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Weathering. Objectives Describe the Rock Cycle. Briefly contrast weathering and erosion. Contrast physical, chemical and biological weathering. List and describe the types of mechanical weathering. List and describe the types of chemical weathering. . The Rock Cycle.

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Weathering

Weathering

Objectives

Describe the Rock Cycle.

Briefly contrast weathering and erosion.

Contrast physical, chemical and biological weathering.

List and describe the types of mechanical weathering.

List and describe the types of chemical weathering.


The rock cycle
The Rock Cycle

  • The three major types of rocks: igneous, sedimentary and metamorphic.

  • Rocks are interrelated by a series of natural processes.

  • Igneous rocks (e.g. granite, basalt) form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments. When Sediments are deposited they form Sedimentary rocks (e.g. sandstone, coal and chalk).



  • With further burial and heating, the metamorphic rocks begin to melt. Partially molten metamorphic rocks are known as migmatite.

  • As melting proceeds with increasing temperatures and depths of burial, eventually the rock becomes molten (magma), which can be erupted onto the Earth's surface as lava, and cools and crystallizes to form volcanic igneous rock.


Complications within the rock cycle include: to melt. Partially molten metamorphic rocks are known as migmatite.

  • Weathering of sedimentary and metamorphic rocks (in addition to igneous rocks)

  • Metamorphism of igneous rocks and repeated metamorphism of metamorphic rocks.


Definition of weathering
Definition of weathering to melt. Partially molten metamorphic rocks are known as migmatite.

  • Weathering is the disintegration (break down) and decomposition (decay) of rocks in situ (in their place of origin) to form sediment.

  • Weathering (unlike erosion) need not involve the movement (transport) of material.


A physical or mechanical weathering
A) Physical or Mechanical weathering to melt. Partially molten metamorphic rocks are known as migmatite.

  • Freeze / Thaw – water expands when it freezes

Talus slope, Lost River, West Virginia

Shale chips, West Virginia


  • Exfoliation or unloading - to melt. Partially molten metamorphic rocks are known as migmatite.

    • rock breaks off into leaves or sheets along joints which parallel the ground surface;

    • caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial.

  • Thermal expansion -

    • repeated daily heating and cooling of rock;

    • heat causes expansion; cooling causes contraction.

    • different minerals expand and contract at different rates causing stresses along mineral boundaries.


  • B chemical weathering
    B) Chemical weathering to melt. Partially molten metamorphic rocks are known as migmatite.

    Rock reacts with water, gases and solutions (maybe acidic). Thus chemical processes can add or remove elements from minerals.

    • Dissolution (or Solution, Carbonation) -

      • dissolving of calcium carbonate (limestone) in acidic rain or ground water.

      • Several common minerals dissolve in water

        (halite, calcite)

      • Limestone and marble contain calcite and are soluble in acidic water

      • Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution.



    • Caves and caverns typically form in limestone to melt. Partially molten metamorphic rocks are known as migmatite. (karst environments)

      • speleothems are cave formations

      • speleothems are made of calcite

        • Stalactites: hang from ceiling

        • Stalagmites: on the ground


    • Karst topography to melt. Partially molten metamorphic rocks are known as migmatite. forms on limestone terrain and is characterized by:

      • caves/caverns, sinkholes (dolines), disappearing streams, springs.


    Sink hole doline formation
    Sink-hole (doline) formation to melt. Partially molten metamorphic rocks are known as migmatite.


    • Oxidation to melt. Partially molten metamorphic rocks are known as migmatite.

      • Oxygen combines with iron-bearing silicate minerals causing "rusting"(olivine, pyroxene, amphibole,biotite).

      • Iron oxides are produced(limonite, hematite, goethite)

      • Iron oxides are red, orange, or brown in color.

      • Mafic rocks such as basalt (which may contain olivine, pyroxene, or amphibole) weather by oxidation to an orange color.

      • "Georgia Red Clay" derives its color from the oxidation of iron bearing minerals.

    Weathering Rind, Wilhite Formation, eastern Tennessee


    • Hydrolysis to melt. Partially molten metamorphic rocks are known as migmatite.

      • Silicate minerals weather by hydrolysis to form CLAY.

      • Feldspar alters to clay (kaolinite) plus dissolved materials (ions)

      • ‘Feldspars’ are stable at high temperatures and pressures (but not at the temperatures and pressures of the Earth's surface)

      • Claysare stable under conditions at the Earth's surface

      • Feldspars and clays are similar in composition.

      • Feldspar readily alters to clay when in contact with acid and water.


    Spheroidal weathering in jointed basalt, Culpeper Basin, Virginia


    C biological weathering
    C) Biological weathering hydrolysis (in addition to iron oxides)

    • Organisms can assist in breaking down rock into sediment or soil.

      • Roots of trees and other plants

      • Lichens, fungi, and other micro-organisms

      • Animals (including humans)


    Roots of trees and plants hydrolysis (in addition to iron oxides)

    Lichens


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