Weathering, Erosion and Transport - MSU Erosion, ... weathering –The disintegration or disaggregation of rocks by ... weathering –Decomposition is most rapid at corners

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  • Weathering, Erosion and Transport

  • Weathering, Erosion, and Transportation

    Rocks exposed at Earths surface are constantly changed

    by water, air, temperature variations and other factors

    Weathering is the group of destructive processes that

    change physical and chemical character of rocks at or

    near Earths surface

    Erosion is physical picking up of rock particles by water,

    ice, or wind

    Transportation is the movement of eroded particles by

    water, ice, or wind

  • The Nature of Weathering

    Weathering is the physical and/or chemical alteration of rocks and minerals where the lithosphere, hydrosphere, atmosphere, and biosphere meet

  • Weathering and Earth Systems

    Hydrosphere Water is necessary for chemical weathering

    Oxygen dissolved in water oxidizes iron and other metals in rocks

    Carbon dioxide dissolved in water creates carbonic acid

    Primary cause of chemical weathering

    Running water loosens and abrades particles

    Glacial ice removes and abrades particles

    Freeze/thaw cycling mechanically weathers

    Biosphere Plant root growth widens cracks

    Animal movement and human activity mechanically weather

    Plant roots decaying organic matter in soils pump carbon dioxide into

    the soil producing acids that dissolve the rock

  • Products of Weathering

    Lithic (Rock) Fragments(granite, basalt, schist, etc.)

    Dissolved Ions(Calcium, Potassium, Sodium, etc.)

    Rust Minerals (Hematite, Goethite, etc.)

    Clay Minerals(Bentonite, Montmorillonite, etc.)

    Residual Minerals(Quartz, Orthoclase, Muscovite, etc.)

  • Two Types of Weathering

    Chemical weathering The decomposition of rocks and minerals as

    chemical reactions alter them into new minerals stable at the Earths surface

    Physical (or mechanical) weathering The disintegration or disaggregation of rocks by

    physically breaking them apart

  • Weathering

    Physical and chemical weathering are two distinct processes

    Usually work in tandem

    Chemical weathering is more significant in warm wet low land environments; Physical weathering is more important in in cold areas and high elevations

  • Physical Weathering

  • Physical Weathering

    Frost action Mechanic effect of freezing (and

    expanding) water on rocks

    Pressure release Removal of overlying rock allows

    expansion and fracturing

    Plant growth Growing roots widen fractures

    Burrowing animals

    Thermal cycling Large temperature changes fracture rocks

    by repeated expansion and contraction

  • But mostly physical weathering is a matter of things just

    falling down. So in a sense, gravity, is the primary cause of

    physical weathering.

  • Physical Weathering in cold high altitude

    environments

  • Physical Weathering by running water

    More on that later

  • Sheeting

    Release of confining pressure on rocks formed deep within the Earth

    Development of fractures and joints caused by expansion

    Rocks break along fractures and joints

  • Sheeting in granite

  • Ice Wedging

    Freeze - Thaw cycles are effective at breaking apart rocks

    Water expands when it freezes

    Volume increases by 9%

    The stress of expansion breaks the rock

    Ice melts and the water percolates deeper into the newly expanded cracks

  • Frost/freeze or Ice wedging

  • Geometry of Weathering

    Spheroidal weathering

    Corners tend to be rounded during weathering

    Decomposition is most rapid at corners

    Rocks shape approaches sphere

    Further weathering reduces size

  • Spheroidal weathering in granites

  • Spheroidal weathering in granites

  • Other Forms of Physical Weathering

    Heat Heat causes rocks (most solids) to expand

    Rocks are poor conductors of heat

    Outer layer of rock that expands breaks off (spall)

    Crystal Growth Minerals precipitate along fractures

    Similar to ice wedging

  • Other Forms of Physical Weathering

    Root Growth Roots may exert enormous forces in growing

    Root tips pressures may exceed 10,000 kg per square meter

    Seeds gather in cracks in rock and germinate

    Growing plant and roots slowly wedge rock apart

  • Geometry of Weathering

    Fractures in rock form from the reduction in load (pressure)

    Generally form in groups

    Parallel joints

    Intersecting joints

    Cut large blocks into smaller blocks

  • Geometry of Weathering

    Surface Area is increased by fracturing

    The increase in surface area, increases the rate of weathering

    Both physical and chemical

    Surface area increases exponentially

    y = 3e0.6931x

    0

    10

    20

    30

    40

    50

    60

    0 2 4 6

    Series1

    Expon.

    (Series1)

  • Chemical Weathering

  • Chemical Weathering

    Minerals are destroyed or altered by chemical reactions

    Dissolution

    Hydrolysis

    Oxidation

  • Chemical Weathering

    Oxidation

    Chemically active oxygen from atmosphere

    Iron oxides are common result

    Soil and sedimentary rocks often stained with

    iron oxides

    Acid dissolution

    Hydrogen cations replace others in minerals

    Carbonic acid from atmospheric CO2 dissolved

    in water

    Sulfuric, hydrofluoric acids emitted by volcanic

    eruptions

    Some minerals, such as calcite, may be totally

    dissolved

    Human activity, such as mining and burning of

    fossil fuels, produces acids

  • Chemical Weathering

    Feldspars

    Most common minerals in crust

    Slightly acidic rain water attacks feldspar

    Clay minerals produced

    K+, Na+, Ca++ ions released into water

    Other minerals

    Ferromagnesian minerals

    Clays, iron oxides, Mg++ ions produced

    More complex silicate bonds lead to lower weathering susceptibility

    Olivine most susceptible, quartz least

    Warm, wet climatic conditions maximize weathering

  • Chemical Weathering

    Most igneous and metamorphic rocks and minerals are formed at high temperatures and pressures They are in a state of equilibrium at the

    Temperature (T) and Pressure (P) of formation

    At the Earths surface, rocks and minerals are subject to chemical weathering

    Secondary minerals formed at the T and P common to the Earths surface

  • Chemical Weathering (cont)

    Sedimentary Rocks: Limestones and Dolomites are formed in

    the ocean and are easily dissolved by water, especially if it is acidic

    Evaporites (Halite, Gypsum and Anhydrite) are precipitated from seawater and easily dissolved in water even if it is not acidic

  • Dissolution

    Some minerals are soluble in water

    e.g., Halite - NaCl

    Minerals dissolve into constituent ions

    Ions removed with water by leaching

    Solubility of compound controls leachability

  • Acid Hydrolysis

    CO2 mixes with water to produce carbonic acid, H2CO3

    Decaying organic matter produces acid

    Roots pump CO2 into the soil producing very high concentrations of carbonic acid

    Anthropogenic sources of acid (CO2 and SO2) Acid rain

  • Acid Hydrolysis

    H+ attacks minerals by replacing other ions in the mineral structure

    Promotes dissolution

    Calcite hydrolysis by carbonic acid solution

    CaCO3 + H2CO3 Ca+2 + 2HCO3

    -

  • Acid Hydrolysis

    New secondary minerals may be created by this process

    H+ ion replaces the K+ ion in the feldspar structure

    K+ ion goes into the water solution

    Kaolinite, a clay mineral, formed

    2KAlSi3O8 + 2H2CO3 + 9H2O =

    2K+ + 2HCO3- + 4H4SiO4 + Al2Si2O5(OH)4

  • Oxidation

    Valence state increases

    Often associated with free O2 in the environment

    Iron is usually found as the Fe+2 ion in silicate minerals

    Exposed to the atmosphere it will oxidize to the Fe+3 ion

  • Oxidation

    Change in valence state disrupts crystal structure

    Oxidation works in combination with hydrolysis and dissolution

    2FeSiO4 + 4H2O + O2 = 2 Fe2O3 + 2H4SiO4

  • Trends in Chemical Weathering

    Alkali and alkaline earth elements removed into solution

    Al and Si are enriched in secondary minerals

    Fe is enriched in insoluble ferric oxides

    Warm wet climates increase chemical weathering rates

  • Weathering of Rocks

    Relative stability of minerals varies widely

    Minerals composition is primary control

    Rock texture influences role of water in weathering

  • Relative stability of minerals

  • Inorganic Carbon CycleWhat controls CO2 concentrations on geologic timescales

    Carbon dioxide present as trace

    gas in atmosphere (380ppm)

    Combines with water to form

    carbonic acid (H2CO3)

    Weathers rocks and provides

    CaCO3 to marine animals and

    plants so they can make shells.

    Returns to the mantle during

    subduction

    Released back to atmosphere by

    volcanic eruptions

  • On geologic timescales volcanism controls CO2 concentrations.

    The negative feedback mechanism on this is the rate of weathering which

    increases because of warmer climates due to higher CO2 concentrations.

  • Climate & Weathering

    Climatic conditions strongly influence weathering reactions

    Amount of rainfall