1. Weathering

2. Weathering: the disintegration, or breakdown of rock material

3. Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces

4. Chemical Weathering: breakdown as a result of chemical reactions
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-

5. Mechanical Weathering
Physical breakup
pressure release
water: freeze - thaw cycles
crystallization of salt in cracks
thermal expansion and contraction
plant roots
friction and impact
All this increases the total surface area exposed to weathering processes.

6. Mechanical Weathering
Exfoliation:
Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.

7. Mechanical Weathering
Many igneous rocks, particularly granite, are formed at great depth.
When they come to the surface, they expand.
Creates a morphology with exfoliation slabs – slabs sheet off in onionlike layers
Stone Mountain, Georgia
Expansion of rocks pose a risk in deep mines – explode off walls
These fractures, or joints can enhance frost wedging and other erosive processes

8. Sheet Joints (Exfoliation)

9. Half Dome, Yosemite, CA Stone Mountain, GA
Summit of Half Dome, Yosemite, shows exfoliation
Half Dome, Yosemite, CA
Stone Mountain, GA

10. Exfoliated Domes, Yosemite

11. Stone Mountain, Georgia, showing the product of exfoliation due to unloading
Summit of Half Dome, Yosemite, shows exfoliation
Stone Mountain, GA

12. Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints

13. Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.
Effects of freeze thaw on granite. Mt. San Jacinto,southern California, Transverse Ranges.  About 8,500 feet.

14. Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments.
Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks.
Weathering
Rain may enhance this process

15. Plant Roots

16. Friction and Repeated Impact

17. Burrowing of Animals

18. Role of Physical Weathering
Reduces rock material to smaller fragments that are easier to transport
2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

19. Surface Area and Weathering

20. Rates of weathering
Joints in a rock are a pathway for water – they can enhance mechanical weathering
Water flowing through joints attacks the rock, rounding corners.
As minerals are converted to clay, the rock expands, breaking outer layers off
Can enhance mechanical weathering

21. Chemical Weathering
Definition: transformation/decomposition of one mineral into another through chemical changes
Agents of chemical weathering:
Water
Oxygen
CO2
Living organisms
Acid Rain
Mineral breakdown
carbonate dissolves
primary minerals --> secondary minerals (mostly clays)
Net loss of elements retained in the soil.

22. Water is the main operator:
Chemical Weathering
Water is the main operator:
Dissolution
Many ionic and organic compounds dissolve in water
Silica, K, Na, Mg, Ca, Cl, CO3, SO4
Acid Reactions
CO2 dissolves in rain water and creates carbonic acid
Carbonic acid easily weathers limestone and marble

23. Dissolution
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-
water + carbon dioxide + calcite
dissolve into calcium ion
and bicarbonate ion
Biological activity in soils
generates substantial CO2
Bicarbonate is the dominant
ion in surface runoff.

24. Chemical Weathering Oxidation Living Organisms
Oxygen dissolved in water promotes oxidation of sulfides, ferrous oxides (rust), native metals
Living Organisms
Plant material makes H+ ions available
Lichens

25. Chemical Weathering
Hydration: attachment of water molecules to crystalline structure of a rock, causing expansion and weakness
Hydrolysis: combination of hydrogen and oxygen in water with rock to form new substances

26. Solution: process by which rock is dissolved in water
Chemical Weathering
Solution: process by which rock is dissolved in water
Is strongly influenced by pH and temperature
When water becomes saturated, chemicals may precipitate out forming evaporite deposits.
Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.

27. Resistance to Weathering
First to
Crystallize
Fast Weathering
Bowen’s
Reaction
Series
Goldrich
Stability
Series
Last to
Crystallize
Slow Weathering

28. Olivine/pyroxene to clay
+ H2CO3 (acid)

29. Feldspars to clay
+ H2CO3 (acid)

30. Quartz to quartz (!)
+ anything

31. Calcite to …….
+ anything
nothing

32. Karst Topography
A type of landscape in rainy regions where there is limestone near the surface, characterized by caves, sinkholes, and disappearing streams.
Created by chemical weathering of limestone

33. ‘Karst’ landforms develop in areas underlain with limestone
This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July.
‘Karst’ landforms develop in areas underlain with limestone
When Lake Jackson’s sinks rupture and the lakewater drains out, the process is referred to as a drawdown. It has occurred many times during the last century, in 1900, 1907, 1909, 1932, 1935, 1936, 1957, 1982, and again in 1999.

34. Erosion
The process by which water, ice, wind or gravity moves fragments of rock and soil.

35. Water Erosion
Rivers, streams, and runoff

36. Ice Erosion
Glaciers

37. Wind Erosion

38. Mass Movements
Landslides, mudslides, slump and creep

39. Rates of weathering Weathering Climate
Temperature and moisture characteristics
Chemical weathering
Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering
Least effective in polar regions (water is locked up as ice) and arid regions (little water)
Mechanical weathering
Enhanced where there are frequent freeze-thaw cycles

40. Mechanical and Chemical Weathering
Fracturing, disintegration caused by mechanical weathering exposes more surface area.
Greater surface area, means more places for chemical action to occur.

42. Alaska
Seattle
Altiplano
Amazon