Transcript Chapter 7
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 Mineral weathering Hypersthene Augite More to least resistant minerals to weathering Chemical composition of igneous vs sedimentary lithologies 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 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 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 processes Coarse 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