Chapter 6   Weathering and Soil

Chapter 6 Weathering and Soil PowerPoint PPT Presentation

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Weathering Mechanical (physical) Rock is broken into smaller pieces by various mechanical processes, providing more surface area for chemical weathering.Chemical weathering Mineral grains are attacked" by water

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Chapter 6 Weathering and Soil

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1. Chapter 6 – Weathering and Soil External processes – Occur at or near surface, energy from solar heat. Part of the Rock Cycle, these process form the components of sedimentary rock. The 3 Processes that break apart rocks and move the debris to lower elevations are: Weathering Mass Wasting Erosion These processes are responsible for shaping the land.

2. Weathering Mechanical (physical) – Rock is broken into smaller pieces by various mechanical processes, providing more surface area for chemical weathering. Chemical weathering – Mineral grains are “attacked” by water & naturally-occurring acids. Microfractures & fractures provide pathways.

3. Mass wasting – The transfer of rock and soil downslope due to the force of gravity overcoming friction and rock/soil cohesion. Water is often a “facilitator” in mass wasting, because water is heavy and it acts as a lubricant. Erosion – The physical removal of “broken down” rock and mineral material. Water is the primary “agent” of erosion. Wind and glaciers are less important.

4. Mechanical Weathering Frost wedging – freeze-expansion of water spreads microfractures, fractures and joints in the rock surface. Over time, repeated cycles can produce a talus slope (see text)

5. Unloading – as erosion removes material above a buried rock unit, expansion of the newly ex-posed rock results in the separation of individual sheets of rock. May happen in quarries, also.

8. Thermal expansion – diurnal heating and cooling may cause “damage” to outer surfaces of rock, especially if rock surface has been weakened by chemical weathering. Biological activity – tree roots grow into existing fractures and during growth, expand the fractures (and thereby cause new fractures). On a “micro” scale, the roots of mosses and other plants may infiltrate micro-fractures in rock surface. Root growth and expansion also results in the fracturing of driveways.

9. Biological activity – tree roots grow into existing fractures - expand the fractures.

10. Tectonic Fracturing and Jointing. Joints are flat and parallel. Fractures are more irregular. Both allow water percolation.

11. Summary of Physical (Mechanical) Weathering Types Frost-wedging Unloading – Exfoliation Thermal Expansion and Contraction Biological Activity – Root Wedging Tectonic Fracturing & Jointing More than one type may be “at work” at a time. All of which create more surface area

12. Chemical weathering – H2O + CO2 = H2CO3 (carbonic acid) As rainfall infiltrates soil, additional CO2 is added from soil bacteria and organics may add additional acids. Polarity of water molecule increases its chemical effects. Hydrogen ions – positive. Oxygen ion – negative.

13. Dissolution – Polarity of the water molecule affects individual salt molecules, e.g. Halides (NaCl, KCl) & Sulfates (gypsum).

15. In our wetter climate, oxidation happens more rapidly than in New Mexico

16. Hydrolysis – reaction of minerals with water, when acids are present, effect is intensified. Hydrogen ion (H+) replaces other cations in mineral lattice (structure), disrupting the original structure = decomposition of mineral and rock structural integrity. Most important silicate minerals (except quartz) are susceptible to hydrolysis, with the dark silicates most susceptible. Basalt in slide 10 “lost” silica to hydrolysis, which facilitated oxidation of Fe-Mg minerals.

17. 2KAlSi3O8 + 2(H+ + HCO3-) + H2O Al2Si2O5(OH)4 + (2K+ + 2HCO3- + 4SiO2) in solution. K feldspar (orthoclase) kaolinite (clay) Hydrolysis products are listed in Table 5.1. Hydrolysis produces the spheroidal (rounded) weathered surface, by “attacking” corners first. When hydrolysis destroys the structure of a rock, but traces of the structures remain – this is “saprolite”.

18. The K Feldspar in this sample has been partially converted to Kaolinite by Hydrolysis.

19. Hydrolysis produces solution pits in granites.

20. When filled with soil, these become their own micro-ecosystems.

21. Hydrolysis produces the spheroidal (rounded) weathered surface, by “attacking” corners first.

22. Over time, with Hydrolysis, the biotite gneiss (left) becomes the saprolite (right). When all traces of structure are lost, the saprolite becomes “residuum” (Ga. Red clay).

23. Summary of Chemical Weathering Types Dissolution Oxidation of Fe-rich minerals Hydrolysis For Fe-rich silicates, Oxidation and Hydrolysis may happen at the same time. Hydrolysis is important in the conversion of Silicates to clays.

28. Rates of weathering – dependent on… Rock Characteristics & Climate Rock Characteristics – quality of chemical bonds in minerals affects solubility. Quartz – strong bonds, stable. Calcite – weaker bonds – less stable. Refer back to Bowen Reaction Series. Minerals that crystallize early (olivine, pyroxene, etc.) have a less organized structure and will chemically weather more easily.

29. Climate – Frequency of freeze-thaw cycles in colder climates result in more frost wedging, i.e., colder climates may favor mechanical weathering. Cold climates keep water “locked up” in ice. Chemical weathering proceeds more rapidly in warm, wetter climates. Temp. is important – every 100 C increase in temp. = doubling of chemical reaction rates. Increases in atmospheric nitrogen and sulfur compounds (from pollutants) can increase the rate of chemical weathering.

30. Plants may play a role in chemical weathering, as acids from decaying plant matter may assist in the breakdown of minerals. The results of physical and chemical weathering are subject to downslope movement by water (erosion), unless held in place by vegetation or soil cohesion. The eroded material becomes sediment in streams and is transported during floods until it reaches it final site of deposition, usually the ocean. The materials that remain behind become part of the soil profile.

35. Mineral types in parent rock play a role. Pure quartz sandstones or quartzites have little that can be chemically weathered to produce nutrients vs. other silicates, etc.. Volcanic ash flows form excellent soils – pulverized nature of ash = more surface area. Glassy nature of ash breaks down easily. CLIMATE – Over time contributes more to soil character. Rate of weathering is dependent on available moisture and temperatures. Also important in nutrient cycling of humus.

37. Description of soil profile (idealized) O Layer – mostly organics A Layer – mineral grains, with up to 30% organics E Layer – light colored horizon, materials removed by “eluviation” washing out of smaller grains and leaching of soluble minerals B Layer – deposition of smaller grains from E layer (subsoil) C Layer – Partially altered parent material

40. Process of Erosion Raindrops dislodge soil particles which are then moved by sheetwash. Surface irregularities concentrate some of flow into “threads of current” which form small channels or “rills”. Rills combine to form “gullies”. Rate of erosion is dependent on quantity of water, slope angle, eroded material, local base level, and other conditions. Gullies erode by down-cutting by water. May widen by sheetwash, rill erosion, and/or by mass-wasting.

45. Problems caused by excessive erosion: Loss of O and A layer of soil Loss of arable farm land when O and A layers are damaged “Silting-up” of streams, i.e., excessive mud hinders “filter feeders” (clams, etc.), gills of fish, etc.. Sand, silt clog stream channel. Growth of deltas into lakes diminishes the water capacity of the lakes, i.e., less water for drinking, recreation, irrigation, electric generation.

46. Problems caused by excessive erosion:

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