wear it down
Mountains - steep landforms of high elevation uplifted by tectonic forces
Plains – flat landforms usually near sea level
Plateaus – broad, flat landforms at high elevations

35. Mountains Youthful – still being uplifted
Rugged shape, sharp peaks & deep, narrow valleys
Mature mountains – no longer rising
Rounded peaks and gentle slopes
Peneplain – old stage, almost featureless
Usually low rolling hills

36. Peneplain
Knobs of hard rock that resist weathering and protrude above peneplain = monadnocks
May be mistaken for depositional plain but underlying rock is folded and tilted

37. Plateaus
Young plateaus have deep stream valleys separating broad, flat regions
Mature plateaus are eroded into rugged hills & valleys
As plateaus age they may form smaller, table-like areas = mesas
Mesas weather into small, narrow topped formations - buttes

38. Landforms In dry regions landforms have steep walls and flat tops.
In humid climates they are more rounded due to increased weathering
Minor landforms = hills, valleys and dunes