Download 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
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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
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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:
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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
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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)