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Earth’s external processes
Down and Dirty: The
Formation of Soils
Weathering
Weathering – the physical breakdown
(disintegration) and chemical alteration
(decomposition) or rock at or near
Earth’s surface
Erosion – the physical removal of
material by mobile agents such as water,
wind, ice, or gravity
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
Mechanical Weathering continued
– Unloading – exfoliation of igneous and
metamorphic rocks at the Earth’s 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)
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
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
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
Increase in surface area by
mechanical weathering
granite
limestone
An obvious example of how weathering differs
with different mineral solubilities
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Weathering
Others factors affecting weathering
• Rock characteristics continued
– Silicate minerals weather in the same order as
their order of crystallization
Others factors affecting weathering, cont’d
•Climate
–Temperature and moisture are the most
crucial factors
–Chemical weathering is most effective in areas
of warm, moist climates
Weathering
Differential weathering
Differential weathering
controlled by jointing patterns
• 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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Joint-controlled weathering
in igneous rocks
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
Typical components in a soil
that yields good plant growth
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
Variations in soil development
due to topography
Soil
Factors controlling soil formation
• Plants and animals
– Organisms influence the soil’s 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
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
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”
– Zone of eluviation and leaching
An idealized soil profile
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
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
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
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 Earth’s 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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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).
A famous example…
Denver
Morrison
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.
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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Deposited first
Layer 4 (dump site soil)
The three inner layers were
obviously different than the lastdeposited layer and had a
mineralogy with broad similarities
to soils from the Rocky Mountain
Front west of Denver, Colorado
Deposited last
Layer 3 (body site)
Colorado
Soil ?
Layer 4 (dump site soil)
Layer 1
Fender Surface
Layer 2
With hopes of locating the victim,
investigators collected over 350 samples
from Rocky Mountain Front, comparing these
samples with soil on suspect’s car.
Sept. 12-14, 1960: Most of Coors’ body
recovered
Layer 3
Soil samples obtained from the fender of the car showed four
distinct layers
Before analyses were completed, Coors’
body was found about 43 km south of
Denver.
Layer 2
Layer 1
Fender Surface
Colorado
Soil ?
Outermost layer (deposited last)
matched soil sampled at the
entrance of the dump
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
Pike’s Peak granite)
Sept. 15, 1960: Coors’ skull located
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Deposited first
Deposited first
Layer 4 (dump site soil)
Layer 3 (body site)
June 8, 1957: Corbett buys a revolver through mail order.
Denver-area soil ?
August 1, 1995: Corbett escapes from California Institution of
Men, Chino, California.
Layer 2 ( Coors ranch)
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.
Layer 1
Some additional evidence that surfaces in the investigation
Fender Surface
Layer 4 (dump site soil)
Layer 3 (body site)
Layer 2 ( Coors ranch)
Fender Surface
Layer 1 Colorado soil ?
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)
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)
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 fourdoor 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.
February 24, 1959: Corbett purchases leg irons from a navy surplus
mail order company.
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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
End of Lecture
The Bottom Line: Together with other evidence, soil was
instrumental in reconstructing the crime.
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