CHAPTER 8 - WEATHERING AND SOIL
Overview
Weathering describes the mechanical (no compositional change) and chemical
(chemical change) destruction of rocks. Erosion involves the picking-up and
physical removal (transportation) of weathering products. Mechanical and
chemical weathering usually occur together, and their effects are ubiquitous.
Spheroidal weathering of boulders and differential weathering of outcrops are
two of the more interesting examples.
Mechanical weathering is caused by frost wedging, frost heaving, pressure
release (producing sheet joints and exfoliation), and to a lesser extent plant
growth, burrowing animals, crystallization pressure, temperature induced
expansion and contraction. Chemical weathering reflects mineral instability.
Oxidation produces iron compounds in particular. Natural acids promote
chemical weathering, with rainwater, as carbonic-acid, being the most
common. Hydrogen easily substitutes for other positive ions in mineralstructures disrupting them and forming new minerals. Minerals, such as
calcite, may dissolve completely, while others, like the feldspars and
ferromagnesians, produce clays. Chemical weathering is most intense where
there is abundant liquid water, and less effective in deserts or cold climates.
Quartz, clays and iron oxide are common solid weathering products, while
dissolution contributes ions to ground water and ultimately the ocean.
Dissolved silica and-bicarbonate are common cementing agents in sedimentary
rocks.
Soil is the weathered, unconsolidated material resting on bedrock. It contains
organic material and promotes plant growth and reflects climate. As soils
mature, distinct soil horizons (O, A, B, and C) appear. Distinctive soil types
such as "hardpans" and laterites may be used to interpret paleoclimatic
changes.
Table 8.3 provides brief descriptions of the 9 soil orders of Canada used for
mapping and classification purposes and their approximate equivalents utilized
by the U.S. Soil Conservation Service.
Learning Objectives
1. Weathering creates sediment and soil by either mechanical or chemical
processes. Erosion is the pick-up and removal of weathering products
(transportation).
2. Mechanical weathering causes physical disintegration without compositional
change. Frost wedging, frost heaving, and pressure release cause most
mechanical weathering. Sheeting and exfoliation domes develop as a result of
pressure release. Plant growth, burrowing organisms and salt crystal
development are additional mechanical weathering processes. Differential
weathering is also a common result of mechanical weathering
3. Chemical weathering causes rock decomposition and new mineral formation
that reflects mineral instability because conditions of formation of the original
minerals are significantly different from those of the earth's surface. Oxygen
and slightly acidic rainwater (carbonic acid) are the agents of chemical
weathering. The feldspars and ferromagnesian minerals weather to clay
minerals, while quartz does not weather chemically. Calcite dissolves adding
calcium and bicarbonate ions to ground water, while some silica is produced by
chemical weathering of feldspars. This dissolved load may eventually be
carried to the ocean.
4. Soil is the layer of unconsolidated weathered material on top of bedrock.
Clay minerals make important contributions in holding water and nutrients on
their surfaces because they are negatively charged.
5. Soil horizons develop as it matures. The LFH horizon (O horizon in the U.S.)
is the top layer and consists of plant litter and other organic material. The A
horizon is the next layer and is characterized by leaching downward. The B
horizon is the zone of accumulation for material leached from the A horizon.
The C horizon is transitional from soil to un-weathered bedrock. Most soils are
residual, but transported soils can be deposited by ice, wind, and running
water. Soils thicken with time.
6. The character of the soil depends on the parent material. Soils forming on
granite are sandy, while those forming on basalt may never be sandy. Soil
types reflect climate. Soils containing large amounts of aluminum and iron
oxides are found in wet climates. Soils formed in arid climates are thinner and
contain higher concentrations of calcite. Hardpans form in either wet or dry
climates and have thick B horizons. Laterites form in tropical regions, have
thick A horizons and may be mined for aluminum. The detailed classification
schemes used for soils in Canada and the U.S. are provided in Table 8.3.
Boxes
8.1 - ENVIRONMENTAL GEOLOGY - ACID RAIN: Acidity, or pH, is measured
on a scale which ranges from 0 to 14, with a value of 7 as neutral. Values
below 7 are acid, and ordinary rain has a pH of 5.5 to 6.0 because it mixes
with carbon dioxide to form carbonic acid. Burning fossil fuels emits carbondioxide, nitrogen and sulfur that contribute to even more acidic rain, with pH
values as low as 2. Acid rain accelerates chemical weathering and adversely
effects plants and animals. The damaging effects of acid rain are most
pronounced in areas of eastern Canada underlain by siliceous rocks of the
Canadian Shield. Many lakes in these regions have significantly reduced fish
populations.
Short Discussion/Essay
1. Explain the difference between weathering and erosion.
2. Cite examples illustrating the difference between mechanical and chemical
weathering.
3. What is the difference between soil and regolith?
4. Why are clays so abundant as weathering products?
5. Why is all rain acid?
Longer Discussion/Essay
1. How do rates of weathering vary among the various rock groups? Why do
they vary?
2. How does the mineralogy and grain size of a parent rock influence its
susceptibility to weathering?
3. Why do rates of weathering vary with climate?
4. Explain why soils reflect climate.
5. How are mechanical and chemical weathering related and how do they
influence each other in the weathering history of any rock sequence?
Selected Readings
Berner, R.A. and Lasaga, A.C. 1989. "Modeling the geochemical Carbon cycle,"
Scientific American 260: 74-81.
Birkeland, P.W. 1998. Soils and Geomophology. 2nd ed., New York: Oxford
University Press.
Agriculture Canada, 1987. The Canadian System of Soil Classification (2nd
edition) Agriculture Canada Publication 1646, 164pp.
Hodder, A.P. W. 1997. "A geotechnical approach to weathering simulation,"
Journal of Geoscience Education, 45(4): 359-362
Railsback, L.B. 1993. "A geochemical view of weathering and the origin of
sedimentary rocks and natural waters," Journal of Geological Education 41:
404-411.
Web sites with information on Canadian soils and classification systems:
http://sis.agr.gc.ca/cansis/
http://interactive.usask.ca/ski/agriculture/soils/soilclass/soilclass_can.html
http://www.pedosphere.com/resources/cssc3rd/