1. Weathering, Soil, Mass Movements, & Erosion
Chapter 12 Section 1

2. Weathering The breakdown of rocks at the Earth’s surface 2 types:
Mechanical
(physical)
Chemical

3. Mechanical Weathering: (disintegration) occurs when rock is split, cracked or broken into smaller pieces of the same material without changing its composition

4. Types of Physical Weathering
Frost wedging: caused by alternating freezing & thawing
also called frost action
Water expands as it freezes and widens cracks in rock
Common where temp. varies from below to above freezing
Occurs mostly in porous rocks & rocks with many cracks

5. Abrasion: rocks rub or bounce against each other when moved by wind, water, ice, or gravity.
Worn away and broken into smaller pieces
By water 
smooth, rounded particles
By wind 
flat, angular surfaces,
cuts, grooves
ventifaction

6. Biologic Activity:
plant roots grow into cracks and expand
animals dig holes in the soil
allow water and air to reach bedrock and weather it
may bring rocks to surface where they weather more quickly

7. Exfoliation or jointing:
As overlying layers of rock are removed, pressure is released and rocks can expand.
Rocks break along curved joints…parallel to surface
rock peels off like layers of an onion

8. Chemical Weathering: (decomposition) occurs when rock is broken down by chemical action and its minerals are changed into new substances
Occurs almost everywhere because of presence of water or water vapor
Other agents include acids and oxygen

9. Water and Chemical Weathering
Hydrolysis: Chemical weathering by reaction of water with other substances
Minerals (such as feldspar, hornblende, augite) dissolve into ions and react with water to form clay minerals
Water’s chemical effects is increased by the presence of acids dissolved in water

10. Rainwater is naturally acidic
carbon dioxide dissolved in water produces carbonic acid
The acid in rainwater reacts with & dissolves some rocks
Ex. Marble & Limestone

11. Pollution Gases: from volcanoes, cars (nitrogen oxides), or power plants (sulfur dioxides) can make rainwater even more acidic (ACID RAIN)

12. Caves are carved out of limestone as water flows through them

13. Other rocks are very resistant to acid and do not dissolve
Ex. Quartzite

14. Organic Lichens: fungi + algae that make acids which can dissolve rock
Acids formed by the decay of dead plants & animals can contribute to chemical weathering.

15. Oxygen and Chemical Weathering
Oxidation: Oxygen reacts with minerals in rock
Ex. Iron + Oxygen = Rust (iron oxide)
Red  hematite
Yellowish-brown  limonite

16. Under normal conditions weathering is slow.
Rates of Weathering
Under normal conditions weathering is slow.
Several factors affect the rate of weathering.
Surface area
Composition of the rock
Climate

17. Surface Exposure & Weathering Rate
Breaking rocks into smaller pieces exposes more surface area and causes them to weather faster. 

18. Composition of Rock
Different types of rocks weather at different rates.
Determined by:
the rock’s mineral composition
type of weathering
More resistant (quartz-based) rocks weather slower than less resistant (feldspar, hornblende, biotite mica, augite, calcite, gypsum-based) rocks

19. Climate & Weathering Rate
Arid (dry):
slow weathering
Mostly mechanical (by wind)
Cold & Humid (wet):
more physical weathering
mostly frost action
Warm & Humid (wet):
more chemical weathering
higher temp. speeds up chemical reactions

20. Weathering, Soil, Mass Movements & Erosion
Chapter 12 Section 2

21. What is soil and how does it form?
Soil is:
the mixture of weathered rock and organic material that covers bedrock and in which plants with roots can grow.
typically composed of 3 layers or horizons.

22. Types of Soils Parent material- the material that soil comes from
Residual Soil- parent material is local bedrock (beneath the soil)
Transported Soil- formed from parent material in different areas and is brought into an area by wind or water (rivers or glaciers)
Most soils in New England are transported soils

23. Stages of Soil Formation
“Mature” soils:
have had a long time to form
have 3 distinct zones
Can be seen in a soil profile
cross section exposed by digging down to the parent material

24. Soil Profile A horizon (topsoil) B horizon (subsoil) C horizon
top layer
rich in organic remains (humus)
gray-black color
B horizon (subsoil)
middle layer
rich in minerals
Washed down from A horizon
iron, clay, calcium, magnesium
red-brown color
C horizon
lowest layer
slightly weathered parent material
color depends on parent material

25. Soil Composition Soil is classified by composition (make up).
1000s of different types
Strong link between composition & climate

26. Weathering, Soil, Mass Movements & Erosion
Chapter 12 Section 3

27. Mass movements and erosion carry away weathered materials.

28. Erosion
Erosion- removal and transport of materials by natural agents such as wind and running water
**can only take place after weathering has occurred

29. Mass Movements
Mass movements- Movement of material down slope under the influence of gravity
Talus- rock fragments that have been weathered from a cliff and pulled down by gravity
Landslide- general name for the movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff
Most likely on steep slopes
Tend to occur after heavy rains or during the spring when large amounts of snow are melting

30. Specific types of mass movements
Fast moving
Flows
Earthflows
Mudflows
Landslides
Avalanches
Rockslides
Slow moving
Creep
Slump

31. Creep
Very low velocity (slow), gradual, steady movement of surface soil
Movement is imperceptible, but effects can be seen
Fence posts, poles, &
trees to lean downhill

32. Slump
Blocks of land tilt and move downhill along a surface that curves into the slope.
Tends to occur because a slope has become too steep for the bottom of the slope to support the soil at the top.

33. Flows Earthflows: soil saturated with water, flows downhill
Slower and less fluid than mudflow
Affected by amount of water present, composition of soil, and steepness of the slope
Mudflows: rapid movement of water that contains large amounts of suspended clay and silt
Can move rocks, boulders, and trees
Tend to occur in drier regions that experience infrequent but heavy rains

34. Landslides/Avalanches & Rockslides
High velocity event, material is loose & dry.

35. Erosion and Landforms
Rivers, streams, glaciers, wind, and ocean waves and currents are all agents of erosion.
Climate and composition of the rock also play an important role
Erosion helps in shaping the landscape
Topography depends on the “balance” between uplifting forces and erosion

36. Soil as a Resource Soil fertility- ability of soil to grow plants
Chapter 12 Section 4
Soil fertility- ability of soil to grow plants
Proportion of minerals, water & organic matter determines fertility
Threats to fertility:
Soil depletion- when nutrients are lacking or taken out of soil to the point where it can no longer grow a usable crop
Reduce depletion by allowing field to lay fallow or by rotating crops
Often add fertilizers instead
Runoff affects ecological balance (ex. water quality)
Salinization- water from irrigation leaves behind minerals as it evaporates
Eventually, soil cannot support crop growth

37. Erosion and Soil Conservation
Erosion is accelerated by human activity.
Farming, construction, mining
Soil erosion must be controlled using soil conservation methods.
Plant windbreaks/shelterbelts (trees reduce wind erosion)
Constructing terraces (“steps”, slows water flow)
Implementing erosion-reducing farming methods
Contour farming (plow parallel to land shape, slows water flow)
Strip cropping (alternate rows, little ground cover/complete coverage, planted perpendicular to dominate wind direction)
No-till (plowing, planting, fertilizing, and weed control done at same time, land not disturbed again until harvest)

38. contour
windbreak
terracing
Strip-cropping