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Down and Dirty: The Formation of Soils

Earth’s external processes

Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surfaceErosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

WeatheringTwo 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

Weathering

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)

WeatheringMajor 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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WeatheringAlterations 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

granite limestone

An obvious example of how weathering differs with different mineral solubilities

Increase in surface area by mechanical weathering

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WeatheringOthers factors affecting weathering

• Rock characteristics continued– Silicate minerals weather in the same order as

their order of crystallization•Climate

–Temperature and moisture are the most crucial factors

–Chemical weathering is most effective in areas of warm, moist climates

Others factors affecting weathering, cont’d

WeatheringDifferential weathering

• Masses of rock do not weather uniformly due to regional and local factors

• Results in many unusual and spectacular rock formations and landforms

Differential weathering controlled by jointing patterns

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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

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

Variations in soil development due to topography

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

Soil

The soil profile• B horizon – zone of accumulation• C horizon – partially altered parent

materialThe O, A, E, and B horizons together are called the solum, or “true soil”

An idealized soil profile A soil profile showing different horizons

OA

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

SoilThree 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 ForensicsAlthough 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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A famous example…

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).

Coors Case

DenverMorrison

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 JerseyWas there a connection ?

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Soil samples obtained from the fender of the car showed four distinct layers

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

Fender Surface

Layer 1Layer 2

Layer 3

Layer 4 (dump site soil)

Deposited first

Deposited last

ColoradoSoil ?

With hopes of locating the victim, investigators collected over 350 samples from Rocky Mountain Front, comparing these samples with soil on suspect’s 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

Fender Surface

Layer 1

Layer 2

Layer 3 (body site)

Layer 4 (dump site soil)

ColoradoSoil ?

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)

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Fender Surface

Layer 1

Layer 2 ( Coors ranch)

Layer 3 (body site)

Layer 4 (dump site soil)

Deposited first

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)

Fender Surface

Layer 1

Layer 2 ( Coors ranch)

Layer 3 (body site)

Layer 4 (dump site soil)

Deposited first

Denver-area soil ?

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)

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.

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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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.

Capture

End of Lecture