Transcript Weathering
Terrigenous Sediments Weathering Sediment Production and Weathering • Sedimentary Cycle – Components of the Sedimentary Cycle • Weathering – Physical • Types – Chemical • Types • Products – clays Sedimentary Cycle • Rock Cycle – Sedimentary Cycle – Mass movement (non- sedimentary) • Components of Sediment Cycle – – – – – – – Weathering Erosion Transportation Deposition Lithification Uplift Weathering again Components of Sediment Cycle • Weathering – Processes which break down rock at the E’s surface to form discrete particles • Erosion – Processes which remove newly formed sediment from bedrock • Transportation – Gravity driven (creep, mass flow, glaciers, rivers) – Segregates/ sorts the weathering products • Deposition – Energy is exhausted • Lithification – Compaction, cementation Physical Weathering – Mechanical fraction of the rock – Aids in Chemical weathering • RETAINS CHARACTERISTICS OF ORIGINAL ROCK – Works best in cold, dry, high relief – Produces mineralogically immature particulate material Physical Weathering and Sediment Production • Physical weathering is a function of: – Climate • Temperature • Precipitation • Vegetation – Slope Angle (gravity) – Area Area Physical Weathering Mechanisms • • • • • Freeze-thaw/ frost- wedging (ice expands) Daily heating/ cooling (deserts, maybe) Plant Roots (expand cracks) Crystallization of salts (salts expand) Release of overburden pressure – Erosion or melting of thick glaciers • Volume changes as primary (original minerals) are converted to clay minerals (secondary) Physical Weathering – Insolation • Large diurnal temperature variations – Hot arid climates: Mohave » Spring 48°F; to 92°F; Summer 71°F to 108°F » Fall 59°F to 100°F, Winter temperature 41°F to 68°F • Expansion/ contraction due to temperature change – Minerals respond differently, aids in generating stress – If it’s rapid, can crack the rock » Rocks can pop and crack after sun sets (cooling) Physical Weathering – Volume changes from hydration/ dehydration • Alternating wet and dry seasons • Clays, lightly indurated shales expand with water • Upon dehydration, shrinkage cracks develop – Increases permeability to aid in chemical weathering – Reduces rock strength Physical Weathering – Stress Release of overburden • At depth, rocks are compressed by overburden – Elastic-- returns to original size after compression • With weathering, erosion of overburden, rock expands – Can fracture – Creep can aid fracturing • Fractures impacted by other weathering processes – Sheeting – Exfoliation domes Stress Release Steven Marshak Orange River, South Africa Christensen Orange River, South Africa Christensen Chemical Weathering • Meachanical weathering produces sediments – Quartz: 25 - 50% of igneous rock • Beach sands: 50 - 99% quartz • Limestones and evaporites Chemical Weathering • Destruction of rock by solution – Therefore dependent upon water (not frozen) • Water itself only really dissolves evaporites • Needs acid! • Groundwater is acidic – Carbonic acid (CO2 from atmosphere) – Humic acids (from soils) – Usually accompanies mechanical weathering Chemical Weathering • Rock broken down into three main constituents – Residua • Often quartz rich • Feldspar and mica dependent upon weathering – Solutes (end up in ocean!) • Na, K (other alkali metals- base soluble in water) • REE, Ca, Mg, Sr – Newly formed minerals • Clays (hydrated aluminosilicates) • Classification on basis of combination with Ca, K, Mg, Fe Chemical Weathering • Volumetrically, most significant process in the production of sediments – Chemical alteration (reaction) under at surface Conditions: • • • • low temperature (slow reaction rates) abundant water high Eh (oxidizing conditions) generally low pH (acidic conditions; especially in the presence of decaying vegetation) Chemical Weathering • Sequence of Rock Weathering – Relative mobility of main rock- forming elements • decreases from Ca and Na, to Mg, Si, Fe and Al. – Rocks undergoing weathering • Depleted in Ca, Na, Mg • Enriched in Fe- oxides, Al, Si – Particulates produced in reverse of Bowen’s reaction series Chemical weathering • Sequence – Early: particulates are produced and altered • Mafic minerals (olivine, amphibole, pyroxene) form chlorite clays (Fe-, Mg- rich) • Feldspars produce smectites, illites, kaolins – Clays are flushed out as colloidal clay particles • Some stay to form residuum • Mg-, Ca- bearing minerals removed if weathering continues – Ultimately, rock residuum is just Q (if present in parent) + kaolin, bauxite, and limonite • requires warm humid climate, slow erosion Types of Chemical Weathering • Hydrolysis • Oxidation • Solution Types of Chemical Weathering • Hydrolysis – hydrogen ion (H+) combines with silicate group Mg2SiO4 + 4H20 ---> 2Mg++ + 4OH- + H4SiO4 (olivine, unstable protolith mineral) (hydroxyl) + (silicic acid) • reaction raises pH, and • releases silicic acid (a weak acid) – In the presence of dissolved CO2 ( increased conc. by 10x to 100x) of biogenic origin – production of carbonic acid (2H2CO3) drives reaction to the right Types of Chemical Weathering • Oxidation – Loss of an electron with positive increase in valence (charge). – Due to the presence of an oxidant which is Reduced (gain of an electron) with negative increase of valence. • Most metals immediately oxidize in the presence of Oxygen (the most famous surface oxidant) especially: – Fe++--->Fe+++, Mn++---> Mn+4, S--->S+6 (SO4--). Types of Chemical Weathering • Common sequential reactions in the surface weathering environment – Hydrolysis + Oxidation Hydrolysis: liberates metal cations: Fe2SiO4 + 4H2CO3(aq) ---> 2Fe++ + 4HCO3- +H4SiO4 (olivine, fayalite) Oxidation: reprecipitates oxides: 2Fe++ + 4HCO3- + 1/2O2 +2H2O --> Fe2O3 + 4H2CO3 hematite or amorphous iron oxide Types of Chemical Weathering • Solution – ionization of ionically bonded metal cations (Ca++, Na+, Mg++, K+) by dipolar water molecule. H2O + CaCO3 --> Ca++ + CO3= + H2O – Produces the metal cations common in natural waters Types of Chemical Weathering • Ions in Solution – Ions introduced into the surface and ground water by chemical degradation of surface exposed rock-forming minerals • congruent solution: only ions in solution • incongruent: ions in solution + new mineral phase – Elements with preference to ionic bonding are generally most soluble Types of Chemical Weathering • Limiting Factors: – Water • facilitates most weathering reactions – Sufficient Activation Energy (Temperature) • initiates chemical reactions – Long residence time in the soil horizon • access to checmial weathering • minimal physical weathering Products of Chemical Weathering • Insitu Minerals (minerals formed in place) – Clay Minerals : hydrous Alumino-silicate minerals (phylosilicates;) • Oxides – – – – Hemitie - iron oxide goetite/limonite - iron hydroxide pyrolusite - mangenese oxide gibbsite - aluminum hydroxide • Amorphous Silica – product of hydrolysis reactions of silicate minerals (see above) Generalized Chemical Weathering • Temperate Climates 3KAlSi3O8 + 2H+ + 12H2O --> KAlSi3O10(OH)2 + 6H4SiO4 + K+ (K-feldspar) (mica/illite) (silicic acid) • Temperate Humid Climates: 2KAlSi3O8 + 2H+ + 3H2O --> 3Al2Si2O5(OH)4 + K+ (K-feldspar) (kaolinite) • Humid Tropical Climate: Al2Si2O5(OH)4 + 5H2O --> 2Al(OH)3 + 2K+ + 4H4SiO4 (kaolinite) (gibbsite) Clays: Important Chemical Weathering Products • Clay Mineral Species are a function of – environmental conditions at the site of weathering – available cations produced by chemical degradation Sheet Silicates: the Mica's and Clay Minerals • Mica and clay minerals are Phyllosilicates – Sheet or layered silicates with – Two dimensional polymerization of silica tetrahedra – Common structure is a Si205 layer Phyllosilicates Si2O5 sheets of silica tetrahedra Structure of Phyllosilicates • Octahedral layer – Layer of octahedral coordinated • magnesium (brucite layer) or • Aluminum (gibbsite layer) – Makes up the other basic structural unit Kaolinite: Al2Si2O5(OH)4 1:1 tetrahedral – octahedral sheets The Major Clay Mineral Groups • Kaolinite group: – 1:1 TO clay minerals • Mica (illite) group: – 2:1 TOT clay minerals – Expandible clays: • Smectite- montmorillonite complex 2:1 clay minerals • Chlorite – Fe- and Mg-rich TOT clays Chemical Weathering Products • As the age of sedimentary rocks increases clay mineral assemblages in the subsurface transform through diagenesis to illite + chlorite – Clay mineral assemblages in the subsurface provide an indication of the time/temperature conditions experienced (enjoyed???) during burial Biological Weathering • Breakdown of rock by organic processes – Biochemical solution • bacteria • humic acids (rotting organic matter) – Physical fracturing • Tree roots • Burrowing (promotes chemical weathering) – Worms ingest up to 1 mm diameter, can reduce size – Up to 107 earthworms/km2 ; around since Precambrian – Bring 104 km (0.5 cm) of soil to surface • Product – soil Weathering • Products – Solute • Soluble fraction of rocks which are carried in water – Residua • Insoluble products of weathering – Boulder to colloidal clay – Colloid » Substance made up of very small, insoluble nondiffusable particles that remain in suspension Erosion • Water – ? environments • Wind – Sandblasting Transportation • Agents vary in effectiveness at sorting • Gravity, ice (avalanches, glaciers) – Competent to transport ALL weathering products – Inefficient at segregation • Water – Competent to carry material in solution – Less efficient transport residua (?boulders) • Wind – Highly selective (< 0.35 mm) • Medium- fine sands (saltation) • Silty loess (suspension)