Lecture 9 Weathering and Soil

Topics:
Weathering
Processes of mechanical weathering
Processes of chemical weathering
Resistance to weathering
Soil profiles
Controls of soil profile development
Soil hazards: expansive soil

* Weathering

Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks to form soil or loose particles at or near Earth's surface. Weathering causes deterioration of building materials. It also weakens rocks, a great concern when weathered rocks are used for foundation.

Mechanical weathering is accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition.

Chemical weathering involves breaking down rock components and internal structure and forming new compounds.

Whereas weathering breaks rocks apart, erosion removes rock debris by mobile agents such as water, wind, or ice.

* Processes of mechanical weathering

unloading: jointing, exfoliation, and sheeting
Upon removal of overburden, the elastic component of rock deformation is recovered and the rock expands. The unloading may occur when the overlying rocks are eroded or rocks are removed from a quarry. The expansion caused by unloading may be sufficient to fracture the rock. Such naturally formed cracks are known as joints.

Typically, large plutons or metamorphic bodies split into sheets that are parallel to the mountain face, a process known as exfoliation. It is also known as sheeting if the expansion from unloading occurs in granite to form rock slabs.

frost wedging

One of the most effective mechanical weathering processes is the wedging action, called frost wedging, of repeated cycles of freezing and thawing of water in rock fractures. Liquid water expands by 9% in volume when freezing.

Conditions for frost wedging include moisture, rock fractures or weakness planes, and temperature fluctuation around the freezing point.

A product of frost wedging is talus slope made of angular rock pieces piling up at the base of steep cliffs.

thermal expansion and contraction

As temperature changes (in deserts or from forest fires), not all parts of a rock or all its minerals expand or contract by the same amount. So when rocks are heated or cooled, the mineral grains are subjected to differential stresses, which may be sufficient to make the rock spall, or break off in sheet-like pieces.

* Processes of chemical weathering

Most rocks originally formed under the conditions very different from the surface. So when exposed, they tend to react chemically with components of the surface and atmosphere, including water, oxygen, carbon dioxide. Water is the most important agent of chemical weathering.

surface area effects

Chemical weathering occurs at the surfaces of rocks, thus, the greater the surface area, the more intense the weathering. Thus the breaking of rock into smaller pieces by mechanical weathering greatly accelerates chemical weathering.

solution

Water is an excellent solvent, capable of dissolving many chemical compounds. This is the result of polar nature of water molecules, i.e., the oxygen end has a small negative charge whereas the hydrogen end has a small positive charge.

For example, halite (HCl) readily dissolves in water. As the water molecules come in contact with halite, the negative ends attack the sodium ions and the positive ends attach the chloride ions.

In addition, CO2 in the atmosphere and soils reacts with water to produce carbonic acid.

H2O + CO2  ->  H2CO3.

The carbonic acid readily reacts with calcite (e.g. in limestone and marble):

CaCO3 + H2CO ->  Ca2+ + 2 H2CO3-.

hydrolysis

Hydrolysis is the reaction of acidic solutions with the most common mineral group, silicates. For example, the weathering of K-feldspar of granite is as follows.

2KAlSi3O8 + 2(H+ + HCO3-) + H2O  -> Al2Si2O5(OH)4 + 2K+ +2HCO3- + 4SiO2
K-feldspar       carbonic acid                            kaolinite                in solution          silica

An product of the chemical breakdown of K-feldspar is clay mineral, kaolinite, which is very stable at the surface. Consequently, clay minerals make up high percentage of soils.

oxidation

Iron-rich minerals is subject to oxidation, which occurs when oxygen (dissolved in the water) combines with iron to form iron oxide.

4Fe + 3O2 -> 2Fe2O3 (hematite)

* Resistance to weathering

rock characteristics

Some minerals are more susceptible to chemical weathering than others. For silicates, the order of weathering (Goldrich's mineral stability series) is the same as the order of crystallization (so called Bowen's reaction series).

climate

Temperature and moisture have strong influences on both mechanical weathering (e.g. frost wedging) and chemical weathering. Thus chemical weathering is ineffective in polar regions or arid regions because of the lack of free water.

* Soil profiles

One consequence of weathering is the formation of the soil profile, a vertical cross section from surface down to the parent materials. A well-developed soil profile shows distinct horizons. The major horizons are A, B, and C horizons.

The A horizon is the top soil. It is a zone where downward percolating water removes soluble soil components into deeper zones (called leaching). It is also commonly rich in decomposed organic mater (humus).

The B horizon is the sub soil or the zone accumulation where the material removed from above accumulates. The accumulation of fine clay particles enhances water retention in the subsoil. Organic matter is less abundant in the B horizon.

The C horizon marks the transition from the soil profile to the unweathered parent material below.

* Controls of soil profile development

parent material

The parent material of soils can be (1) the underlying bedrock -- in this case, the soils are termed residual soils; or (2) transported deposits -- in this case, the soils are termed transported soils.

The natural of parent material affect soils in two way. (1) The type of parent materials affects the rate of weathering. (2) The chemical make up of the parent material affects the soil's fertility, which affects vegetation.

climate

Climate is perhaps the most important in soil profile development. (1) As we pointed out above, temperature and precipitation have great influence on weathering. (2) The amount of precipitation affects how much various materials are leached from the soil, thereby affecting soil fertility. (3) Climate affects the type of plant and animal life present.

Soil types: The prevailing climate has controlling influences on soil types. The soil types in the U.S. can be roughly described as pedalfers in the eastern half (east of a line from northwest of Minnesota to south-central Texas) of the U.S. and pedocals in the drier western U.S.

Pedalfer = pedon(soil)+Al(aluminum)+Fe(iron) in Greek.
Pedalfers are found in the eastern U.S. with high precipitation. In pedalfers the soluble carbonates are removed and Al-rich clays and Fe oxides are carried downward to the B horizon.

Pedocal = pedon(soil)+CALcite.
Pedalcals contain an accumulation of calcium carbonate in the B horizon. Pedalcals are found in the drier western U.S. with grassland and brush vegetation.

time

In general, the longer a soil has been forming, the thicker it becomes and the less it resembles the parent material.

topography

Steep slopes encourage runoff and erosion. Thus the soil profile thins near hilltops and thickens in the low lands.

vegetation

Plants holds soil in place with their roots. Plants also provide organic matter to the soil, contributing to soil fertility or water retention.

* Soil hazards: expansive soil

Soils containing swelling clays, primarily smectite, expand when absorbing water and shrink when losing it. Damage to structures caused by expansive soils is one of the most costly natural hazard in the U.S..