Unit 1 - Chapter 3 - Changing Rocks into Soil - Weathering

The exposed surfaces of rocks, gravel, sand, and silt particles are constantly being broken down by physical and chemical weathering. Weathering is the chemical alteration and physical breakdown of rock during exposure to the atmosphere, hydrosphere, and biosphere. As a result of weathering, the Earth is often covered by an irregular blanket of loose rock debris. The weathering of rocks results in the making of the soil's Parent Material.

Physical Weathering involves six processes.

1) Freezing and thawing- The expansion force of water as it freezes is sufficient to split any mineral or rock. Freezing water can exert a pressure of 150 tons per square foot. Freezing and thawing can occur on a daily cycle. 

2) Heating and cooling- Differences in temperature in a rock or soil mass give rise to differential expansion and contraction. The resulting stresses can fracture the minerals. Temperature changes also can bring about exfoliation, where a thin layer of an entire rock is removed--often making the rock round. This rock in the California desert is heated and cooled every 24 hours.
3) Wetting and drying-The disruption of soil by wetting and drying results in the swelling and contracting of soil peds and particles. Abrasion among particles within the soil makes the particles finer. The soil shrinks when dry, and cracks develop, creating an irregular boundary between horizons. {short description of image}

4) Grinding or rubbing- Grinding action, or the rubbing of moving rock or soil particles against each other, also results in the disintegration of the rock or soil particles. In soils high in clay (Vertisols), during the dry season the soil cracks and fills with soil particles from above. During the wet season the soil swells shut and the expanding forces cause the soil peds to have slick, smooth surfaces called Slickensides  {short description of image}which are evidence of soil movement below the surface of the soil.

5) Organisms- Action of organisms, including animals and plants reduces the size of rocks and minerals. Plant roots are capable of splitting the hardest rock. (See Gneiss )Digging by animals or plowing by humans result in a slow breaking of rocks into finer particles. 

6) Unloading- Unloading is the removal of thick layers of sediments overlying deeply buried rocks by erosion or uplift. The response of the rock to this reduced pressure is to expand, and cracks and fissures are created. Unloading is a physical process which can also result in chemical weathering, because the temperatures are less in the soil environment than where the rock was formed, and exothermic chemical reactions occur among minerals, water, oxygen, and carbon dioxide in the soil. Tree roots are often able to penetrate the rock fractures that resulted from unloading. In Minnesota unloading (or rebounding) is taking place but it was from the ice sheet that covered much of the state during the last glacial event 10,000 years ago.

Chemical Weathering involves six processes

Chemical Weathering, that is the decomposition of rocks and minerals as chemical reactions transform them into new chemical combinations that are stable at or near the Earth's surface. Chemical weathering occurs because minerals are made more soluble and are changed in structure, causing easy fragmentation. Solubility changes are caused by: a) solution (usually in water); b) hydrolysis (the reaction of elements with water); and c) carbonation (the reaction with HCO3-). Changes in the chemical structure of elements are brought about by hydration, oxidation, and reduction. Chemical weathering is faster where temperatures are higher and water is present.

1) Dissolution- Dissolution is the dissolving of a solid in a liquid, changing solid materials into separate ions (for instance, sodium chloride (NaCl) dissolves into Na+ and Cl- ions). This permits more independent and greater chemical changes than in an non-ionized (usually solid) state. Minerals have been dissolved in the runoff from the surrounding hills and are precipitated in this dry lake bed. The minerals look like white snow but actually are crystals of NaCl . 

2) Hydrolysis. Hydrolysis is the process of minerals reacting with water to form hydroxides, which usually are more soluble than the original mineral. Hydrolysis is one of the most important weathering processes causing soil profile changes.

3) Acidification- Weathering is accelerated by the presence of the hydrogen ion in water, such as that provided by carbonic and organic acids. Carbonic acid, a weak acid produced when gaseous carbon dioxide is dissolved in water. Thus acidification is a form of dissolution. The carbon dioxide comes partially from the atmosphere, but mostly from biological respiration and from the decomposition of plants. Carbonic acid dissolves minerals more readily than water alone and forms the more soluble bicarbonates. Acidification is responsible for the weathering of limestone in southeastern Minnesota. Sinkholes are created when the limestone bedrock dissolves. Sinkhole Picture For more information and more pictures on sink holes got to Sinkhole story.

4) Hydration- Hydration is the combination of a solid mineral or element with water. When the water molecules are chemically bonded to the mineral, the size of the chemical structure is increased, thereby making a softer, more stressed, and more easily decomposed mineral.

5) Oxidation- Oxidation, as used in mineral weathering, is both the chemical combination of oxygen with a compound and the change in oxidation number of some chemical element (electrons are lost in oxidation). Oxidized minerals have a volume increase and are usually softer. If an element's oxidation number is changed, this can also unbalance the mineral's electrical neutrality, making it more easily weathered by water and carbonic acid. Oxidation is most evident in the weathering of iron-bearing minerals. 

6) Reduction. Reduction is the chemical process in which electrons are gained. In soils, reduction usually takes place when oxygen is scarce, as in stagnant water conditions. Reduction in minerals may result in electrically unstable compounds, more soluble ones, or more internally stressed ones, which eventually decompose more rapidly.

Poorly drained soils (see Aquoll) will have reduction and oxidation reactions taking place throughout the profile. 

This ped exhibits reduction as evidenced by the gray colors, and the red mottles are from the oxidized iron. 
In the lab you will have opportunities to see various stages and kinds of weathering of rocks.

Hydrolysis and carbonation are usually the most effective of the six chemical weathering processes.

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