Survey
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Download Training
Transcript
Weathering and Soil Resources Nancy A. Van Wagoner Acadia University Introduction Why 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 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 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 Crop residue – foliage, stubble, straw – left on soil by crops – before and after harvest Decreases surface runoff – absorbs energy of wind rain Summary – global food security requires understanding of soils proper soil management – WWW resource on Soil Erosion http://soils.ecn.purdue.edu/~wepphtml/wepp/wepptu t/jhtml/intro.html 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 combine What is erosion – The removal of weathered rocks and minerals from the place where they formed water wind glaciers gravity 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 Mechanical Weathering Facilitates Chemical Weathering increases surface area exposes more surfaces to chemical attack Mechanical Weathering Major – – – – – – Mechanisms frost wedging salt cracking abrasion biological activity thermal expansion and contraction pressure release fracturing 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 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 Mechanical Weathering Abrasion – breakup of rock by friction and impact glaciers (fig. 10-8) wind (fig. 10-7) running water (fig. 10-6) waves Mechanical Weathering Biological Activity (fig. 10-9) – plants growing in cracks in rocks – burrowing animals – humans blasting for roads, development, exploration, etc.. Mechanical Weathering Pressure release fracturing (fig. 10-10) – buried rocks are under confining pressure – when exposed they expand due to release of confining pressure erosion surface exfoliation joints cracks dev. parallel to erosion surface – problem for miners (underground)--causes rock bursts 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 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 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 rain Fe-pyroxene oxygen from atm. limonite hydrated Fe-oxide dissolved silica Chemical Weathering (solution) solution of soluble substances, such as salt in water 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 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 Carbonic Acid and Limestone carbonic acid reacts with limestone to dissolve it draw equation result is dissolved Ca++ and HCO3effect on neutralizing acid Certain minerals react with acid solutions to neutralize them Examples are: – Calcite (limestone) – minerals of mafic igneous rocks Ca-rich Olivine feldspar 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 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 Other acids are formed by industrial and automotive emissions The emissions are – SO2 and – gases of nitrogen (NO2, N2O) draw reactions on board 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 Hydrolysis results in: constituent mineral growth, increase in mineral volume puts pressure on the framework of the rock resulting in: – gruss – spheroidal weathering Gruss: is a pile of hydrated minerals form where hydrated minerals fall off and collect at the base of a weathering rock 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 Factors Controlling Rates of Weathering Particle Size Porosity and Permeability Climate – optimum environment for chem. weathering – optimum environment for mech. weathering Mineral Stability
