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Down and Dirty: The Formation of Soils

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1 Down and Dirty: The Formation of Soils

2 Earth’s external processes
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

3 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

4 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

5 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)

6 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

7 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


9 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

10 Weathering Rates of weathering Others factors affecting 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

11 An obvious example of how weathering differs
granite limestone An obvious example of how weathering differs with different mineral solubilities

12 Increase in surface area by mechanical weathering

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

14 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

15 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

16 Differential weathering controlled by jointing patterns

17 Joint-controlled weathering in igneous rocks

18 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

19 Typical components in a soil that yields good plant growth

20 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

21 Soil Factors controlling soil formation Time Climate
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

22 Soil Factors controlling soil formation Plants and animals Slope
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

23 Variations in soil development due to topography

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

25 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

26 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”

27 An idealized soil profile

28 A soil profile showing different horizons

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

30 Soil Three very generic soil types Pedalfer Pedocal Laterite
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

31 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

32 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

33 A famous example…

34 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

35 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.

36 A car belonging to a suspect (Joseph Corbett, Jr
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 ?

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

38 Deposited first Layer 1 Layer 2 Layer 3 Layer 4 (dump site soil)
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

39 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 , 1960: Most of Coors’ body recovered Sept. 15, 1960: Coors’ skull located

40 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 Pike’s Peak granite) Layer 3 (body site) Layer 4 (dump site soil) Fender Surface Colorado Soil ?

41 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

42 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 ?

43 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.

44 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.

45 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.

46 End of Lecture

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