5. WEATHERING, EROSION, AND SOIL
-weathering- physical breakdown (disintegration) and chemical alteration (decomposition)
of rocks near the earth's surface. Rocks and minerals are altered to be more stable at surface
conditions (more in equilibrium with new set of environmental conditions).
-erosion- physical removal of rock particles (weathering makes it easier to erode).
-transportation- movement of eroded particles by wind, water, etc. (may become sedimentary rock).
-differential weathering- rocks usually are not homogeneous throughout, so in same area/outcrop
they weather at different rates, producing uneven surfaces (figs. 5.1, 5.2, 5.3).
Two ways rocks can be weathered: mechanical (physical) and chemical weathering.
I. Mechanical Weathering- physical disintegration of rock into smaller pieces (fig. 5.4); chemical composition doesn't change. The physical processes responsible for mechanical weathering are:
1- Frost Action- repeated freezing and thawing of water in rocks (water expands ~9% when
frozen; most important in areas where the temperature commonly fluctuates above and below freezing). The two phenomena of frost action are:
frost wedging-opening/widening of fractures in rocks by repeated freezing of water(fig.5.5). This process usually forms talus-thick accumulation of mechanically weathered rock debris at base of slopes (fig. 5.6).
frost heaving- layer of sediment or soil lifted by freezing (expanding) water.
2- Pressure Release- reduction of pressure on body of rock formed deep in earth under high
pressure (granite batholiths). Pressure release usually resulted:
a) sheet joints- large fractures that form parallel to rock surface due to expansion (layers break off in loose, rounded slabs) (figs. 5.7, 5.9).
b) exfoliation dome- large rounded dome of rock resulting from removal of slabs of rock bounded by sheet joints (granite) (fig. 5.8).
3- Thermal Expansion and Contraction- rock volume changes during heating and cooling. -differential expansion- different rock types / minerals expand at different rates.
4- Growth of Salt Crystal- similar to frost wedging, crystal growth from solution can cause disaggregation of rocks. Effective in hot arid areas.
5- Activities of Organisms- burrowing; plant roots (figs. 5.10, 5.11)
II. Chemical Weathering- rock decomposed by chemical alteration of parent material. Important chemical agents are oxygen, water, and acids. Common weathering processes include:
1- solution- ions in a solid become dissociated from one another in a liquid, and solid dissolves; some mins. dissolve in water (halite; fig. 5.12) and some in acid (calcite).
2- oxidation- reactions with oxygen to form oxides; in rocks, combining Fe with oxygen most
-(ex. iron + oxygen-->hematite [iron oxide]) (fig. 5.14); (important for ferromagnesian silicates).
3- hydrolysis- hydrogen ions in water (H+) replace +ve ions in silicate minerals (K+, Na+, Ca++).
Many silicate minerals (KAlSi308) weather by this process & break down to clay minerals.
III. Controls on the rate of chemical weathering:
1) particle size- chemical weathering works on the surface of particles.
-the more surface area exposed, the more effective chemical weathering is.
-expose more surface area by breaking rock into smaller pieces (mechanical weathering) (fig. 5.16).
-spheroidal weathering-corners attacked on 3 sides, edges on 2, sides on 1; becomes rounded (fig. 5.18).
2) climate- chemical weathering facilitated by (5.17):
a) higher temperatures- most chemical processes occur faster at higher temperatures.
b) presence of liquids (water)- many chemical processes occur faster in water.
3) parent material- some mineral more stable than others.
-minerals weather at rates proportional to position on Bowens reaction series (Table 5.1).
-quartz most stable mineral at Earth's surface; not subject to chemical weathering, because:
a) formed at temperatures most similar to Earth's surface.
b) Quartz (SiO2) doesn't have any ions (Na, K) that can be replaced by H+ (hydrolysis).
-regolith- layer of unconsolidated rock and mineral fragments; covers most of Earth's land surface.
-soil- regolith consisting of weathered material, water, air, and organic matter that can support life.
-soil profile- 4 horizons usually form that differ in color, texture, composition, and structure (fig. 5.19).
-O-horizon- thin layer of organic material and humus (decayed organic material).
-A-horizon- zone of leaching (topsoil); intense biological activity (roots, bacteria, etc.)
-B-horizon- zone of accumulation (subsoil).
-C-horizon- partially weathered or unweathered parent material.
-residual soil- soil formed in place by weathering of underlying bedrock (fig. 5.19a).
-transported soil- formed from material brought in from some other region (fig. 5.19b).
-Factors controlling soil formation (fig. 5.20):
a) climate- (most important) 3 major soil types depending on climate:
-humid (E.U.S. & Canada)-pedalfers B high in Al, Fe because of downward leaching by water.
-arid (western U.S.)- pedocals upward movement of water (evaporation); caliche forms
in B horiz.
-tropical (very hot & humid)- laterite- highly leached soil (only least soluble minerals left
behind); very rich in Fe and Al oxide minerals (fig. 5.22); hematite = Fe ore, bauxite = Al ore
b) parent material- different rocks in same climate can yield same soil (fig. 5-23a).
c) organic activity- the more organic activity, the more rapidly soil will develop.
d) slope- weathered material eroded from a steep slope faster than it can form 5.23b).
e) time- generally, longer a soil is developing the thicker it is (~2.5 cm/100 yrs.).
IV. Expansive soils
Soils containing clay minerals that increase in volume when wet, and shrink when dry (6% considered highly expansive); $6 billion damage annually in U.S (fig. 5.24).
V. Soil Degradation-
Decrease in soil productivity or loss of soil due to erosion.
-erosion- wind and water (together ~85% of all soil degradation; Perspective 5-2); result
of agricultural practices, overgrazing, deforestation; can be minimized by crop rotation,
contour plowing (fig. 5.27), no-till planting, and terracing (Table 5.2).
-chemical deterioration- depletion of nutrients, pollution, salinization.
-physical deterioration- compaction.