Geology 1001-section 4
Last updated:
02/11/981. Chemical weathering -
2. Mechanical weathering -
* There is a feedback between these two types of weathering i.e.
SOILS - the part of the regolith that can support rooted
plants - mixture of - IMPORTANT NATURAL RESOURCE that is essential f
- mineral stability is roughly the reverse of
2. ROCK STRUCTURE - rocks with a greater
3. CLIMATE -the higher the
(a) water - results in both chemical and mechanical weathering
(b) vegetation - chemical weathering through release of
- mechanical weathering through root wedging
(c) animal life - burrowers
(d) temperature - enhanced chemical weathering at
(e) soil - affected by all of the above
4. TIME - the longer the time period the greater amount of weathering until
5. TECTONICS - movements of rocks on a large scale produce
MECHANICAL WEATHERING
This process produces a general increase in surface area.
1. Joints - closely spaced fractures with
- develops within 50 meters of the surface
2. Crystal Growth - evaporative crystals can grow in
3. Frost Wedging - in environments with many freeze-thaw cycles water
4. Heat Effects - possible effects due to daily heating and cooling
- certain effects due to rapid heating
5. Roots - act as
CHEMICAL WEATHERING
minerals developed at high T, P may be unstable at the surface of the Earth.
1. Chemical weathering reactions
(a) Development of carbonic acid by solution of CO2 in rainwater:
H2O + CO2 = H2CO3
(b) Dissolution of carbonate minerals by carbonic acid
CaCO3 + H2CO3 = Ca2+ + 2(HCO3)-
(c) Hydrolysis - H+ and OH- replace ions in minerals
e.g. hydrolysis of K-feldspar to kaolinite:
4 KAlSi3O8 + 4 H+ + 2 H2O = 4 K+ + Al4Si4O10(OH)8 + 8 SiO2
(d) Leaching - removal of soluble material by water (analogous to coffee-making).
(e) Oxidation - change of oxidation states of transition elements (esp. Fe)
e.g. Oxidation of Fe2+ in silicate minerals to goethite
4 FeO + 2 H2O + O2 = 4 FeO(OH)
2. Concentrates stable minerals - especially
3. Exfoliation and spheroidal weathering - spalling of
Basis for the stability of terrestrial hydrosphere and biosphere
- support of
- storage of
- pollutant trap
Origin - chemical/mechanical breakdown of
-organic matter is derived from
The following is a simplified soil profile:
A-Horizon - upper dark zone,
B-Horizon - brown-to-red zone,
C-Horizon - weathered
Other soil horizon zones that are localized or regional
O-Horizon - uppermost
E-Horizon - light-colored zone due to lack of
K-Horizon - Ca-carbonate-rich horizon common in
Pedalfers - soils rich in
- generally with a thin
- typical of soil in areas of moderate -to-high rainfall
- generally good agricultural soils
Pedocals - soils rich in Ca (typically as CaCO3)
- thin A- and B-horizons commonly with a crust of soil cemented by
- typical of soils developed in
- generally poorer soils (little organic matter)
Laterites - deep, red soils stripped of all silicates leaving
- very thin soil organic-rich humus layer due to constant recycling to the surface plants
- typical of equatorial rain forests (e.g. Brazil)
-results in poor soils subject to rapid damage
SOIL-FORMING FACTORS
1. Climate - warm, humid climates have
- lack of water and cool weather inhibit soil formation
2. Vegetation cover - generally
3. Soil organisms - produces more
4. Composition of parent material - responsible for the proportions of
5. Topography - regions with steep topography generally
6. Time - the more time the greater
SOIL EROSION
As population increases there is pressure on agricultural land use; generally resulting in more rapid soil erosion
Rates of soil loss
- erosion removes topsoil at 5 times its rate of formation
- » 6.7 kg of soil is lost for each 1 kg of food produced
Control of soil erosion
- effective controls exist, but must be used extensively
- as much land as possible should be in grasses
- crop rotation of row crops and solid cover crops
- steep slopes should not be farmed
World economy
- agriculture is the basis of the world economy
- on a short term, soils are non-renewable