Geology 229 Engineering Geology Lecture 23 by ezk20114

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									   Geology 229
Engineering Geology
     Lecture 23


    Rock Weathering

      (West, Ch. 8)
Outline

 Introduction of weathering
 Mechanical weathering
 Chemical weathering
      Comparison of surface and
        subsurface conditions
Subsurface                 Surface

• High temperature but     • low temperature, and
  constant at which          highly variable
  minerals reach           • little or no confining
  equilibrium                pressure (stress)
• high confining           • abundant of water
  pressure (stress)        • abundant of oxygen
• less water or no water
• no oxygen
Conclusion can be drawn from the comparison:

Rock at the surface will undergo changes

---- This change is called Weathering

  Weathering is the physical breakdown
  (disintegration) and chemical alteration
  (decomposition) of rocks to form soil or loose
  particles at or near Earth's surface. Weathering
  causes deterioration of building materials. It also
  weakens rocks, a great concern when weathered
  rocks are used for foundation.
Two types of weathering

 Mechanical weathering:
   Physical disintegration or degradation of
   rock pieces without a change in composition
         --size reduction

 Chemical weathering:
    decomposition whereby one mineral species is
    changed into another through various chemical
    processes. Water plays a major role, through:
         1, provide oxygen,
         2 provide mobility for moving ions.
   Mechanical weathering always involves
fracturing--but that can occur by a whole
host of causes.

   Chemical weathering tends to weaken
rock, thereby making it easier to break.
Likewise, mechanical weathering creates
additional surface area that is prone to
chemical attack. In this way, the two
processes work together.
Weathering is controlled largely by climate.
The more water available, the more likely
that chemical processes can proceed.
Additionally, if temperatures are warm, then
chemical weathering can proceed even
faster. Then, mechanical weathering can
move more quickly also. In arid climates,
however, weathering processes move very
slowly. Mechanical weathering will be the
dominant process in arid climates; however,
because of its reliance on chemical
weathering, it will also be quite slow.
mechanical weathering:

 With or without water makes distinguished
 difference in rock weathering process, in
 arid region, since no water or little water
 presence, even the carbonate rocks (e.g.,
 limestone) are not subject to solution but
 persist as resistant rocks. The rock mass
 may be sharp angular, and topography
 developed.
Mechanical Weathering (cont.):

 Mechanical weathering processes include:

 1) freezing & thawing (frost wedge)
 Example:
 Pottery container with water in winter time,
 when water is frozen and become ice it can
 have 9% of relative volume change, i.e.,
     dv/v = 9% = 0.09
 This is a very large strain! Similarly if there is
 water in rock fractures it will force the fracture
 to propagate into further depth.
Mechanical Weathering (cont.):

 The stress generated by frozen water is
 about 550 psi, check with Table 6.1 you can
 find out that this is a value comparable to
 the tensile strength of most rock types. Or
 only one order of magnitude less. But
 remember rocks are experiencing many
 freezing-thawing cycles before they are
 finally breaking down.




                         σ
Mechanical Weathering (cont.):

2) differential expansion and contraction
As temperature changes (in deserts or from
forest fires), not all parts of a rock or all its
minerals expand or contract by the same amount.
So when rocks are heated or cooled, the mineral
grains are subjected to differential stresses,
which may be sufficient to make the rock spall, or
break off in sheet-like pieces.
#

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.
The thermal expansion coefficient αL and αV
       1 ∂l             1 ∂V
  α L = ( )P        αV = ( ) P
       L ∂T             V0 ∂T
characters how much a mineral change its
dimension in response to a unit degree
increase in temperature . Using a finite
expression

        ∆L=LαL∆T
Mechanical Weathering (cont.):

 The linear thermal expansion coefficient αL is
 on the order of 10-5/ºC, and it could be
 anisotropic, i.e.,

         αLx ≠ αLy ≠ αLz

 For example, αmax is in the elongation axis (C-
 axis).
Differential weathering
Processes of mechanical weathering
unloading: jointing, exfoliation, and sheeting
Upon removal of overburden, the elastic component of rock
deformation is recovered and the rock expands. The
unloading may occur when the overlying rocks are eroded or
rocks are removed from a quarry. The expansion caused by
unloading may be sufficient to fracture the rock. Such
naturally formed cracks are known as joints.
Typically, large plutons (bodies of igneous rock) or
metamorphic bodies split into sheets that are parallel to the
mountain face, a process known as exfoliation. It is also
known as sheeting if the expansion from unloading occurs in
granite to form rock slabs.
3), Sheeting and exfoliation

 When subsurface confining stress is
 released fast, and the rock exposed to the
 surface, if the rock has large residual stress,
 it will be broken into blocks. The fracture in
 vertical direction form exfoliation. If the
 fracture is in horizontal direction, it calls
 sheeting.
The famous examples of exfoliation are the
 North Dome and Half Dome in the Yosemite
 National Park, CA.
joints are parallel cracks in which rocks on either
side are not offset; Sheeting rock layers peel like
layers of an onion
Chapter 5:   2) Mechanical weathering        Unloading
                                        Another “sheeting” example




                                            See also T&L Figure 5.8
North Dome
North Dome
Half Dome
Half Dome
Vertical columns from
magma cooling &
shrinking

“columnar jointing”
                                    Columnar jointing in basalt




Glacially polished basalt columns
(end view)
4), Spheroidal weathering

  Spheroidal weathering is caused by the
 combination of any of the following causes:

    pressure relief, frost wedging, expansion,
 wind, and chemical weathering.
Spheroidal Weathering. Granite illustrates weathering forms quite well.
Chemical weathering attacks to granite along joints and makes rounded
boulders (Alabama Hills near Lone Pine) .
“Devil’s Marbles”   Australia
5), Other mechanical weathering

  erosion by wind and water
            (only the mechanical part);
  forest fires;
  plants’ rooting;

Fire and water quenching was used till 16th
  century to do rock quarrying.
Chemical weathering
Chemical weathering=chemical processes that
dissolve and decay earth materials;
Chemical weathering needs the rock exposes to
air and water. Mechanical weathering could
enhance chemical weathering by disintegration,
i.e., increase the surface area of rock blocks and
debris and this will greatly accelerates chemical
weathering.
Chemical weathering rate depends on
  1. Temperature
  2. Amount of surface area
  3. Availability of water or natural acid

     Thus, rocks in tropical environment
     experience most severe chemical
     weathering.
Because of its dipolar nature
water is able to dissolve many
chemical compounds. In
addition to the solution effect,
water aids decomposition
through acid action,
oxidation, and hydrolysis.
Acidity of Natural Waters
             Water is a good solvent.
             Acidic water is better!
             pH of most natural
               waters ranges from 4
               to 9
             pH > 9 or < 4 occurs in
               extreme
               environments
Chemical Weathering of silicate
  minerals by carbonic acid

 feldspar + water + carbonic acid = clay
        minerals + dissolved ions

      2KAlSi3O8 + H2O + 2H2CO3 =
Al2Si2O5(OH)4 + 2K+ + 4SiO2(aq) + 2HCO3-
 Why is rainwater naturally acidic?
Rainwater contains dissolved CO2 from
  atmosphere.
Dissolved CO2 reacts with water to form
  carbonic acid (H2CO3)
            CO2 + H2O ⇔ H2CO3
Carbonic acid dissociates to produce
  hydrogen ion (H+) and bicarbonate
           H2CO3 ⇔ H+ + HCO3-
Chemical weathering (cont.)
Common chemical weathering processes are
solution (dissolution), oxidation, and hydrolysis.
Rock reacts with water, gases and
solutions (may be acidic); will add or
remove elements from minerals.
Solution (or dissolution)
  * Several common minerals dissolve in
water
     i), halite; ii), calcite
  * Limestone and marble contain calcite
and are soluble in acidic water.
Marble tombstones
and carvings are
particularly
susceptible to
chemical
weathering by
dissolution. Note
that the urn and
tops of ledges are
heavily weathered,
but the inscriptions
are somewhat
sheltered and
remain legible.
Photo taken in one
New Orleans
graveyard.
A 16th-century
monastery in Mexico
shows the ravages of
weathering mostly from
wind and wind-driven
rain. The rock is volcanic
tuff.
Karst landscape of Guilin, China, caused
dissolved Carbonate rocks.
Oxidation
Oxidation - Oxygen combines with iron-bearing silicate minerals causing "rusting".
Iron oxides are produced that are red, orange, or brown in color. Iron-bearing
silicate minerals that undergo oxidation include the following:
  * olivine
  * pyroxene
  * amphibole
  * biotite
Iron oxides are produced by oxidation of iron-bearing silicate minerals. These
minerals are iron oxide minerals:
  * 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
Broken piece of fine-
grained basalt from a dike
near Stone Mountain, GA.
Note the black color of the
unweathered rock, and the
weathering rind colored by
iron oxides. The
weathering rind has two
distinct layers, an inner
yellowish layer and an
outer orange layer. Sample
is about 10 cm in width
(below) .



                              Mafic rocks such as basalt (which
                              may contain olivine, pyroxene, or
                              amphibole) weather by oxidation
                              to an orange color (above)
Hydrolysis: Hydration-reaction between mineral and water


Since many igneous rocks contain feldspar minerals (K-feldspar and
plagioclase) as a major component in mafic, intermediate, and felsic rocks,
clays are common products of chemical weathering of igneous rocks. Clays are
a group of minerals that are all sheet silicates with various amounts of water in
their crystal structures (added to feldspar structure, which is a framework
silicate). An example is kaolinite (a chalky white mineral)


  Feldspar + water -> kaolinite + silica + potassium ions


  KAlSi3O8 + H2O -> Al2Si2O5(OH)4 + SiO2 + K+
The difference between Weathering and Erosion
Weathering involves two processes Chemical weathering
and Mechanical weathering, that often work in concert to
decompose rocks. Both processes occur in place. No
movement is involved in the weathering.
If a particle is loosened, chemically or mechanically, but
stays, call it weathering.
Once the particle starts moving, it is erosion.
God’s Garden, Colorado
Twin Tower, God’s Garden, Colorado
Summary
Mechanical weathering is accomplished by
physical forces that break rock into smaller
and smaller pieces without changing the
rock's mineral composition.
Chemical weathering involves breaking down
rock components and internal structure and
forming new compounds.
Whereas weathering breaks rocks apart,
erosion removes rock debris by mobile agents
such as water, wind, or ice.

								
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