1. WEATHERING, EROSION,
AND SOIL
The Walker School
Geology

2. Weathering vs. Erosion
 Weathering the decomposition of earth rocks, soils
and their minerals through direct contact with the
planet's atmosphere. Weathering occurs in situ, or
quot;with no movementquot;, and thus should not to be
confused with erosion, which involves the movement
and disintegration of rocks and minerals by agents
such as water, ice, wind, and gravity.

3. Formation of the Grand Canyon
http://www.youtube.com/watch?v=ktf73HNZZGY
Debris flows
shown in this clip
erode rock along
the walls of the
canyon.

4. Arches National Park
http://www.nps.gov/arch/National
Erosion takes
place at different
rates – called
differential erosion
Produces:
hoodoos, spires,
arches, and
pedestals
Fig. 6-CO, pp. 168-169

5. Hoodoos, Bryson Canyon National Park
http://www.nps.gov/brca/

6. Types of Weathering
 Mechanical
 Chemical
 Biological
Weathering of Granite
Fig. 6-1a, p. 170

7. PHYSICAL WEATHERING

8. Physical Weathering
 Mechanical or physical
weathering involves the
breakdown of rocks and
soils through direct
contact with atmospheric
conditions such as heat,
water, ice and pressure.
Badlands, SD

9. Abrasion
The primary process in
mechanical weathering
is abrasion (the
process by which clasts
and other particles are
reduced in size).
Talus at the base of Rocky Mountains in Canada

10. Physical Processes of Weathering
 Frost Action
 Pressure Release
 Thermal Expansion and
Contraction
 Salt Crystal Growth
 Activities of Organisms

11. Frost Action
Fig. 6-3b, p. 172

12. Frost Heaving in New York City

13. Pressure Release (Unloading)
Granite (igneous
rock) crystallizes far
below the surface, so
when it is uplifted
and the overlying
material is eroded, its
contained energy is
released by outward
Exfoliation at Stone Mountain, Georgia
expansion.

14. Thermal Expansion (Exfoliation)
Slabs of granitic
rock bounded by
sheet joints in the
Sierra Nevada, CA
Rock is a poor
conductor of heat, so
its outside heats up
more than its inside;
the surface expands
more than the
interior.
Often occurs in areas, like
deserts, where there is a large
diurnal temperature range.
Fig. 6-4a, p. 173

15. Salt Weathering (haloclasty)
 Mechanical
 Derives from an external source
(capillary rising ground water,
eolian origin, sea water along
rocky coasts, atmospheric pollution).
 Favored by dry conditions in arid
climates.
 The expanding salt crystals exert a
pressure on the walls of the rock
pores that exceeds the tensile Marine Abrasion of Granite.
strength of the rock.

16. BIOLOGICAL WEATHERING

17. Biological Weathering from Plants
Trees and other plants in
Lassen Volcanic National
Park, CA help break down
parent material into smaller
pieces and contribute to
mechanical weathering.
Fig. 6-6b, p. 174

18. CHEMICAL WEATHERING

19. Chemical Weathering
 Chemical weathering, involves the direct effect of
atmospheric chemicals, or biologically produced
chemicals (also known as biological weathering), in
the breakdown of rocks, soils and minerals.

20. Organisms
Lichens are part
fungi and part
algae. They derive
their nutrients from
the rock and
contribute to
chemical
weathering.
Fig. 6-6a, p. 174

21. Decomposition of Earth’s Materials
 Dissolution (minerals dissolve in water,
limestone dissolves to form caves)
 Hydrolysis (hydrogen ions in water
dissociate and attack minerals in rocks, ex.
forms hard water)
 Oxidation (minerals react with oxygen, ex.
formation of rust)

22. Factors Affecting Chemical Weathering
 Presence of Fractures
 Particle Size
 Climate
 Parent Material
Granite rocks in Joshua Tree
National Park, CA. Chemical
weathering is more intense along
fractures.

23. Mechanical and Chemical
Mechanical weathering speeds chemical weathering by increasing
the surface area of the rock.
Fig. 6-10, p. 180

24. Temperature and Chemical Weathering
Chemical
processes proceed
more rapidly at
high temperatures
and in the
presence of
liquids.
Chemical
weathering can
extend to
depths of 10s of
meters in the
tropics, but only
a few inches in
arid or cold
climates
Fig. 6-11, p. 180

25. Stability is the
opposite of
Bowen’s reaction
series.
Table 6-1, p. 180

26. SOIL AND ITS ORIGINS

27. Regolith
A collective
term for
sediments.

28. Soil Production through Weathering
Fig. 6-1b, p. 170

29. Soil Production through Infiltration

30. Soil Composition
Fig. 6-14a, p. 183

31. Soil Profile
Parent material, in soil
science, means the
underlying geological
material (generally bedrock
or a superficial or drift
deposit) in which soil
horizons form.
Fig. 6-14b, p. 183

32. Variables in Soil Production
 parent material
 time
 climate
 atmospheric composition
 topography
 organisms

33. Climate and Soil Formation
Fig. 6-15, p. 184

34. Pedocal
Fig. 6-16a, p. 185

35. Pedalfer
Fig. 6-16b, p. 185

36. Laterite
Fig. 6-16c, p. 185

37. Soils in the Field
Shows Laterite in
Madagascar, a
deep red soil that
forms in
response to
intense chemical
weathering.
Fig. 6-18a, p. 186

38. 12 Soil Types
http://soils.ag.uidaho.edu/soilorders/orders.htm

39. World Soils

40. U.S. Soil Orders

41. Most of the World’s Soils are Under Threat
38% of the
world’s
croplands have
serious topsoil
erosion.

42. Activities of Organisms
Churn soil; Break
down nutrients;
Provide
pathways for
gases to escape

44. SOIL PROBLEMS

45. Soil Expansion
$6 billion in damage a
year to foundations,
roadways, sidewalks
and other structure.
Fig. 6-21b, p. 188

46. Erosion
Rill
Gully
Fig. 6-22a, p. 189

47. Stalinization
Stunts Crop Growth;
Lowers Crop Yields;
Eventually Kills Plants;
Ruins the Land

48. Slash and Burn Agriculture
Practices deplete tropical
soils of nutrients for
agriculture.
Fig. 6-19a, p. 187

49. Desertification
70% of the world’s dry
lands are suffering.

50. Causes and Consequences of Desertification

51. Soil Conservation
Table 6-2, p. 190