Weathering
revised 2/11/04
Weathering figures
Weathering
- Weathering - Process of breaking down and
alteration of materials at the earth’s surface
- Types of Weathering
- Mechanical (or Physical) Weathering
- materials broken down into smaller fragments
- Ex: qtz in granite
-> qtz sand
- Chemical Weathering - Chemical alteration
or dissolution
- Ex: feldspar
-> clay; calcite
-> Ca2+ + ________
- Erosion
- Weathering plus transport
- Transporting Agents
- __________________ - bedload, suspended
load, dissolved load
- __________________
- __________________
Mechanical Weathering Processes
- Processes of Mechanical Weathering serves
to increase surface area which enhances chemical weathering
- Abrasion - collision, grinding of particles
- Running water and waves
- Wind
- Glaciers
- Thermal expansion and contraction -
Hot, dry regions (T > 30ºC);
No experimental confirmation, but . . .
- Frost wedging
- Develops talus slopes - large piles
of angular rocks beneath cliffs
- Root wedging (organic activity)
- Fracturing
- ___________ - regular pattern of fractures
in rock; due to crustal stresses; process called ___________
- ____________ - type of joint believed
to be caused by release of pressure (unloading)
- Exfoliation Dome
Figure. Exfoliation dome in granite in
NH.
Rock Mineralogy and Weathering
- Why this distribution of sedimentary minerals?
Factors Controlling Chemical Weathering
- ___________
- Chemical weathering more rapid in
warm , humid climate
- Chemical reactions faster at higher
temperature (rates double per 10°C increase in temperature)
- Water important for weathering reactions
- ___________
- Soil: surface accumulation; rock and
mineral matter, organic matter (humus), water, and air
- Stores water, biologically active, acidic
Factors Controlling Chemical Weathering (cont.)
- Why is water a key player in weathering?
- Serves as chemical highway
- carries ions in and out; O2
in
- Universal solvent (polar)
- More effective if acidic
- What is the pH of water exposed to the
air?
- Carbonic acid: CO2 + H
2O = __________
Chemical Weathering Processes
- Three important chemical weathering processes
- __________________
- __________________
- __________________
Chemical Weathering Processes
Dissolution
- Dissolution - minerals dissolve into soluble
species (mostly ions)
- Halite dissolution NaCl =
Na+ + Cl-
- Quartz =
quartz grains and dissolved SiO2
- SiO2 + 2H2O
= ______________ [silicic acid]
- Mg-pyroxene dissolution
- MgSiO3 + 2H2CO
3 + H2O =
Mg2+ + 2 HCO3- + H4SiO
4
- Calcite dissolution:
- CaCO3 + H2CO
3 = Ca2+
+ 2____________ (bicarbonate)
Chemical Weathering Processes
Oxidation
- Oxidation - ____________ transferred from
one element to another; involves change of valance
- "Rusting" 4Fe° + 3O2
= 2Fe2O3
- Fe° donates electron, O2
accepts electron
- Weathering of Fe-pyroxene:
4FeSiO3 + O2 + 8H
2O = 2___________ + 4H
4SiO4
- Oxidation of organic matter:
CH2O + O2 =
CO2 + H2O = ______________
Chemical Weathering Processes
Hydrolysis
- Hydrolysis - H+ substitutes
for a ____________ in a silicate mineral
- Key process in silicate weathering
- Weathering of a feldspar
- K-spar weathers to ___________________
(clay), potassium ions, and dissolved silica
- 2KAlSi3O8 (K-spar)
+ 2H+ + H2O =
Al2Si2O5(OH)4 (kaolinite)
+ 2K+ + 4SiO2
Combined Chemical and Mechanical Weathering
- Spheroidal weathering - weathering processes
which tend to give rock spheroidal shape.
- Salt cracking - growing of salt crystals
in cracks causes expansion of crack
