WEATHERING AND SOIL

GEOL 1121 (Written by T. Weiland)

*A couple hundred years ago, people believed that the physical features of the earth such as mountains, rivers, lakes, and shorelines were permanent and unchanging features on the earth's surface. We now know these physical features and environments on earth are constantly changing. Volcanic and tectonic processes are constantly building up mountain ranges; whereas, erosion, weathering and mass wasting are simultaneously lowering these elevated areas. One aspect of weathering is the formation of soils. This involves the modification of bedrock to the point that it will sustain life. The evolutionary transition of life from the oceans to the land could not have occurred without the presence of soils. We tend to take the 'dirt' beneath us for granted; however, it is a valuable resource which takes hundreds to thousands of years to form. Once it is lost, as is now occurring throughout the tropics, we not only drive many species into extinction, but we also severely alter weather patterns and ecosystems.

Weathering - the disintegration and decomposition of rock at the earth's surface. (an in situ change in character). In contrast, erosion is the process where rock material is removed from one location by water, wind, or ice.

A. Mechanical Weathering - physical breaking without a chemical change. It produces a greater surface area for chemical weathering to attack.

1. Frost Wedging - the freezing and thawing of water in cracks and voids which breaks the rocks into smaller pieces. *Water expands 9% when it freezes (example - capped bottle in the freezer). Frost wedging is important in mountainous regions of the middle latitudes where a daily freeze-thaw cycle exists. The process can be noticed by potholes in the roads and talus slopes (large piles of fragmental rock at the base of outcrops).
2. Unloading - erosion of overlying and surrounding rocks of igneous bodies results in a reduction of confining pressure. The outer layer then expands more than the lower areas forming onion-like fracture sheets (sheeting). Example - Stone Mountains and Mt. Arabia where they quarry the granitic sheets without having to blast. ("Exfoliation domes")
3. Thermal Expansion - the repeated contraction and expansion and rocks due to daily temperature changes (esp. in deserts) which appears to weaken rocks. Experimental work indicates this is not a very important agent.
4. Organic Activity - physical breakdown of rock due to the activities of plants, animals and man. Root wedging, burrowing animals and man's machines all break rock up.

B. Chemical Weathering - decomposition of minerals which become unstable at the earth's surface. *Water is the most important agent of weathering.

1. Solution - the change of matter from solid into liquid by its combination with water.
   1. *Dipolar nature of water - oxygen has a small residual negative charge and the hydrogen have a small residual positive charge. This is why water is such a good solvent. Example - NaCl solution
   2. Acidic Solutions - solutions which have an excess of reactive H atoms. The solutions are much more corrosive than water. Example - Calcite in an acidic solution (HCl)

CaCO₃ + 2[HCl+H₂O] –-- Ca²⁺ + CO₂ + 3H₂O+ 2Cl^-

This process can be in the formation of caves by acidic groundwater and weathering of calcite building stone by acid.

Types of Acids

• Carbonic acid - forms when carbon dioxide is dissolved in the atmosphere.
• Organic acids – forms by the decay of organisms.
• Sulfuric acids - forms by the weathering of sulfides.

2. Oxidation - process of combining oxygen with other cations in the mineral. This is especially important in iron oxides. 4Fe + 3O₂ -- 2Fe₂O₃ (hematite) *Serves to decompose the ferromagnesian minerals and produces a rusty surface on the rock. It is most important in wet moist areas.
3. Hydrolysis - reaction with water. Hydrogen (H) and hydroxl (OH) ions dissociate and replace other positive ions in the crystal structure.

2KAlSi₃O₈ + 2(H⁺+HCO₃^-) ----- Al₂SiO₅(OH)₄ + 2K⁺+2HCO₃^- +2SiO₂

K-feldspar in acid produces wate, kaolinite and several ions in solution. *Clay minerals are the end product of weathering. These minerals are very stable under surface conditions. They are also the major components of soils.

II. Rates of Weathering

1. Rock Structure and Composition
   1. Mineral Stability - Most stable minerals are the bottom of Bowen's Reaction Series (lower temperature). Example: quartz versus olivine.
   2. Grain size - finer-grained material is more resistant to weathering.
   3. Structure - fractures, cracks, foliation increase weathering rates
2. Climate Temperature and Rainfall. (more water - greater weathering)
3. Topography - controls the amount of rock exposed to the agents of weathering.
4. Plant and Animal Life – produce organic acids and can physically break up rock material

III. Soil - earth material which has modified to the point that it will support life. It is a mixture of rock fragments, organic material, air and water.

1. Factors Influencing Soil Formation
   1. Source Material (Parent material) - chemical composition determines the soils fertility. Residual vs. Transported Soils
   2. Time - longer time is better. Farming tends to deplete soil in many minerals (fertilize) and erodes it much faster (dust bowl).
   3. Climate - determines the style of weathering and thickness of soil horizon.
   4. Plant and Animal Life - furnish organics, organic acids, breakup soil, etc.
   5. Slope - determines the amount of erosion, water content (lower slope – better soil).
2. Soil Profile - vertical differences separate the soil into horizons.
   1. O horizon - upper layer of mostly organic material (humus)
   2. A horizon - upper layer which consists of about 30% organics and the remainder is minerals without the fines (esp. clays) which are pushed downward by percolating water. "Zone of leaching" - soluble minerals have been removed.
   3. B horizon (subsoil) - area where the soluble minerals and fine clays are reprecipitated - "Zone of accumulation". An abundance of clays in this zone can form a hardpan.
   4. Saprolite Zone - area of partially broken and altered rock debris.
3. Soil Types - determined predominantly by climate.
   1. Pedalfer - Al- and Fe-rich soil. Contains abundant iron oxides and aluminum-rich clays in the B horizon. *Form in the middle latitude areas where moderate rainfall carries the soluble material away and the less soluble iron oxides and clays are left. Red color soil is common. This is the most common soil type in this area.
   2. Pedocal - calcite-rich soils formed in areas of low rainfall. The rain does not penetrate deep enough such that the soluble minerals (esp. CaCO) are dissolved from the uppermost layer and redeposited in the lower levels. *Pedocals form in arid and semi-arid areas.
   3. Laterites - intense chemical weathering depletes the upper soil in silica and other soluble materials and instead concentrates iron and aluminum in the upper zones. *Common in the tropical areas - poor soils to cultivate because the minerals have all been washed out. Bauxite is the principal ore of aluminum.