1. Chapter 7: WEATHERING AND EROSION

2. Objectives Describe the two major kinds of rock weathering.
Identify three end products of weathering.
Explain the difference between weathering, erosion, and mass wasting.
Describe how ice, water, and air transport regolith across Earth’s surface.
Define and give examples of mass wasting by slope failure and/or sediment flow.

3. Weathering-The First Step in the Rock Cycle

4. Weathering-The First Step in the Rock Cycle
How rocks disintegrate
Weathering
The chemical and physical breakdown of rock exposed to air, moisture and living organisms
Regolith
A loose layer of fragments that covers much of Earth’s surface
Soil
The uppermost layer of regolith, which can support rooted plants
The rock in the photo has weathered in place with little erosion, forming soil

5. Weathering: Two Types Mechanical engineering Chemical weathering
The breakdown of rock into solid fragments by physical processes
Chemical composition of rock NOT altered
Chemical weathering
The decomposition of rocks and minerals by chemical and biochemical reactions

6. Joints
A fracture of rock , along which no appreciable movement has occurred
Sheet jointing or exfoliation
Frost wedging
Abrasion
The gradual wearing down of bedrock by the constant battering of loose particles transported by wind, water or ice
The jointing in these rocks has exposed new surface area which has broken and smoothed due to wind, water and ice.

7. Weathering exfoliation and frost wedging

8. Frost wedging and biomechanical weathering

9. Chemical weathering Dissolution
The separation of materials into ions in a solution by a solvent, such as water or acid
Rainwater acts as weak solution of carbonic acid
Anthropogenic actions influence acidity of rainwater
The marble grave marker has been attacked by acidic rain because of the calcite composition. The grave marker on the right, while old, has not been dissolved because of its granite composition

10. Chemical weathering: ion exchange and the chemical breakdown of feldspar

11. Factors affecting weathering
Tectonic setting
Young, rising mountains weather relatively rapidly
Mechanical weathering most common

12. Factors affecting weathering
Rock composition
Minerals weather at different rates
Calcite weathers quickly through dissolution
Quartz is very resistant to chemical and mechanical weathering
Mafic rocks with ferromagnesian minerals weather more easily

13. Factors affecting weathering
Rock structure
Distribution of joints influence rate of weathering
Relatively close joints weather faster

14. Factors affecting weathering
Topography
Weathering occurs faster on steeper slopes
Rockslides

15. Factors affecting weathering
Vegetation
Contribute to mechanical and chemical weathering
Promotes weathering due to increased water retention
Vegetation removal increases soil loss
Vegetation can both hold water
And increase weathering. If removed
Rocks may also be vulnerable to abrasion

16. Factors affecting weathering
Biologic activity
Presence of bacteria can increase breakdown of rock

17. Factors affecting weathering
Climate
Chemical weathering is more prevalent in warm, wet tropical climates
Mechanical weathering less important here
Mechanical weathering is more prevalent in cold, relatively dry regions
Chemical weathering occurs slowly here
Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering, i.e. New York area

18. Factors affecting weathering: color dots on map match colors on chart

19. Products of Weathering
Clay
Tiny mineral particles of any kind that have physical properties like those of the clay minerals
Clays are hydrous alumino-silicate minerals

20. Products of Weathering
Sand
A sediment made of relatively coarse mineral grains
Soil
Mixture of minerals with different grain sizes, along with some materials of biologic origin
Humus
Partially decayed organic matter in soil

21. Erosion and Mass Wasting
Erosion is the removal of weathering products from the source and most often occurs by water
Erosion
The wearing away of bedrock and transport of loosened particles by a fluid, such as water
Example: Sediment moved along the bottom of a stream

22. Erosion and Mass Wasting
Erosion by wind
Particles of sand are transported close to the surface.
finer particles of silt and clay can be transported great distances
Erosion by ice
Glacier
A semi-permanent or perenially frozen body of ice, consisting of recrystallized snow, that moves under the pull of gravity
Wind-blown fine sediments such as this dust cloud can
Be transported across oceans

23. Erosion and Mass Wasting
Left: deposits of unsorted glacial till from glacier Right: rock polished and striated by glacier

24. Erosion by ice: glacier removes, breaks and transports rock pieces glaciers scour valleys and deposit piles of debris as moraines

25. Erosion and Mass Wasting
The downslope movement of regolith and/or bedrock masses due to the pull of gravity
Slope failure
Falling, slumping or sliding of relatively coherent masses of rock

26. Erosion and Mass Wasting: Rock slide, rock fall, and slumping result in downhill transport of broken rock

27. Erosion and Mass Wasting
Flow: If water or air combines with the downward movement, the regolith can “flow” downhill
Creep
The imperceptibly slow downslope flow of regolith
Unstable slopes move very slowly over long periods of time

28. Erosion and Mass Wasting

29. Why do major landslides occur near plate boundaries?
Tectonics and mass wasting
World’s major historic landslides clustered near converging lithospheric plates
High mountains undergo rapid weathering
Earthquakes near plate
boundaries can trigger landslides
This massive slide was triggered by
A magnitude 9 earthquake in Alaska
near a subduction zone.

30. Critical Thinking
On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why?
Why are some granite bodies extensively jointed, while others are essentially joint free?

31. Critical Thinking
On Earth, clay minerals are the most common products of weathering. Samples from the Moon do not contain any clay minerals. Why?
Why are some granite bodies extensively jointed, while others are essentially joint free?