weathering, erosion & soil

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    Weathering, erosion &


    Dr. Aman Zalama


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    Alteration of Minerals and Rocks

    Mechanical Weathering

    Frost Wedging

    Pressure Release

    Thermal Expansion and Contraction

    Growth of Salt Crystals


    Organisms and Weathering

    Chemical Weathering Decomposition of Earth Materials




    Sand Dunes

    Expansive Soils and Soil Degradation

    Weathering and Natural Resources

    Soil and Its Origin

    The Composition of Soil and Soil Horizons

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    Exogenetic processes deals with

    weathering, erosion, and soil.

    Weathering is a pervasive phenomenon

    that alters rocks and minerals so that

    they are more nearly in equilibrium

    with a new set of environmental


    Erosion removes weathered materials

    which are transported elsewhere and

    deposited as sediment that may be

    converted to sedimentary rock.

    Earth materials are not structurally

    and compositionally homogeneous, so

    they are attacked by weathering

    processes at different rates, even in

    the same area.

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    Differential Weathering In this example

    of differential weathering, the rock

    layers have been tilted so that the

    planes separating adjacent layers are

    vertical. Weathering and erosion along

    these surfaces have yielded the

    pillars, spires, and isolated knobs at

    this rock exposure in Montana.

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    Mechanical weathering processes

    break parent material into

    smaller pieces but do not change

    its composition.

    Frost action, pressure release,

    thermal expansion and

    contraction, salt crystal

    growth, and the activities of

    organisms account for mechanical


    is a form of mechanical weathering

    (that is, weathering that involves

    physical, rather than chemical change).

    Frost wedging is caused by the repeated

    thaw cycle of water in extreme -freeze

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    ll cracks climates. Most rocks have sma

    in them, called joints (or, tectonic

    joints). When it rains, rainwater seeps

    into these joints. As the day cools and

    temperatures at night drop below

    freezing, the water inside the joints

    freezes. As water freezes into ice, it

    ing ice places expands. The expand

    pressure on the joints in the rock.

    Finally, when the pressure is too much,

    the joint expands. In some cases, the

    rock will split, though this usually

    happens after repeated freeze and

    thaws. As new water is added during the

    e ice is created at warmer days, mor

    night, wedging the joints apart


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    Some rocks form at depth and are

    stable under tremendous pressure.

    Granite, for instance, crystallizes

    far below the surface, so when it

    is uplifted and the overlying

    material is eroded, its contained

    energy is released by outward

    expansion, a phenomenon called

    pressure release.

    Outward expansion results in the

    origin of fractures called sheet

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    joints that more or less parallel

    the exposed rock surface.

    Sheet-jointbounded slabs of rock

    slip or slide off the parent rocka

    process known as exfoliation

    leaving large, rounded masses of

    rock called exfoliation domes.

    During thermal expansion and

    contraction, the volume of rocks

    changes in response to heating

    and cooling.

    In a desert, where the

    temperature may vary as much as

    30C in one day, rocks expand

    when heated and contract as they


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    Rock is a poor conductor of

    heat, so its outside heats up

    more than its inside; the

    surface expands more than the

    interior, producing stresses

    that may cause fracturing.

    Under some circumstances, salt crystals

    that form from solution cause

    disaggregation of rocks.

    Growing crystals exert enough force to

    widen cracks and crevices or dislodge

    particles in porous, granular rocks

    such as sandstone.

    Even in crystalline rocks such as

    granite, salt crystal growth may pry

    loose individual minerals.

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    Animals, plants, and bacteria

    all participate in the

    mechanical and chemical

    alteration of rocks.

    Burrowing animals, such as

    worms, termites, reptiles,

    rodents, and many others,

    constantly mix soil and sediment

    particles and bring material

    from depth to the surface where

    further weathering occurs.

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    Chemical weathering processes include

    solution, oxidation, hydrolysis, and

    the activities of organisms. They

    result in a change in the structure,

    composition, or both of parent


    Particle size, climate, and type of

    parent material determine how quickly

    chemical weathering takes place.

    To a chemist, oxidation is any

    chemical reaction in which a compound

    or ion loses electrons, whether oxygen

    is present or not.

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    But in chemical weathering it refers to

    reactions with oxygen to form oxides

    (one or more metallic elements combined

    with oxygen) or, if water is present,

    hydroxides (a metallic element or

    radical combined with the hydroxyl, OH


    For example, iron rusts when it

    combines with oxygen and forms the iron

    oxide hematite:

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    When hydrolysis takes place, hydrogen

    ions (H) or the hydrogen in (OH) of

    water reacts with and replaces positive

    ions in minerals, thereby changing

    their composition and liberating

    soluble compounds and iron that may

    then be oxidized.

    Potassium feldspars such as orthoclase

    (KAlSi3O8) and plagioclase feldspars

    (which vary from CaAl2Si2O8 to

    NaAlSi3O8) are framework silicates, but

    when altered by hydrolysis they yield

    materials in solution and clay

    minerals, which are sheet silicates.

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    Erosion is the removal of weathered

    materials from their source area.

    Erosion is the removal of weathered

    rocks and minerals by moving water,

    wind, ice, and gravity.

    Deflation is erosion by wind. Silt and

    sand are removed selectively, leaving

    larger stones on the surface and

    creating desert pavement.

    The Western Desert of Egypt is the land

    of wind.

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    Sand grains are carried short distances

    and a meter or less above the ground by

    saltation, but silt can be transported

    great distances at higher elevations.

    A dune is a mound or ridge of wind-

    deposited sand. Most dunes are

    asymmetrical, with gently sloping

    windward sides and steeper slip faces

    on the lee sides.

    Types of dunes include barchan dunes,

    transverse dunes, longitudinal dunes,

    and parabolic dunes.

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  • 17 | P a g e

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    dan slake rnea ro sdesert ni dfoun


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    Expansive soils contain clay minerals

    that expand when wet and shrink when

    they dry.

    Soil degradation is any loss of soil or

    decrease in soil fertility resulting

    from erosion, compaction, and

    contamination by pollutants.

    Weathering is responsible for the origin of

    some natural resources and for the

    concentration of others, such as bauxite

    and other residual concentrations. And of

    course soils are one product of weathering.

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    Soil is made up of weathered materials,

    air, water, and organic matter and can

    support vegetation.

    Soil profiles for pedalfer, pedocal,

    and laterite show horizons designated

    O, A, B, and C that differ in

    structure, composition, and color.

    Climate, parent material, organisms,

    angle of slope, the direction a slope

    faces, and time are the factors that

    control the rate of soil formation.

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    A Horizon

    B Horizon

    C Horizon

    Bed rock


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