Down and Dirty: The Formation of Soils

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Down and Dirty The Formation of Soils
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Earths external processes

• Weathering the physical breakdown
  (disintegration) and chemical alteration
  (decomposition) or rock at or near Earths
  surface
• Erosion the physical removal of material by
  mobile agents such as water, wind, ice, or gravity

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Weathering

• Two types of weathering
• Mechanical weathering breaking of rocks into
  smaller pieces
• Four types of mechanical weathering
• Frost wedging alternate freezing and thawing of
  water in fractures and cracks promotes the
  disintegration of rocks

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Weathering

• Mechanical Weathering continued
• Unloading exfoliation of igneous and
  metamorphic rocks at the Earths surface due to a
  reduction in confining pressure
• Thermal expansion alternate expansion and
  contraction due to heating and cooling
• Biological activity disintegration resulting
  from plants and animals

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Weathering

• Chemical Weathering
• Breaks down rock components and internal
  structures of minerals
• Most important agent involved in chemical
  weathering is water (responsible for transport of
  ions and molecules involved in chemical processes)

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Weathering

• Major processes of chemical weathering
• Dissolution
• Aided by small amounts of acid in the water
• Soluble ions are retained in the underground
  water supply
• Oxidation
• Any chemical reaction in which a compound or
  radical loses electrons

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Weathering

• Major processes of chemical weathering
• Oxidation continued
• Important in decomposing ferromagnesian minerals
• Hydrolysis
• The reaction of any substance with water
• Hydrogen ion attacks and replaces other positive
  ions

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Weathering

• Alterations caused by chemical weathering
• Decomposition of unstable minerals
• Generation or retention of materials that are
  stable
• Physical changes such as the rounding of corners
  or edges

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Weathering

• Rates of weathering
• Advanced mechanical weathering aids chemical
  weathering by increasing the surface area
• Others factors affecting weathering
• Rock characteristics
• Rocks containing calcite (marble and limestone)
  readily dissolve in weakly acidic solutions

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granite
limestone
An obvious example of how weathering differs
with different mineral solubilities
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Increase in surface area by mechanical
weathering
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Weathering

• Others factors affecting weathering
• Rock characteristics continued
• Silicate minerals weather in the same order as
  their order of crystallization

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Others factors affecting weathering, contd

• Climate
• Temperature and moisture are the most crucial
  factors
• Chemical weathering is most effective in areas of
  warm, moist climates

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Weathering

• Differential weathering
• Masses of rock do not weather uniformly due to
  regional and local factors
• Results in many unusual and spectacular rock
  formations and landforms

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Differential weathering controlled by jointing
patterns
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Joint-controlled weathering in igneous
rocks
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Soil

• Soil is a combination of mineral and organic
  mater, water, and air
• That portion of the regolith (rock and mineral
  fragments produced by weathering) that supports
  the growth of plants

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Typical components in a soil that yields good
plant growth
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Soil

• Factors controlling soil formation
• Parent material
• Residual soil parent material is the underlying
  bedrock
• Transported soil forms in place on parent
  material that has been carried from elsewhere and
  deposited

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Soil

• Factors controlling soil formation
• Time
• Important in all geologic processes
• Amount of time for soil formation varies for
  different soils depending on geologic and
  climatic conditions
• Climate
• Most influential control of soil formation
• Key factors are temperature and precipitation

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Soil

• Factors controlling soil formation
• Plants and animals
• Organisms influence the soils physical and
  chemical properties
• Also furnish organic matter to the soil
• Slope
• Steep slopes often have poorly developed soils
• Optimum terrain is a flat-to-undulating upland
  surface

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Variations in soil development due to
topography
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Soil

• The soil profile
• Soil forming processes operate from the surface
  downward
• Vertical differences are called horizons zones
  or layers of soil

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Soil

• The soil profile
• O horizon organic matter
• A horizon organic and mineral matter
• High biological activity
• Together the O and A horizons make up the topsoil
• E horizon little organic matter
• Zone of eluviation and leaching

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Soil

• The soil profile
• B horizon zone of accumulation
• C horizon partially altered parent material
• The O, A, E, and B horizons together are called
  the solum, or true soil

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An idealized soil profile
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A soil profile showing different horizons
O
A
B
C
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Soil

• Soil types
• The characteristics of each soil type primarily
  depend on the prevailing climatic conditions
• Three very generic soil types
• Pedalfer, Pedocal, Laterite

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Soil

• Three very generic soil types
• Pedalfer
• Best developed under forest vegetation
• Accumulation of iron oxides and Al-rich clays in
  the B horizon
• Pedocal
• Associated with dry grasslands and brush
  vegetation
• High accumulations of calcium carbonate
• Laterite
• Hot and wet tropical climates
• Intense chemical weathering

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Significance of Soil to Forensics
Although soils can be grouped together in various
categories according to characteristics, no two
soils are truly identical. This is partly a
function of the fact that no two parent materials
(e.g. rocks) are truly identical. Also, the
amounts and types of organic material contained
in soils differ according to climate and
vegetation type
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To trace the transfer patterns or ultimate source
areas of particular soils, forensic geologists
focus on the unusual, rather than the
usual. Features unique to soil of a particular
area can include distinctive minerals, texture,
and colour Soils are the most common materials
used in forensic geology because of their
dominance on the Earths surface (soils are most
likely to be picked up and transferred), and
their diverse characteristics. In some cases,
the concept of superposition is also useful
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A famous example
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Coors Case
Adolph Coors III (age 44, grandson Adolph Coors
I, the founder of Coors brewery), disappeared on
the morning of February 9, 1960 near Morrison,
Colorado (near Denver).
Denver
Morrison
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Coors car was later found with the motor still
running Coors glasses, hat, and spots of blood
were found at the scene, suggesting murder of the
victim and removal of the body.
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A car belonging to a suspect (Joseph Corbett,
Jr.) was discovered February 17, burning in a
garbage dump in Atlantic City, New Jersey Was
there a connection ?
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Soil samples obtained from the fender of the car
showed four distinct layers
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Deposited first
Outermost layer (deposited last) matched soil
sampled at the entrance of the dump The three
inner layers were obviously different than the
last-deposited layer and had a mineralogy with
broad similarities to soils from the Rocky
Mountain Front west of Denver, Colorado
Layer 1
Layer 2
Layer 3
Layer 4 (dump site soil)
Fender Surface
Colorado Soil ?
Deposited last
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With hopes of locating the victim, investigators
collected over 350 samples from Rocky Mountain
Front, comparing these samples with soil on
suspects car. Before analyses were completed,
Coors body was found about 43 km south of
Denver. Sept. 12-14, 1960 Most of Coors
body recovered Sept. 15, 1960 Coors skull
located
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Layer 1
Layer 2
Further soil comparisons indicated similarities
between layer 3 on fender and soil where body was
found (south of Denver) material included
distinctive pink feldspar from Pikes Peak
granite)
Layer 3 (body site)
Layer 4 (dump site soil)
Fender Surface
Colorado Soil ?
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Deposited first
Further soil comparisons indicated similarities
between layer 2 on fender and soil from Dakota
Hogback where Coors ranch was located (material
included grains derived from sandstone, gray,
green and maroon shales, and traces of limestone)
Layer 1
Layer 3 (body site)
Layer 2 ( Coors ranch)
Layer 4 (dump site soil)
Colorado soil ?
Fender Surface
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Deposited first
The origin of layer 1 was inconclusive, but was
determined to have been sourced from somewhere in
the Denver area (grains diagnostic of weathered
Front Range granites in Denver area)
Layer 1
Layer 3 (body site)
Layer 2 ( Coors ranch)
Layer 4 (dump site soil)
Fender Surface
Denver-area soil ?
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Some additional evidence that surfaces in the
investigation
March, 1951 Corbett pleads guilty to
second-degree murder in San Rafael County,
California -sentenced to a term of five years to
life -eventually jailed at California Institution
for Men, Chino, California. August 1, 1995
Corbett escapes from California Institution of
Men, Chino, California. June 8, 1957 Corbett
buys a revolver through mail order. February
24, 1959 Corbett purchases leg irons from a navy
surplus mail order company.
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April 25, 1959 Corbett purchases handcuffs by
mail-order October 8, 1959 Corbett buys
typewriter December 23, 1959 Corbett sells his
1957 Ford. January 8, 1960 Corbett registers
1951 yellow Mercury four-door car under the name
Walter Osborne. February 10, 1960 Early
Morning. Corbett moves out of his apartment in
Denver. Ransom note demanding 500,000 arrives
addressed to Mrs. Adolph Coors, III. Postmark is
February 9, 3PM. Ransom is never collected.
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Capture
Corbett was eventually captured in October, 1960
in Vancouver, Canada, convicted in Colorado
court, and sentenced to life in prison. 1978
Corbett paroled The Bottom Line Together with
other evidence, soil was instrumental in
reconstructing the crime.
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End of Lecture