Weathering and Soil Resources

1. WeatheringandSoil Resources Nancy A. Van Wagoner Acadia University

2. IntroductionWhy Should I Care? • Soil is a Critical Resource • World’s farmers must feed an additional 90 million people every single year • At the current rate of population growth • Limiting constraint = availability of fertile land • i.e.. good quality soil, and proper soil management

3. Soil is a Nonrenewable resource on the human time scale • How long does it take to produce a 10 cm thick layer of soil from bedrock? • 100 years to 10,000 years!!! • What are current rates of soil loss • India • 4.3 billion tons per year • USA • 3.9 billion tons per year • *1987 National Resources Inventory, USDA SCS

4. Processes that contribute to the loss • contamination • removal of surface vegetation and residue • agricultural cultivation • forest harvesting • rangeland grazing • surface mining • urbanization (hwy, building construction) • degradation

5. Crop residue foliage, stubble, straw left on soil by crops before and after harvest Decreases surface runoff absorbs energy of wind rain

6. Summary • global food security requires • understanding of soils • proper soil management • WWW resource on Soil Erosion • http://soils.ecn.purdue.edu/~wepphtml/wepp/wepptut/jhtml/intro.html

7. Soil Formation and Weathering • What is weathering? • The decomposition and disintegration of rocks and minerals at the Earth’s surface by mechanical and chemical processes • converts rock to gravel, sand, clay and soil

8. combine

9. What is erosion • The removal of weathered rocks and minerals from the place where they formed • water • wind • glaciers • gravity

10. Types of weathering • mechanical weathering • The physical disintegration of rock into smaller pieces each retaining their original characteristics • Example • chemical weathering • The decomposition of rocks and minerals as a result of chemical reactions (removal and/or addition of elements • Example

11. Mechanical Weathering Facilitates Chemical Weathering • increases surface area • exposes more surfaces to chemical attack

12. Mechanical Weathering • Major Mechanisms • frost wedging • salt cracking • abrasion • biological activity • thermal expansion and contraction • pressure release fracturing

13. Mechanical Weathering • Frost Wedging (fig. 10.4) • When water freezes it expands • Example • Volume increases by about 9% • Water migrates into cracks in rocks • Ice crystal growth puts tremendous pressure on surrounding rock • Enough to break rock • Most effective in mountainous areas where daily freeze/thaw • Talus slopes • Dangers to hikers

14. Mechanical Weathering • Salt Cracking (fig. 10.5) • salts crystallize in cracks in rocks • puts pressure on surrounding rock • important in • dry climates (arid regions) • ground water is salty, salts precipitate out of solution • coastal areas • salt spray blows into cracks in rocks

15. Mechanical Weathering • Abrasion • breakup of rock by friction and impact • glaciers (fig. 10-8) • wind (fig. 10-7) • running water (fig. 10-6) • waves

16. Mechanical Weathering • Biological Activity (fig. 10-9) • plants growing in cracks in rocks • burrowing animals • humans blasting for roads, development, exploration, etc..

17. Mechanical Weathering • Pressure release fracturing (fig. 10-10) • buried rocks are under confining pressure • when exposed they expand due to release of confining pressure • problem for miners (underground)--causes rock bursts erosion surface exfoliation joints cracks dev. parallel to erosion surface

18. Chemical Weathering • WATER = main agent of chemical weathering • pure water by itself is relatively inactive, but • pH = 7 • with small amounts of dissolved substances it becomes highly reactive • many of these substances are found in the atmosphere • and soil

19. Composition of Clean Dry Air • 78% Nitrogen • 21% Oxygen • 1% other • inert gases = 0.93% • carbon dioxide CO2 • methane CH4 • Hydrogen • oxides of Nitrogen • carbon monoxide • ozone O3

20. Chemical Weathering (Oxidation) • reactions with oxygen • common, ~21% of atm. = oxygen • example, Iron bearing minerals oxidize to form rust • 4FeSiO3 + 2H2O + O2 > 4FeO(OH) + 4SiO4 dissolved silica rain oxygen from atm. limonite hydrated Fe-oxide Fe-pyroxene

21. Chemical Weathering (solution) • solution of soluble substances, such as • salt in water

22. Chemical Weathering (acids and bases) • CO2 dissolved in water, rain or snow, produces • Carbonic Acid • Remember, pure water is neutral (not acid or base) • If we increase the number of H+ ions in water, it becomes an acid, pH < 7 • If increase the number of Hydroxyl ions (OH-) it becomes a base • Acids and bases are more corrosive than pure water

23. Chemical Weathering (acids and bases) • All natural rain water is “acid rain” • Why • as rain drops fall through the atmosphere and through soil • react with carbon dioxide in the air, and produced by decaying organisms in soil • to form carbonic acid • H2O + CO2 H2CO3 H++HCO3

24. Carbonic Acid and Limestone • carbonic acid reacts with limestone to dissolve it • draw equation • result is dissolved Ca++ and HCO3- • effect on neutralizing acid

25. Certain minerals react with acid solutions to neutralize them • Examples are: • Calcite (limestone) • minerals of mafic igneous rocks • Ca-rich feldspar • Olivine

26. Carbonic Acid and Silica-rich rock • idealized by the reaction with the mineral orthoclase, a common mineral found in granite • EQUATION (draw on board) • What has happened • The feldspar is weathered to clay. • Ions are released to be soil nutrients. • Silica goes into solution. • H+ replaces K in the crystal structure as OH- ions = hydrolysis • this disrupts and expands the crystal structure • Al is retained

27. Other important points • Only one hydrogen ion is neutralized for each mole of feldspar consumed • because clay minerals are by-products of weathering • form at surface conditions • very stable at surface conditions • comprise a high percentage of the inorganic component of soil

28. Other acids are formed by industrial and automotive emissions • The emissions are • SO2 and • gases of nitrogen (NO2, N2O) • draw reactions on board

29. High silica rocks are wide spread: • Canadian Shield • Appalachians • New England • Nova Scotia • Therefore lakes in these geographic settings have a poor buffer against the effects of acid rain. • Soils in these settings also have a poor natural buffer and farmers must add lime (CaCO3) to the soil. • figure 23.2

30. Hydrolysis results in: • constituent mineral growth, increase in mineral volume • puts pressure on the framework of the rock resulting in: • gruss • spheroidal weathering

31. Gruss: is a pile of hydrated minerals • form where hydrated minerals fall off and collect at the base of a weathering rock

32. Spheroidal weathering: also caused by chemical weathering • sequence of events: • pressure release forms orthogonal joints • water percolates through cracks • -hydrated minerals disrupt the framework of the rock • put pressure outward • weathering first reacts more intensely at corners, producing a rounded shape • finally, onion-skin pieces of rock flake off • end up with what looks like giant pile of marbles

33. Factors Controlling Rates of Weathering • Particle Size • Porosity and Permeability • Climate • optimum environment for chem. weathering • optimum environment for mech. weathering • Mineral Stability