Transcript Training
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