CHAPTER 6 (Weathering and Soil)
1. Weathering is a term which describes the general process by which rocks are broken down at the Earths surface into such things as sediments, clays, soils and substances that are dissolved in water.
2. The process of weathering typically begins when the earths crust is uplifted by tectonic forces. After the physical breakup and chemical decay of exposed rocks by weathering, the loosened rock fragments and alterations products are carried away through the process of erosion.
3. Erosion relies on transporting agents such as wind, rivers, ice, snow and downward movement of materials to carry weathered products away from the source area. As weathered products are carried away, fresh rocks are exposed to further weathering. Over time, that mountain or hill is gradually worn down.
4. There are two types of weathering:
(a) Chemical Weathering results from chemical reactions between minerals in rocks and external agents like air or water. Oxygen oxidizes minerals to alteration products whereas water can convert minerals to clays or dissolve minerals completely.
(b) Physical Weathering is when rocks are broken apart by mechanical processes such as rock fracturing, freezing and thawing, or breakage during transport by rivers or glaciers.
Factors Which Control the Rates of Weathering
Properties of the Parent Rock
1. The mineralogy and structure of a rock affects its susceptibility to weathering.
2. Different minerals weather at different rates. Mafic silicates like olivine and pyroxene tend to weather much faster than felsic minerals like quartz and feldspar. Different minerals show different degrees of solubility in water in that some minerals dissolve much more readily than others. Water dissolves calcite more readily than it does feldspar, so calcite is considered to be more soluble than feldspar.
3. A rocks structure also affects its susceptibility to weathering. Massive rocks like granite generally to not contain planes of weakness whereas layered sedimentary rocks have bedding planes that can be easily pulled apart and infiltrated by water. Weathering therefore occurs more slowly in granite than in layered sedimentary rocks.
1. Rainfall and temperature can affect the rate in which rocks weather. High temperatures and greater rainfall increase the rate of chemical weathering.
2. Rocks in tropical regions exposed to abundant rainfall and hot temperatures weather much faster than similar rocks residing in cold, dry regions.
1. Soils affect the rate in which a rock weathers. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil. Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering.
2. Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air.
Length of Exposure
1. The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup. Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time.
Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved.
Some Examples of Chemical Weathering
(a) Conversion of silicates into clays
(b) Dissolving of minerals
Conversion of Silicates to Clays
1. Silicates comprise almost all minerals in igneous rocks and are also important components in metamorphic rocks. Not all silicates, however, survive weathering processes to become incorporated into sedimentary rocks.
2. Silicates + water = Clays + dissolved SiO2 and dissolved cations ( K+, Na+, etc.)
Silicates = felspar, mica and amphibole (but not quartz)
3. Figure 6.3: Chemical weathering may involve the disintegration of a rock into rock fragments due to conversion of silicates into clays. For example, interlocking silicate grains in fresh granite gradually decay along crystal boundaries due to conversion to clays. Eventually cracks open around the boundaries, the rock weakens and easily disintegrates.
4. Figure 6.4: The process by which silicates decay is analogous to the brewing of coffee. Water dissolves some of the solid, leaving behind an altered material and producing a solution containing substances drawn from the original solid (coffee grounds).
5. The type of clay formed depends on the composition of the original silicate
(a) Feldspars + water = Al-rich clay (kaolinite) + SiO2 + K+ + Na+
(b) Fe-, Mg-rich silicates + water = Fe- Mg-rich clays + dissolved constituents
6. Figure 6.6: The conversion of silicates to clays is enhanced when the water is slightly acidic. Carbonic acid forms through the mixing of rainwater with carbon dioxide in the atmosphere via the reaction CO2 + H2O = H2CO3. The acid rainwater than reacts with minerals on the exposed rock face.
7. Reaction of silicates with carbonic acid and water produces clays and also releases Si and certain cations into water as dissolved constituents:
silicate + carbonic acid + water = clay + dissolved SiO2 + dissolved cations (K+, Na+, etc.) + dissolved bicarbonate
8. The dissolved cations are carried away by rain and river waters and ultimately transported to the oceans.
9.In tropical regions, clays can further react with water to form Bauxite (Al-hydroxide), an ore which is a major source of Al.
silicates + water = clays + more water = Bauxite
10. Figure 6.5: As weathering breaks down a rock into smaller particles, the surface area increases so that the process of chemical weathering is accelerated.
Dissolving of Minerals
1. Slightly acidic rainwater can also react with non-silicates in a rock or soil. For instance, carbonic acid can dissolve carbonates such as calcite so that all constinuents go into solution.
CaCO3 + H2CO3 = Ca2+ 2HCO3-
2. Halite dissolves directly into water
NaCl = Na+ + Cl-
Chemical Weathering Through Oxidation
1. Oxidation involves the combining of certain metals (Fe in particular) with oxygen in the process of stealing electrons. During oxidation, metals like Fe lose one or more electrons to oxygen.
metal + oxygen = metal oxide
4Fe + 3O2 = 2Fe2O3
2. Iron can also dissolve in water as cations. Dissolved Fe can exist in two oxidation states; Fe2+ - Fe3+ (highest).
3. Figure 6.8: Fe-bearing silicates like pyroxene, when dissolved in water, releases Fe2+ into solution. The Fe2+ is then oxidized by O2 in the water to Fe3+, which in turn combines with oxygen in the water and precipitates out of solution as the iron oxide hematite.
4. Figure 6.9: Iron oxide minerals are widespread and have the characteristic red and brown colors seen in desert sediments and red soils in humid regions.
Stability of Common Minerals Under Weathering Conditions
1. Table 6.2: Iron oxides, Al-hydroxides, clay minerals and quartz are the most stable weathered products whereas highly soluble minerals like halite are the least stable. Silicates fall within the middle range. The stability of silicates is opposite Bowens reaction series where the last minerals to crystallize (quartz and K, Na rich feldspars) being more stable than the early crystallized minerals (olivine and pyroxene).
2. The most common silicates in clastic sedimentary rocks are quartz, K-, Na-feldspars and micas. Amphiboles, pyroxene, olivine and Ca-feldspars are almost never found in sedimentary rocks.
Physical weathering is when rocks are broken apart by mechanical processes.
(a) Fig 6.11: Natural Zones of Weakness: bedding planes, fractures, joints. Rocks sometime expand when exhumed.
(b) Fig. 6.12: Activity of Organisms: tree roots can invade and widen cracks in a rock.
(c) Fig. 6.13: Frost Wedging is breakage resulting from expansion of freezing water in cracks.
(d) Mineral Crystallization: minerals (calcite, gypsum, etc.) crystallize from solutions in rock fractures, forcing the fractures to expand.
(e) Alternating Heat and Cold: common in desert regions that experience hot days and cold nights. Repeated expansion and contraction of the rock during heating and cooling.
(f) Fig 6.14: Exfoliation is the physical process where large flat or curved sheets of rock are fractured and detached from an outcrop.
(g) Fig 6.15: Spheroidal Weathering involves the cracking and splitting of curved layers from spherical boulders. Sometimes these curved layers fall away like skin on an onion.
1. Soils can form in place as residue left behind after weathering.
2. Soils may also form from transported material derived from elsewhere and deposited in a lowland or basin.
3. Residual soils develop on plains and lowlands with moderate to gentle slopes and consist of loose, heterogeneous material left behind from weathering. This material may include particles of parent rock, clay minerals, metal oxides and organic matter. This loose material is collectively called regolith, whereas the term soil is reserved for the topmost layer which contains organic matter.
The Soil Profile
1. Fig. 6.17: Soils can be grouped into three principle horizons.
2. The A-horizon is the topmost layer and is usually a meter or two thick. The upper portion of the A-horizon is often rich in organic matter, called humus, and may also contain inorganic material like insoluble clays and quartz. The A-horizon may take thousands of years to develop depending on the climate and acitivity of plants and animals. This is the layer that supports crops and other types of vegetation.
3. The soluble minerals leached from horizon A are precipitated in the B-horizon as calcite, quartz, gypsum, salts and/or iron oxides. These precipitated minerals often accumulate in small pods, lenses and coatings. Organic matter is sparse in the B-horizon.
4. The lowest layer constitutes the C-horizon and is comprised of cracked and variably weathered bedrock mixed with clays.
Different Types of Soils
1. Soils can vary significantly in color and composition. The particular type of soil that is produced in a region depends on the available materials, climate and also time.
1. Fig. 6.18a: Laterite is a deep red soil found in tropical regions and often developed on mafic igneous bedrock.
2. The high temperatures, heavy rainfall and humidity of tropical regions have driven chemical weathering to the extreme. As a result, feldspars and other silicates have been completely altered while silica and calcite is extensively leached from the soil.
3. The upper zone of laterite consists of insoluble precipitated iron and other oxides along with some quartz.
4. At best, only a very thin layer of organic matter resides at the top of the soil to support the jungle vegetation. When the jungle vegetation is cleared, the humus oxidizes quickly and soon disappears. For this reason, laterite can only be farmed extensively for a few years after clearing and afterwards must be abandoned.
1. Fig. 6.18c: Pedocals are the dominant soils in arid regions where rainfall and vegetation are sparse. As a result, very little chemical weathering occurs to alter the original mineralogy. Pedocal soils are generally very thin.
2. The A-horizon is typically leached and can only supported limited, desert vegetation.
3. Much of the soil water is drawn up near the surface and evaporates between rainfalls, leaving behind precipitated nodules and pellets of calcium carbonate mostly in the B-horizon.
1. Fig. 6.18b: Pedalfer soils occur in temperate climates experiencing moderate to high rainfall. The relatively thick, organic rich layer makes pedalfers favorable to agriculture. Pedalfers typically form on granitic bedrock, the principle rock type in these regions.
2. The upper and middle layers of pedalfers contain abundant insoluble minerals such as quartz, clay minerals and iron alteration products. All soluble materials, especially calcium carbonate, have been leached away.
1. The classification of soils is actually more complex than presented, especially when taking into account significant differences in bedrock. Some of these categories can be related to the soil types previously discussed.