Chapter 12 Weathering and Soil
This chapter deals with the breakdown of rock. You can think of weathering processes as what
connects igneous rocks to sedimentary rocks. Why? Because, as we’ll see, the end products of
weathering are the raw material for sedimentary rocks.
Think of weathering processes as falling into two categories: (1) mechanical weathering—which
involves physical disintegration without significant chemical change, and (2) chemical
weathering—which involves decomposition, or chemical alteration. You should become familiar
with several examples each of mechanical and chemical weathering processes.
Mechanical Weathering Processes
Mechanical weathering processes include pressure release, frost action (ice wedging), and plant
growth (root action).
Pressure release causes deeply buried rocks to crack up as they move from depth, up to the
surface. This is a particularly important process for granitic rocks. Once a felsic magma cools
to produce a granitic rock at depth, the rock, having cooled, is somewhat brittle, but still under
enormous pressure because it’s down deep. Under certain conditions, the rock mass continues
to rise, since even though solid, it still may be less dense than the surrounding rock.
Decompressiong, expanding, and cracking all occurs as the granitic rock mass buoyantly rises.
The decompression process can produce a series of parallel sheets of broken rock, roughly
parallel to the ground surface, a feature called sheeting, as shown here:
Unloading joints in granodiorite.
Image courtesy of Marli Miller, University of Oregon
Image source: Earth Science World Image Bank
(http://www.earthscienceworld.org/images)
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Another interesting weathering process is called spheroidal weathering:
Spheriodally weathered granitic boulders, South Africa
(courtesy Wikipedia; http://en.wikipedia.org/wiki/File:South-africa-sphere-weath.JPG)
As granitic rock cools, it typically develops at least three sets of mutually perpendicular cracks
(joints), and the rock mass breaks up into angular blocks while still underground. As overburden
is removed and the granitic rock decompresses, it continues to break up as it’s exposed.
Chemical weathering processes now attack the feldspar minerals in the granitic rock, turning
them to clay. This process is more effective at the corners of the jointed blocks (which have a
higher surface area/volume ratio) than along the edges, as shown here:
Rapid corner weathering of an
originally angular joint block
produces a rounded corestone
(gray).
Corestone formation.
Once exposed, the weathered debris falls away to reveal a rounded “corestone”.
So, the thing that’s interesting about spheriodally weathered granitic rock is that the rounded
shapes didn’t form by rolling over and over in a stream. Rather, they formed “in situ” (in place)
while still underground…pretty cool!
Frost action (freeze-thaw) should be pretty obvious in terms of how it works. As water freezes,
it expands approximately 9% in volume. So, as water seeps into an already mechanically
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weathered rock mass and freezes, the expanding ice forces the cracks farther apart,
disintegrating the rock mass in the process.
A True Story: how I learned that water expands upon freezing…
Back in 1994, I was hired on as a staff geologist at a local environmental consulting firm. One of my first
jobs was to meet a drilling crew up in Bakersfield to collect some groundwater samples for contamination
testing. After a couple days of drilling, I’d collected about a dozen samples of potentially contaminated
groundwater, which I placed in small, glass viles, following standard protocol. Glass is used because it’s
chemically inert. In order to keep any dissolved chemicals from seeping out of the viles en route to the
laboratory, standard protocol also required the samples to be chilled…which I did.
Wanting to make a good impression, my field assistant and I drove back to San Diego late one
afternoon, shortly after we finished collecting the last groundwater sample, rather than spending an extra
$100 of our project budget for another night’s hotel stay…big mistake, because we hit LA rush hour
traffic on the way back.
Finally, about 1 AM the next morning we pulled into our office parking lot, exhausted and bleary eyed.
My field assistant suggested we just leave our groundwater samples in the truck on ice, and drop them
by the lab the next morning. I objected, saying they’d be better off stored in my home fridge until
morning (bad idea).
Once I got home, I opened the fridge and thought to myself, “Hey, these very expensive groundwater
samples would be nice and cool in the fridge, but how much cooler they’d be in my freezer.” So, I stuck
the glass water viles in the freezer…Well, you can imagine what happened…All the viles burst as the
water turned to ice, rendering the samples worthless!
I gotta tell you, that was the worst day of my professional career, having to explain to my new boss what
happened to the groundwater samples we’d just collected. Amazingly, I wasn’t fired on the spot…I think
they felt sorry for me…After all, who would hire a guy who puts water in the freezer and expects it not to
freeze?…Glass water viles in the freezer?! What was I thinking?!!!!
On the company nickel, I went back to Bakersfield to re-collect the groundwater samples. I’m still
surprised they didn’t dock my pay. Luckily, the groundwater came back clean, free of any contaminants,
when tested. The moral of this story? Er, water expands upon freezing (no duh!). For years afterwards,
my colleagues at work called me the “ice man.” 
Plant growth (root action) also contributes to the mechanical weathering of a rock mass.
Here’s a link to some nice examples of frost and root action (click each image for a larger
view).
Chemical Weathering Processes
Chemical weathering processes include oxidation—chemical combination with oxygen, solution
weathering (dissolution), and hydrolysis—a decomposition reaction involving the components of
water (H+ and OH-).
Although your book doesn’t use the term hydrolysis, equation D in Table 12.1 is in part a
hydrolysis reaction (notice the water). In this case, the potassium feldspar (K-spar) doesn’t
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simply absorb water; rather, the water chemically combines with the K-spar to create a clay
mineral.
Here are some examples of important chemical weathering reactions (this list is by no means
exhaustive):
(1) 4Fe3+ + 3O2
(iron)
2Fe2O3 (hematite)
(oxygen)
(2) CaCO3
+
(calcite)
CO2
+
(carbon dioxide)
(acid)
Ca++
H2O
(water)
(3) 2KAlSi3O8 + 2H+ + 2HCO3- + H2O
(K-spar)
OXIDATION
(iron oxide mineral)
(calcium)
DISSOLUTION
(bicarbonate)
Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2
-
(H+ and OH )
DISSOLUTION
2HCO3-
+
(clay mineral)
(ions in solution)
HYDROLYSIS
You don’t have to memorize these reactions. I just included them so you can see in detail
what’s going on.
In equation (1), above, iron is combining with oxygen to produce a common iron-oxide mineral,
hematite…literally, rust. Ferromagnesian silicate minerals like olivine, pyroxene, hornblende,
and biotite rust to produce hematite and other oxide minerals, as in the picture below:
Weathering Rind
Oxidized granitic boulder, Angelica, New York
(modified from Wikipedia; http://en.wikipedia.org/wiki/File:Weathering_9039.jpg)
In equation (2), above, calcite, a common non-silicate mineral, is dissolving in acidic water. To
keep things simple, I haven’t shown how carbon dioxide breaks down to carbonic acid
(H2CO3 >>>> H+ + HCO 3), but when it does, acidic water is produced that can dissolve other
minerals, like calcite. When rain falls, carbon dioxide in the atmosphere reacts with the falling
water to produce carbonic acid, which can then dissolve minerals like calcite and feldspar when
it lands.
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Equation (3) above is quite complicated, but in general, a feldspar mineral (K-spar) is
undergoing two chemical weathering processes, dissolution and hydrolysis, to produce a clay
mineral (in this case, kaolinite). Various dissolved ions (charged atoms), and compounds
(bicarbonate and silica) are also created as chemical weathering products.
Here’s a fascinating animation of a feldspar crystal weathering to clay.
Remember Bowen’s reaction series? Click this link to Bowen’s Reaction Series—Fig. 11.19
for a quick review. It turns out that the order of crystallization in a cooling magma is also the
order of chemical weathering in various igneous rocks. For example, in a mafic rock like basalt,
the olivine crystals (which formed at a higher temperature) will typically decompose via chemical
weathering before the pyroxene crystals (which formed at a slightly lower temperature).
In a felsic rock like granite, the darker minerals like biotite mica and hornblende (an amphibole)
will typically weather out before lower temperature, lighter minerals like muscovite and quartz.
Quartz, a low-temperature mineral at the bottom of Bowen’s Reaction series, is very resistant to
chemical weathering due to an abundance of strong, silicon-oxygen bonds that resist
breakdown.
Why is the order of chemical weathering important? Well, for one thing, it gives us a general
way to predict which types of rocks will be more resistant to chemical weathering in which
climates. For example, which rock, granite (felsic) or gabbro (mafic) would you expect to be
more susceptible to chemical weathering in a moist environment, all else being equal? Probably
gabbro! Why? Because gabbro typically contains an abundance of high-temperature minerals
like olivine and pyroxene, which decompose (chemically break down) rapidly in moist
environments.
In fact, you can see this effect in eastern San Diego County. As you drive east on Highway 8,
past El Cajon, toward the Cuyamaca and Laguna Mountains, you might have noticed that some
of the mountain peaks appear smooth and reddish, with few outcrops of exposed rock--like
Viejas Mountain (below). This is because the rocks of Viejas Mountain are gabbro, and have
undergone extreme chemical weathering, producing thick, reddish soils.
Gabbro
Viejas Mountain, San Diego County, CA.
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Notice what you don’t see in the above photo: extensive exposures of fresh rock. Instead, the
whole mountain is mantled by a thick, reddish/brownish soil—the chemical residue left by the
decomposition of the gabbroic rock.
On the other hand, other mountain peaks, like the one below, are irregular, bumpy, and contain
abundant bedrock exposures.
This is because such peaks are composed of granitic (i.e., intermediate to felsic) bedrock, which
contains minerals like quartz, K-spar, and low-temp plagioclase that are more resistant to
chemical weathering.
Granitic rock
Small peak east of Viejas Mountain, San Diego County, CA.
We can relate mechanical and chemical weathering processes in a general way to climate, as
shown in this diagram:
Increasing
rainfall
Strong chemical
weathering
Moderate chemical
weathering
Very slight chemical
weathering
with frost action
Very slight chemical
weathering
Increasing temperature
Weathering intensity and climate.
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Chemical weathering processes tend to dominate in warm, wet climates, where abundant water
is available for chemical reactions and solar heat is available to drive these reactions (upper
right portion of above diagram). In contrast, mechanical weathering processes such as frost
action dominate in cold climates, where liquid water is less available but where ice freezes and
thaws (lower left portion of above diagram).
Finally, chemical weathering is very slight in dry climates, which lack liquid water (lower right
portion of above diagram). This is why many geologists, including myself, like to hang out in dry
climates…the rocks in such climates tend to be “fresher” and therefore easier to study than
those in wet climates, where they might be covered by a thick mantle of soil. Imagine trying to
study bedrock geology in the middle of the Brazilian rainforest…you’d have to dig pretty deep in
many places just to get to the rocks!
In addition to common mechanical and chemical weathering processes, you should also
become familiar with the end products of weathering, which include:




rock fragments
clay minerals
oxide minerals
dissolved ions and compounds (including silica, calcium, sodium, potassium, bicarbonate)
All of these products except rock fragments are produced as a result of chemical weathering,
with rock fragments obviously a result of mechanical weathering. These end products are
important because they are the raw material for sedimentary rocks.
Feel free to just skim the section on soils. The only thing I’d like you to know from this section is
that soil is itself a weathering product, or rather a mixture of weathering products. Don’t worry
about knowing the various soil horizons. If you’re interested in soils, many universities offer
entire courses that just focus on soils.
Although we’ve considered mechanical and chemical weathering processes separately, they
tend to act together on a rock mass, reinforcing one another. Chemical weathering weakens a
rock mass (as the feldspars in granitic rock turn to clay, for example), which makes it more
susceptible to mechanical breakdown. Mechanical weathering, in turn, increases the surface
area of a rock mass and gives a boost to chemical weathering processes (which require access
to the interior of a rock mass to work effectively). Here’s an interesting Flash animation
demonstrating how mechanical weathering increases rock mass surface area.
Here’s a chart of the process described above, which one of my former students nicknamed the
weathering cycle:
Mechanical Weathering
(MW)
CW accelerates MW by
weakening rock mass…
Chemical Weathering
(CW)
The Weathering Cycle.
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MW accelerates CW by
increasing rock mass
surface area…