Weatheringand

Soil Resources

Nancy A. Van Wagoner

Acadia University

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

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/

wepptut/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

– problem for miners (underground)--causes rock bursts

erosion surface

exfoliation jointscracks dev. parallelto erosion surface

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

Fe-pyroxenerain oxygen

from atm.limonitehydrated Fe-oxide

dissolvedsilica

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 HCO3- effect on neutralizing acid

Certain minerals react with acid solutions to neutralize them

Examples are:– Calcite (limestone)– minerals of mafic igneous rocks

Ca-rich feldspar Olivine

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