Chapter 3 Weathering Processes Two major categories Mechanical

Chapter 3 - Weathering Processes Two major categories Mechanical (or Physical) processes physical breakdown of larger particles into smaller particles Chemical processes Chemical reactions that facilitate soil profile development Removal of chemical elements Addition of altered chemical elements Addition of new chemical elements

Mechanical processes aid Chemical processes Increases surface area to volume ratio Chemical processes dominate soil development Mechanical processes breakage can occur along bedding planes Along stress induced fractures or joints Micro or macro scale Along crystal boundaries

Pressure release Caused by unroofing of deeply buried rock- typically igneous, but can be any kind of rock

Sheet joints due to pressure release create an exfoliation dome known as Enchanted Rock in Enchanted Rock State Park, Texas

Freeze Thaw Water, when frozen, expands and can fracture rock if the cracks have the appropriate geometry and the climate is suitable. Salt Crystal Growth Similar to freeze thaw fracturing, crystal growth occurs as chemicals dissolved in the water precipitate out as water evaporates- creating salt crystals that pry the rock apart e. g. , K-horizons with high percentage of salt in sandy soils push sand grains apart so they are held in salt suspension Can also induce chemical reactions on mineral grainschemical weathering

This boulder in Death Valley, California has disintegrated in situ due to saline waters entering fractures, and prying apart the cracks Best developed early in pedogenesis

Thermal Fracturing Implication of freeze thaw expansion and contraction due to changes in temperature and thermal properties of the parent material Also relates to fire related actions as well Spalling occurs when fire superheats the surface Also vaporizes water which exerts force Can be influenced by micro -environmental conditions

Influence of fire and environmental conditions on weathering of boulders

Sand in the bottom of a 5 m deep weathering pit at Cookie jar Butte in Utah

13 m deep weathering pit in Utah Likely formed from a variety of mechanical and chemical processes

Sand accumulated at the bottom of a 18 m deep weathering pit in Utah.

Chemical weathering is dominant factor in pedogenesis Requires mechanical weathering to facilitate continued chemical reactions Surface area to volume ratio…

Congruent vs Incongruent weathering Congruent- no by products from the chemical reaction in an water-based solution i. e. , the stuff goes into solution and completely dissolves e. g. , salt (Na. Cl) or limestone (Calcite {Ca. CO 3} Na. Cl + H 2 O ------->Na+ & Crystalline & liquid aqueous Claqueous Ca. CO 3 + CO 2 + H 20 ----->Ca 2+ + 2 HCO 3 Crystalline & gas & liquid aqueous Solubility of calcite is dependent upon CO 2 and H+ concentration

Soil environment is loaded with acids and gases that can supply the either of the two things that drive the dissolution of calcite organics create organic acids CO 2 is abundant in soils and is greater than in the actual atmosphere The reactions are self sustaining up to a point saturation of solution with respect to Ca. CO 3

Soils over limestone are influenced by insoluble materials in the rock eolian influx biological components Complete dissolution of pure limestone produce very little in the way of soil. Incomplete dissolution creates a special kind of material called terra rosa red earth

Terre Rosa fills sinkholes in karst terrane

processes of chemical weathering • Oxidation/Reduction - a function of available free oxygen and the Eh (redox potential) - Oxygen strips electrons away from other atoms • the process is reversible - often related to water levels in streams and groundwater systems

Incongruent chemical weathering creates weathering by-products or materials Especially commonplace when considering silicates Why? - silica is only slightly soluble in most aqueous solutions with lower p. H values (0. 0007 g/liter or 7 ppm in most solutions less than p. H of 9) That means that some other elements will dissolve but in so doing cause secondary chemical reactions that create new minerals, especially those that can react with the silicate structures Examples clay minerals; oxidation processes

Oxidation • interaction between substances and oxygen (and in some cases water) 4 Fe. O + O 2 2 Fe 2 O 3 The iron goes from the ferrous reduced state (Fe 2+) to the ferric (Fe 3+) state Mg. Fe. Si. O 4 + 2 H 2 O Mg(OH)2 + H 2 Si. O 3 + Fe. O • E. G. , olivine magnesium hydroxide 2 Fe. S 2 + 7 H 2 O + 15 O pyrite silicic acid 2 Fe(OH) + 4 H 2 SO 4 sulfuric acid

Clay minerals • occur as layers or sheets arranged in a variety of ways - layers consist of sheets of aluminum and silica that alternate between discrete layers > 1: 1 - clay minerals include Kaolinite, Allophane, and Halloysite > 2: 1 - clay minerals include Illite, Smectite, Montmorillonite, and Glauconite • usually derived from the decomposition of silicate minerals like feldspars, micas, and hornblende • Can have unique properties that are important for good construction > shrink-swell clays like Smectite

Examples of incongruent chemical weathering

Solubility of materials determines which things remain to be able to create alternative minerals in the soil, most notably clay minerals Polynov’s Ion mobility series Cl- > SO 42 - > Na 2+ > Ca 2+ > Mg 2+ > K 2+ > Si 2+ > Fe 3+, 2+ > Al 2+ Phase I--><--II--><---III---><--IV--><---V----

Other factors • Mobility-how susceptible atoms are to movement due to normal chemical processes • Leaching - removal of elements from upper portions of the weathering profile to locations lower in the weathering profile • Chelation- mobilization of relatively immobile ions by encapsulating them in a complex molecular structure - common way to move things like Aluminum, Iron and Calcium - usually involves organic compounds that fix the metal into a mobile compound

Chelation structures

Abrasion p. H’s for common minerals As cations are released in the grinding process, the solution, which begins as a neutral p. H becomes increasingly more basic However, in most environments, this tendency is offset by production of more H+ cations

H+ ions attack weaker spots in the crystalline structure of aluminosilicate minerals

Etching • related to chemical weathering acting and pieces breaking off along cleavage surfaces - keeps rates from achieving some steady state

processes of chemical weathering • solution - removal of atoms from mineral structures - causes minerals to fall apart • related to p. H of the solvent - typically the water

chemical processes • Ion exchange - usually a function of a polarized molecular arrangement > causes a propensity to try and balance the charges by replacing ions with other ions through a process called adsorption > esp. Cations - often measured as CEC (Cation Exchange Capacity) > a measure of the adsorption ability of a surface - particularly common in clay minerals

Measurement of weathering Total chemical analysis is one way given as oxides- oxygen is a common balancing ion