Chapter 7- Parent Material influences on Pedogenesis Primary influence is on mineralogy Some minerals more resistant than others e.g., feldspar vs quartz.

Chapter 7- Parent Material influences on Pedogenesis

Primary influence is on mineralogy

Some minerals more resistant than others

e.g., feldspar vs quartz

e.g., olivine vs quartz

Chapter 7- Parent Material influences on Pedogenesis

Primary influence is on mineralogy

Some minerals more resistant than others

e.g., feldspar vs quartz

e.g., olivine vs quartz

Measuring influence• Measure the weathering of key

minerals– Compare those in profile to those that

are unweathered (i.e., parent material)– Look for depletion of elements, etching,

or clay alteration•Hornblende etching

• Measure the weathering of key minerals– Compare those in profile to those that

are unweathered (i.e., parent material)– Look for depletion of elements, etching,

or clay alteration•Hornblende etching

Mineral weathering

AugiteHypersthene

More to least resistant minerals to weathering

Chemical composition of igneous vs sedimentary lithologiesChemical composition of igneous vs sedimentary lithologies

Chemical composition influence

Chemical composition influence

• Rocks vary in mineralogy – Changes the chemical composition of

the material– Changes the resistance to weathering

• Resistant minerals = harder to weather = thinner or less well developed soils when compared to those developed in rocks with less resistant mineralogy and chemical composition

• Rocks vary in mineralogy – Changes the chemical composition of

the material– Changes the resistance to weathering

• Resistant minerals = harder to weather = thinner or less well developed soils when compared to those developed in rocks with less resistant mineralogy and chemical composition

Influence of texture on soils

Can refer to consolidated or unconsolidated material

consolidated

porosity or fractures are key

fine grained with highly permeable and porous conditions is preferable

Influence of texture on soils

Can refer to consolidated or unconsolidated material

consolidated

porosity or fractures are key

fine grained with highly permeable and porous conditions is preferable

Depth of leaching varies with permeability and porosity of parent material

Depth of leaching varies with permeability and porosity of parent material

Extreme control• Podzolization or not Podzolization. That is

the question!– Common in sandy material formed from

crystalline rock– Not common in glacial till formed from

ground up sedimentary rocks– Common in soils with ultramafics– Not common in soils with lesser amounts of

ultramafics– Sandy (common) clayey (uncommon)

Limestone soils- terra rosa• Often contain soils that are nothing like

the parent rock– Four common interpretations

• Residual materials in carbonate host rock• Fluvial or colluvial from higher positions on

landscape• Ash sources• Eolian dust sources

– Lab analysis and field observation can assist in source determination• Mass balance, chemical signatures, topographic

expression

Ash soils• More control over soil formation than

any other substance– So unique that they have their own soil

order!•Andisols- melanic epipedon

– Often have unique subhorizons and weathering materials•Volcanic glass- weathers into clays like

allophane– Often mistaken for albic subhorizons

– Simple chemical tests often assist in determining origins

Uniformity

• Parent material is very important for assessing development– e.g. PDI relies heavily on parent material

values

• Needed to separate pedogenic processes from sedimentary processes– Bedding vs horizonation

Numerous ways to mix up the parent materials

• Frost heave• Shrink swell clays• Colluvial washdown• Bioturbation• Preferential weathering

Dilution by eolian processesCoarse fractions fine upward due to input from eolian sources

Dilution by disintegration

• Preferential disintegration of smaller sized fractions relative to larger clasts– Results in more fines being produced

as large stuff gets left behind• Surface to volume ratio dictates this

River deposits

• Problematic due to episodic variation in deposition– Alternating energy of deposition creates

stratified materials– Must separate strata from pedo processes

when evaluating