Weathering and Erosion

1
Weathering and Erosion
2
Weathering

• Changes that occur in sediments and rocks near
  Earths surface
• Two main types chemical and physical

Chemical minerals are chemically altered or
dissolved
Physical fragmentation
3
Physical and chemical weathering go hand in hand
4
Factors Controlling Rates of Weathering
5
Chemical Weathering
6
Chemical Weathering

• This process occurs because minerals formed deep
  in the Earths interior are not stable under
  surface conditions.
• Stability is generally the reverse of Bowens
  reaction series.
• The principle agent of chemical weathering is
  water.

• Quartz very stable
• Feldspars form clay minerals
• Mafic minerals alter to oxides

7
Controls on the rate of weathering
Chemical stability Measure of a substances
ability to remain in a given chemical form rather
than to react to form a different chemical
substance. Two controls on chemical stability
solubility and rate of dissolution. Solubility
Amount of the mineral dissolved in water when the
solution is saturated. Higher solubility lower
weathering stability. Rate of dissolution The
amount of mineral that dissolves in an
unsaturated solution in a given amount of time.
8
Weathering and Making Coffee
Water dissolves some of the solid, leaving behind
an altered material, .
.and produces a solution containing substances
drawn from the original solid
Fig. 7.4
9
Etched and corroded feldspar in the soil zone
Fig. 6.2
Fig. 7.2
10
Importance of fractures
Fig. 7.5
11
Microscopic view of disintegration of granite
Cracks form along crystal boundaries. Feldspar,
biotite and magnetite start to decay, while
quartz does not.
The decay progresses, and as the crack opens, the
rock weakens and disintegrates
Fig. 7.3
12
Weatheredgranite
13
Carbon Dioxide
Fig. 7.6
14
Bicarbonate reactions
Fig. 7.6
15
Weathering reactions

• Atmospheric carbon dioxide dissolves in
  rainwater
• CO2 (carbon dioxide) H2O (water) --gt H2CO3
  (carbonic acid)
• Weathering of silicates uses carbonic acid,
  which reduces the carbon dioxide level in
  the atmosphere
• 2KAlSi3O8 2H2CO3 H20 --gt Al2Si2O5(OH)4
  4SiO2 2K 2HCO3-
• feldspar carbonic acid water --gt
  
  kaolinite silica potassium ions
  bicarbonate ions

16
Chemical weathering of carbonates

• Carbonates weather fast, especially in the
  presence of acid.
• Calcite acid carbonic ions --gt calcium
  bicarbonate ions

Weathered limestone in 2500-year-old Greek ruins
in Italy (Fig. 7.7)
17
Formation of iron oxide
18
Weathering oxides provide color to the desert
landscape
Fig. 7.9
19
Mechanical Weathering
20
Some notes

• Mechanical weathering physical weathering.
• Physical weathering is most prominent in arid
  regions (chemical weathering is minimal).

21
How Rocks Break

• Natural zones of weakness
• Joints Formed in massive rocks. Large cracks.
  Formed below surface. Meters apart.
• Natural foliation in rocks (surfaces along which
  rocks part).
• Activity by organisms
• Frost wedging Breaking that results from the
  expansion of water as it freezes after
  infiltrating into crack.
• Mineral crystallization
• Alternating heat and cold cycles (desert
  temperatures drop 43 to 15 in an hour at
  twilight)
• Exfoliation Physical process where large flat
  or curved sheets of rock detach from an outcrop.
• Spheroidal weathering Same as exfoliation, but
  on a smaller scale.
• Other forces (glaciers, waves, etc.).

22
Exfoliation Dome in Yosemite
Fig. 7.13
23
Joint-controlled weathering
Fig. 7.10
24
Role of Organisms in Weathering
Fig. 7.11
25
Gneiss Boulder Fractured by Frost Action
Fig. 7.12
26
Spheroidal Weathering
Fig. 7.14
27
Soil
28
Soil Terminology

• Regolith A layer of loose, unconsolidated
  material overlying bedrock (soil is top
  layers).
• Humus Organic matter in soil.
• Horizon Layer of soil.

29
Typical soil make-up
30
Fig. 7.16
31
Climate dependence
32
U.S. Soil Distribution