Geomorph Lec 2 – Chap 3 – Weathering

Geomorph Lec 2 – Chap 3 – Weathering
Intro –
a. Wthrg defined: disintegration & decomposition of rx & mins as a result of
physical & chemical processes (does NOT encompass “erosion” and
b. Changes primarily in situ (in place)
c. “Wthrg prepares the way for erosion by weakening the rock & making it
susceptible to mass mvmt & removal”
i. IMPORTANT concept: materials will continue to break down until
they produce end products that are in equilibrium with the
Why is this important? Think about stability field of rx &
minerals..and that in nature, the drive toward stability controls a great
many processes.
Ign, mm
Are rx & mins that are precipitated out of a melt at very high P & T going to be
stable at low P & T?? What do YOU think?
Much like living things, mins have a “comfort zone”, the zone in which they were
created…if external environmental conditions start to change, the mins respond
by Weathering
d. Wthrg zone represents intersection of all Earth’s spheres: bio, hydro,
litho, and atmosphere…lots of stress on a mineral that was formed 10 km
under Earth’s surface!
e. Weathering creates diff “profiles”, depending upon climate:
i. Arid climates (no water) produce:
1. angular slopes
2. steep cliffs
3. loose debris at base
4. lots of bedrock exposure
ii. Humid climates (much water) produce:
1. smooth, rolling hills
2. gently rounded slopes
3. deep accumulation of weathered material (soil)
4. little exposure of bedrock
Dominant wthrg processes change from climate to climate
Mechanical weathering
a. Intro: Wthrg Defined: “breakdown of rx by physical processes, w/ no
change in chemical or mineral composition”
i. Primary drivers: stresses inside rx, stresses outside the rx
ii. Internal stress of rx tend to press outward as overburden stress is
released, confining pressure (cp) is reduced (“unloading”).
iii. Other stresses include:
1. “Wedging” from ice, salt xls, plant growth
2. Plucking action of colloids
3. Earth-moving by organisms
Unloading (rock exfoliates)
i. High cp results from rock overburden:
1. F=ma
2. Wt=mg
3. Pressure = Force / Unit Area
Surface time 1
Lots weight,
Surface time 2
Less weight, low cp
high cp
Rk compressed,
high internal
forces push out
Same high internal
forces push against
weaker cp, rk expands
Fracturing accompanies unloading…occasionally in mining, a face will
explosively “fly apart” when unloaded rapidly by a mining operation.
Quick aside – Hartmann’s Law – In the strain ellipse caused by differential compression
by σ1, σ 2, σ 3, predictable pattern of shear and extension fractures is produced.
σ1 (maximum prin stress, always
(minimum prin stress,
sometimes extensional)
Shear fractures form at 30o to max principal stress
Extension fractures form parallel to max principal stress
(You will see this much more in Dr. O’Brien’s Structural Geology course…)
BOTTOM LINE: If there are not enough fractures along which a rk can expand when cp
drops, more fracturing will occur to accommodate the need for the rk to expand in the
shallow subsurface
See fig 3-1…fracs become more abundant [spacing decreases] the closer the rk gets to
the surface….
And more fracs mean easier penetration by water, salt, plant roots, all of which wedge
the cracks open even more. Once the cycle starts, it is self-perpetuating until the rk is
reduced mechanically in size to tiny pieces, all of which fall off a cliff face or hill side and
fall or slide to the base of a hill.
Particularly interesting…exfoliation planes, seen often in granite, and often parallel to
the surface…look at the geometry of the local stress field at the near surface:
Lack of overburden produces vertical σ3, so rock will “unload” in direction of least stress,
just like a mineral xl will grow in the direction of least stress, producing foliations during
m-m. See Fig 3-2 – classic!
You can even find places where these sheeting joints will be parallel to surface in
concave upward patterns, like in U-shaped valleys and cirques…
c. Freeze-thaw cycle of H2O – VERY IMPORTANT – think…. “potholes”
mechanism is simple…when water freezes it expands 9%. So observe a filled crack
with water:
And the crack when the water freezes:
Now thaw the ice, fill the crack with water again…
Then freeze it again…..
Notice how much the original fracture has expanded.
See Fig 3-4 showing a shattered boulder
This is an exceedingly important phys wthrg mechanism in the mid latitudes where
water is available. Less prevalent at equator or at the poles…good exam question
d. Crystal growth (salt, carbonate, etc) – mechanism of wthrg is essen. same
as growth of ice xls…expansion forces rk to expand.
Note fig 3-5 – fence post shattered by salt xl growth nr Great Salt Lake
Interesting story of two Egyptian obelisks w/ hieroglyphics. One moved to
NYC in 1879. Now after 100 yrs in humid climate, the NYC obelisk writing is
illegible, while that in Egypt still legible after 3000 yrs.
But wthrg can occur in arid climates too. See Fig 3-6 of Egyptian pyramid
states of wthrg.
e. Thermal expansion / contraction – several interesting experiments suggest
this mechanism is minor, except in cases of major forest fires, lightning,
strikes, etc.
f. Wetting and drying – can help disintegrate shale. This is because a
number of clay minerals swell with add’n of water, then contract when H2O
dries. This can be a significant wthrg force…swelling in montmorillonite
can be up to 100% ! Author mentions pedestals in SW US.
g. Colloidal plucking – soil colloids pluck of pieces of rk...importance of
process “remains to be determined”.
h. Organic activity – “difficult to measure directly….” So its importance as a
PHYSICAL weathering agent is subject to debate. However, author
admits that heaved sidewalks and rocks pried apart by roots “attest to the
force of root growth”. Even lichens attach firmly to rock.
Chemical weathering
a. Intro – defined: “decomposition of rx by processes that change the chem
composition of original material”
In reality, chem & mech wthrg work together to break up rx at the surface
Most vivid example – fractures dramatically increase surface area that is
exposed to chem processes – see Fig 3-8
Equilibrium is once again mentioned…most rx at surface are in major
disequilibrium relative to the processes at depth under which they
originated. As a result, chem processes are at work to change the rock
mineralogy and chemistry to products that are stable in Earth’s surface
b. Role of water – as in phys wthrg, role is hard to overstate….water is of
huge importance in wthrg. Roles include a medium of exchange of
elements between rx and atmosphere, participation in actual rxns, and
washing away of rxn products to make way for next round of chem rxns.
Chem wthrg rates are essentially proportional to amt of precip in an area,
so chem wthrg is typically most active in wet climates.
Water has unique dipolar structure (Mickey Mouse) that creates + and –
electrical poles, making it attractive to many minerals, especially those
with OH-:
H2O can act in the same way a bar magnet does: that is, “opposites
Water is a powerful solvent (dissolver) because:
1. Water’ electrical charge can also pull ions off of mineral surfaces,
2. H2O can surround ions, isolate them, keep them in solution
Also, the Hydrogen bonds act to keep the H2Os attracted to each other (+
attracted to -), so there is a lot of surface tension
c. Solutions – mineral constituents get dissolved into water, then are moved
with surface and groundwater flow. Local chemical conditions (Eh and
pH) dictate when and where solution occurs, and when deposition occurs
“virtually all chem wthrg involves some solution, during which the original
material is broken down chemically”
The basic mechanism is: “attraction of dipolar water for incompletely
bonded ions” (that is, those that have some excess electrical charge that
attracts the H2O), and then water enters the xl structure.
H2O essentially latches onto (or surrounds) ions, then carries them off in
2 types of dissolution:
 Congruent – all ions stay in solution
o Examples –
o Gypsum
o Halite (salt)
o Quartz
Another example of congruent…perhaps the most important controlling
rxn on the planet….dissolution of calcite. Why important? Linked with
atmospheric CO2 (as in global warming…)
In terms of wthrg significance, Carbonated water (eg, H2CO3, carbonic
acid) is usually found in nature, not just pure H2O, and this has more
dissolving power than pure H2O
Very important rxn is shown on p.25, the carbonate equilibrium rxn….this
controls a lot of transfer of CO2 from atmosphere into ground- and ocean
water, and vice versa
Incongruent dissolution also very important to wthrg, because new
minerals (wthrg products) are formed:
o Examples
o Feldspar breaks down to form clay minerals
o Limestone and marble are dissolved in humid regions of the
country, creating caves & sinkholes. After dissolution, these
mins are often re-precipitated downstream.
o Silica can also be dissolved, but with much more difficulty
than limestone
d. oxidation-reduction reactions
i. these rxns involve electron transfer
1. oxidation = lose an electron
2. reduction = gain an electron
ii. most famous example is O, which is eager to attract electrons in
order to fill its shells, and reduce its state from O to O2iii. very common rxn: ferrous iron Fe2+ oxidized to ferric iron Fe3+ while
in the presence of O through H2O (the rusting process). This can
be a very fast (overnight) reaction, resulting in color changes in
“earth tone” range (yellow, brown, red)
iv. these rxns also involve many common mins with ferrous (Fe2+) iron:
1. pyroxene
2. amphibole
3. olivine
4. biotite
these mins wthr to form various ferric oxide mins:
1. hematite
2. limonite
3. goethite
example: olivine in presence of wtr plus oxygen forms hematite
oxidation often takes place in presence of wtr because of oxygen in
however, water can also have oxygen deficient conditions, which leads to
Important note, p.26:
Removal of Fe from many ferromagnesian silicates (e.g., olivine,
pyroxene, amphibole, biotite) greatly WEAKENS them and makes them
Note too that reduction occurs in the near surface, but not to the extent
that oxidation does, because O2 is abundant in the zone of aeration
above the water table.
Locally, organic decay in swamps, deep ocean, quiet water, can lead to
oxygen deficiency and reducing conditions. These conditions produce,
among other things, oil and gas accumulations.
e. Ionic exchange
This is an extremely important component to chemical wthrg.
This is the substitution of one ion for another…requirements are
usually similar size, similar charge.
Na ----- K
Ca <-> Mg <-> Si <-> Al
 Interesting rule of thumb re: Ionic potential (IP) = Z = charge
r radius
if IP < 3, ions will remain in solution
these include: K+, Na+, Ca++, Fe++, Mg++
Ions with IP > 3 include:
Fe+++, Al+++, Si+++
 Contrast the ionic potential with cation exchangeability: depending upon the
host, cations may be tough to pry out (in feldspars), easy to attract and hold
(in clay mins).
Sidebar here: clay mins are excellent sites for “fixing” (holding) some types of
nasty heavy metals in the xl lattice, keep them in the soil, rather than letting them
stay mobile and enter the groundwater supply.
f. Hydrolysis – addition of H+ and OH- to mins
g. Carbonation – carbonate equilibria equation very important
Add’n of CO2 to water in the system will drive to create carbonic acic, which will
further dissociate and attack CaCO3:
CO2 + H2O  H2CO3  H+ + HCO3A companion reaction: H2CO3 + CaCO3  Ca++ + 2HCO3In essence, as you push CO2 into these equations from the left, the rxns get
pushed to the right…that is, adding CO2 to groundwater tends to create more
H2CO3 (upper eqn), which in turn dissolves CaCO3- (lower eqn) into Ca++ and
If you start to pull CO2 out of the system, rxn will move toward the left, and tend
to precipitate CaCO3 (lower eqn).
h. Hydrolysis and carbonation – essentially stating that hydrolysis
(production of H+) is facilitated by the carbonation rxn
This happens a lot in the shallow subsurface around plant roots, which are
the sites of a lot of CO2 - the more CO2 available, the more the tendency
to produce carbonic acid, which then dissociates to H+ ions and
bicarbonate ions (HCO3-)
i. Hydration – things are hydrated when water is combined with them…you
hydrate yourself when you drink fluids after a workout….
Classic geol case is change from anhydrite (CaSO4) to gypsum (both
typically assoc with sedimentary evaporate deposits)
Addition of wtr causes expansion and weakening of xl lattice (important
phys wthrg effect)
Another example of hydration is the swelling of clays mins such as
j. Chelation – this involves “grabbing” of metal ions by another compound
This is important because metal ions such as Fe and Al, normally
immobile during wthrg, can become mobilized, usually by organic
Controls on the rate & character of weathering (combination of factors)
a. Parent material – what do you start with?
i. the resistance of mins to chem & phys wthrg control the resistance
to wthrg in general
ii. other phys factors also important:
1. joints
2. bedding planes
3. porosity
Table 3-2 says it all – the REVERSE of Bowen’s Rxn Series!
b. Mineral stability –
soluble mins least resistant to chem weathering:
4. Halite
5. Gypsum
6. Calcite
7. Aragonite
8. Dolomite
(so these are primarily evaporite and carbonate mins that precipitated out
of water to begin with…. Makes sense that they would be easily
dissolvable, yes?)
chem wthrg products such as the clay minerals & iron oxides
are RESISTANT to wthrg, because they are the end products
heavy access mins (rutile, ilmenite, corundum) are inert & resistant
silicate min resistance is controlled by bonding (e.g., Si very strongly bonded
to O, creating very stable SiO2)
and more complex breakdown of sheet and chain silicates involves breaks
between other bonds, not the one between Si and O.
c. Climate
climate very important to weathering…”establishes the temperature and
moisture conds at / near Earth surface”
Number of examples of importance of climate
Climate controls freeze-thaw (v. important phys wthrg process) doesn’t
happen unless you have moisture and cold temps
Climate controls soil types, even to the point where similar soils can develop
from diff parent rock types
Climate controls the amt & distribution of water, which is extremely important
to chem wthrg
o Moist tropical regions (near equator) experience very deep wthrg (Fig
o Arid regions (30o N & 30o S, also high polar latitudes) experience little
chem wthrg, because there is no water (Fig 3-22). What little water
there is comes down torrentially, and is not effective at infiltrating the
soil….so most rainfall in arid regions results in large amt runoff, not
infiltration and increased soil moisture
Lots of info on Fig 3-22 to ponder
 In tropics, wthrg is intense & deep, progression develops:
Fresh rk > little chem alteration >
illite-montmorillonite > kaolinite > alum & iron oxides
In deserts, not much wtr, and not much wthrg, except for some carbonates
Mid latitudes have mod rainfall and temp, so you see moderate wthrg and soil
Polar regions – little wtr or temp changes, so little wthrg
d. Vegetation – also a control on weathering, but very much linked to precipitation /
an important function of veg is to slow down the rate of precip runoff, and
increase infiltration
another important function is to contribute – a lot – to soil development in several
diff ways:
the roots break things up physically
as organic material dies, it accumulates on surface, triggering several
- releases organic acids (H+) that attack rx
- releases organic colloids (particles) that promote ion exchange
and chelation
- H+ ions nr root tips exchange with rx & mins for nutrient cations for
plants: Ca++, K+, Na+, etc.
These nutrients act as fertilizers for plants, and this process must
be taken into acct when growing crops:
 must rotate crops or you will quickly draw certain nutrients out of
 if you deplete natural cations, must use fertilizer or plants /
crops won’t grow
e. Topography
are the two important topo-related factors that control wthrg
at really high altitudes above treeline (i.e., above vegetation), freeze-thaw can be
important for phys breakdown of rx
at lesser altitudes, vegetation holds wtr & promotes chem wthrg
slope controls water table position, and amt of water avail for wthrg
typically, steep slopes are not the site of much vegetation…slope must flatten out
in order for wtr to accumulate and wthrg to occur to any signif depth to create
how many steep cliff faces have YOU seen covered with soil???
Also, direction (aspect) of slope faces can be important locally…in N hemi.
South-facing slopes are warmer, have less snow that can translate to grdwtr
during thaw
These can be seen in some famous rule-of-thumb examples:
One way to tell “North” in the woods is that moss only grows on the North
sides of trees
Same (sometimes) for lichen on rx
f. Time – also an important control on wthrg
Easterbrook suggests a direct relation between time and wthrg
(this is true if all other factors are equal, but not true if you start considering
presence or absence of wtr, etc).
Remember the earlier example of Egyptian hieroglyphics in Egypt (3000 yrs w/
no major wthrg) vs NYC (trashed after 150 yrs in Central Park)
(ooh…good exam question!!)
g. Weathering Rates
summary of wthrg controls:
function of 5 controlling factors:
 parent material
 climate
 vegetation
 topography
because of wide variability in the way these factors interact, generalizations
about wthrg are difficult
but attempts have been made to msr wthrg rates of rx of known ages:
 exptl lab studies (subject to serious questions, like can you replicate field
conditions in the lab?)
 wthrg dated materials used by man (tombstones, monuments, building stone)
 study amt of dissolved elements in streams
one thing DOES appear to be apparent:
“rates of chem wthrg processes decrease with time”
however, you MUST consider factors like atmospheric pollution (e.g., acid rain, high
levels of CO2 which could promote creation of carbonic acid, H2CO3)
Effects of wthrg
OK, so now it’s time to look at the actual results of processes….how does it
change things?…what changes to what?
Processes combine to produce a VARIETY of Geomorphic Features –
- clay formation
- granular disintegration
- wthrg pits
- spheroidal wthrg
- chem wthrg & stripping
- cavernous wthrg
- soils
A. Clay Mineral formation
 These are sheet minerals with lots of cations and anions
 Sheets can be organized differently
1:1 (Si sheet:Al sheet),
2:1 (Si sheet:Al sheet)),
resulting in many diff minerals
 Fig 3-31 shows how cations and anions are “wedged” in between sheets,
causing changes in mineralogy & xl structure
These sheets also control how things wthr:
1:1 sheets like kaolinite do NOT have room for excess cations or wtr….so with
low cations & anions, they have low CEC (electrically neutral), and do not exert a
strong draw to other cations & anions. So these clays do NOT swell.
2:1 sheets include micas, smectite
In smectites, Fe2+ and Mg2+ frequently substitute for Al3+
Also, exchange of cations and water is easy and widespread
So smectites are known as “swelling clays”…montmorillonite is most well-known
In micas, illite & vermiculite are most significant. Illite is v. similar to
montmorillonite, so it can alter to that.
Chlorite is also a 2:1 sheet, frequently found in soils and in low-grade M-M
terrane. Since bonding is strong, no swelling occurs.
First-generation clays frequently alter to other clay mins, and mixed-layer clays
2:1:1 are also common
Jackson et al (1948) proposed this sequence:
Mica > vermiculite > montmorillonite > kaolinite , but this is a bit too simplistic
Easterbrook also notes that, with enough time, granites can COMPLETELY
weather to clay mins…..
B. Granular disintegration – primarily in granites and sandstones – essentially grainby-grain failure and removal from an initially hard rk
C. Wthrg pits and etched knobs – common in granitic terranes
D. Spheroidal wthrg- produces rounded, boulder-like forms (Fig 3-35)
When fspar changes to clay, incr in volume takes place, so xline rk can be
broken up (in same way that wtr increases in vol when it changes to ice)
Whtrg takes place preferentially along joint planes, which start the rounding
process. These look a lot like stream-rounded boulders…!
E. Cavernous wthrg (or honeycomb wthrg) Fig 3-36 – mostly a product of granular
F. Deep chem wthrg and stripping – postulated evolutionary sequence that can
produce inselbergs (protruding bedrock from a peneplain of wthrd rk) is:
Wthrg front pushes downward
Pulse pf stripping occurs
Process repeats itself
Time 1
Time 2
G. Soil development – “perhaps the most important effect of wthrg” (especially if you
like to eat….)
Soil defined: “a wthrg residue that has become differentiated with depth into
So how do horizons develop?
 accumulation of organic material at or near surface
 leaching of parent material to the point that large amts of one or more
minerals are removed
 accumulation of organic material in upper part of profile
 accumulation of wthrg products with depth
what are the horizons?
O horizon (decomposing plants, little mineral content)
A horizon (dark due to decaying organics, generation of organic acids, and
leaching of mins downward – A is known as zone of leaching
(occasional E horizon here)
B horizon (Zone of accumulation) – material that has accumulated by
downward mvmt from O & A horizons, and the parent material below has
also been severely weathered – one characteristic is that there is more
clay here than above or below
There are several variations on the B horizon, including Bt and Bk
C horizon (decayed rock horizon)
R horizon (unweathered parent rock)
H. Soil maturity – some generalizations:
 the more erosion, the thinner and less mature the soil
 the less mature the soil, the more it reflects parent rk composition
I. Paleosols (ancient soils)
Typically preserved by burial
They are often associated with uncomformities (makes sense, as unconf is an
erosion surface)
They are useful in interpretation because they indicate enough time exposed
subaerially to have soils develop
They can also help us interpret ancient climates
J. Soil classification – important skill for soil scientists, especially those who aid farmers
illite > montmorillonite
vermiculite > chlorite
smectite > chlorite > kaolinite