WEATHERING AND SOILS Chapter 6
A.
Introduction: By breaking up rock, weathering makes all
other surfaces processes (such as streams, glaciers, waves, wind, etc.)
effective erosional agents. See p. 124
1.
Definition: Weathering is the chemical
alteration (and decomposition) and the physical = mechanical disintegration
(and breakup) of rock materials in the surface environment.
a.
Weathering is a stability issue just as is
diamond/graphite
b.
"Surface environment" extends down a
few 10's of feet below the actual land surface
2. Chemical
weathering exists on the earth's surface but not on the lunar surface or at
depth within the earth's crust because of: not
emphasized in textbook, but know
a.
Prevalence of water
in the surface/near surface environment
b.
Prevalence of
atmospheric oxygen in the surface environment
c.
Abundance of biologic
activity in the surface/near surface environment
3. Relationship
of Weathering and Erosion: The rate of erosion CANNOT exceed the rate of
weathering; however, the rate of weathering may greatly exceed the rate of
erosion
4. Mechanical
(=Physical Weathering) and Chemical Weathering
The weathering process is a single integrated process that is generally divided
into 2 aspects for purposes of discussion.
a. Physical:
breaking large blocks of surface materials into small fragments by physical
means
b. Chemical:
decomposing surface materials by chemical reactions.
1. Pressure
Release= Removal of Overburden: first
process to occur as rock slowly enters the surface environment (due to erosion
of the "overburden") p. 125
2. Frost
Action: involves cycles of freeze/thaw of water: very effective in most
temperate climates see p. 125
a. Water is
highly unusual in that at the freezing point the liquid is denser than the
solid (ice) form
b. Frost
Wedging: prying apart rock
c.
Frost Heaving: disrupting soil (freezing the soil moisture)
3. Plant Activity: particularly root activity sees p.125
4.
Repeat cycle of
heating/cooling (similar to 2, but does NOT involve water). Thermal
expansion/contraction stress and may break rocks---may occur in some desert
areas; otherwise, not too important on
earth
1.
Factors effecting chemical weathering rates
a. Increase in surface area increases
effectiveness see Fig 6.9, p.132
b. Most
reaction rates will double with a 10-degree increase in temperature
c. Availability of water is extremely
important because while water sometimes is a reactant, it is almost always a
catalysis d. Chemical
Weathering is most effective in a hot, humid (wet) climate and least effective
in a cold, dry climate
e. Note that natural rainfall is
somewhat acidic which increases the effectiveness of the chemical weathering
process. See p. 114
---------------DIGRESSION: ACID RAIN----------------See “GEOFOCUS” ---pp.130-131
2.
Chemical Weathering Processes pp. 127-129
a.
Hydrolysis p.129
mineral + water = new mineral +
dissolved material
b. Solution p.127
mineral + water = dissolved material
c. Oxidation p.128
mineral + atmospheric oxygen = new
mineral
3.
Mobility of specific ions: K, Na, Ca, Mg, Fe, Si,
and Al Not
specifically covered in text so pay special attention to notes about this
Chemical
weathering attacks the "weakest link" in a mineral structure.
The elements above constitute most rock-forming minerals and are listed
from left to right in approximate order of ease of removal from a mineral
structure
a.
K, Na, Ca: most easily removed from
minerals. Once removed K & Na tend to go into to solution and remain
in solution; K tends to be taken up by plants and clay minerals and remains in
the area.
b.
Mg, Fe: next most easily removed
elements. Like K & Na, Mg tends to
go into solution and remain there.
However, Fe combines with atmospheric oxygen to form stable iron oxides
and therefore, is not that mobile
c. Al and Si: most difficult to remove. Of the two, Al is most persistent. A typical igneous rock has a ratio of about
1:3 Al to Si, but with weathering to clay minerals,
that ratio is about 1:1
4.
Chemical Weathering of Specific Minerals Not specifically covered in text so pay special attention to
notes about this
a.
Feldspar weathers to clay minerals by Hydrolysis
b.
Olivine, pyroxene, amphibole, biotite weathers
to
Clay
minerals by Hydrolysis and to Fe-oxides
c.
Quartz is extremely resistant to chemical
weathering
(“quartz does not undergo chemical weathering”)
d.
Carbonates (calcite, dolomite) dissolve by Solution
e. Sulfides weather to sulfates by Oxidation
1.
Soil Horizons: see Fig. 6.11, page 134
a.
O-Horizon (organic rest layer—topsoil)
b.
A-Horizon
(Zone of Leaching)
c.
B-Horizon (Zone of Accumulation)
d.
C-Horizon
(incompletely weathered parent material)
2.
Effect of Climate on Soils: Humid (wet)
vs. Arid (dry)
a.
b.