Download Variability in Weathering

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C H A P T E R 1 5 • W E AT H E R I N G A N D M A S S W A S T I N G
The specific type of clay mineral produced by hydrolysis depends
on the composition of the preexisting mineral and other substances that may be present and active in the water. Because water
is the weathering agent, hydrolysis is not limited to rocks that are
exposed at the surface, but can occur in the subsurface through
the action of soil and groundwater; in tropical humid climates, it
can decay rock to a depth of 30 meters (100 ft) or more.
The chemical weathering process of hydrolysis differs from the
physical weathering process of hydration, in which water molecules
join and leave a substance, causing it to swell and shrink without
changing its inherent chemical formula. Both weathering processes,
however, may involve clay minerals because those clay-sized substances are often the product of hydrolysis, and many clay minerals
swell and shrink substantially during hydration and dehydration.
Low-latitude regions with humid climates consequently experience the most intense chemical weathering. In the tropical rainforest, savanna, and monsoon climates, chemical weathering is more
significant than physical weathering, soils are deep, and landforms
appear rounded. Although chemical weathering is somewhat less
extreme in the mid-latitude humid climates, its influence is apparent in the moderate soil depth and rounded forms of most landscapes in those regions. In contrast, the landforms and rocks of both
arid and cold regions, where physical weathering dominates, tend
to be sharper, angular, and jagged, but this depends to some extent
on rock type, and rounded features may remain in an arid landscape as relicts from prehistoric times when the climate was wetter
( ● Fig. 15.17). Comparatively low rates of chemical weathering
are reflected in the thin (or absent) soils found in arid, subarctic, and polar climatic regimes. We have already noted the role of
daily temperature ranges in weathering processes, including thermal expansion and contraction weathering in arid climates and
freeze–thaw weathering in areas with cold winters.
Air pollution that contributes to the acidity of atmospheric
moisture accelerates weathering rates. Extensive damage has
Variability in Weathering
How and why types and rates of weathering vary, at both the
regional and local spatial scales, are of particular interest to physical geographers. In addition to climate, the type
of rock (lithology) and the nature and amount
● FIGURE 15.16
of fractures or other weaknesses in it are major
This diagram of weathering regions summarizes the relationships between climate and weathinfluences on the effectiveness of the various
ering processes. Physical weathering is most active where temperature and rainfall are both
low. Chemical weathering is most active in regions of high temperature and rainfall. Most world
rock weathering processes. How specific rocks
regions experience a combination of both physical and chemical weathering.
weather in different natural settings is imporIn which weathering region would we find a site that has an annual mean temperature
tant for explaining the amount and formation
of 5°C (41°F) and an annual rainfall of 100 centimeters (40 in.)?
of regolith, soil, and relief, but it also impacts
the cultural environment because weathering
THEORETICAL WEATHERING REGIONS
affects building stone as well as rock in its natAnnual rainfall (in.)
ural setting.
80
60
40
20
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g
ron
St
–10
ys
ph
te
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ys
ph
l
ica
d
Mo
Outside normal
climatic range
14
al
era
Mean annual temperature (°C)
In almost all environments, physical and chemical
weathering processes operate together, but usually one of these categories dominates. Although
water plays a role in all but two of the physical
weathering processes, it is essential for all types
of chemical weathering. Also, chemical weathering increases as more water, comes in contact
with rocks. Chemical weathering, then, is particularly effective and rapid in humid climates
( ● Fig. 15.16). Most arid regions have enough
moisture to allow some chemical weathering,
but it is much more restricted than in humid
climates. Arid regions typically receive sufficient
moisture for physical weathering by salt crystal growth and the hydration of salts. Abundant
salts, high humidity, and contact with seawater
make salt weathering processes very effective in
marine coastal locations.
The other principal climatic variable,
temperature, also influences dominant types
and rates of weather ing. Most chemical
reactions proceed faster at higher temperatures.
l
ica
Moderate physical
ht
ys
ph
ig
Sl
0
32
Moderate chemical
and physical frost action
10
50
Almost
no chemical
or
physical
weathering
Moderate chemical
20
Strong chemical
200
150
Mean annual temperature (°F)
Climate
68
100
Annual rainfall (cm)
50
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V A R I A B I L I T Y I N W E AT H E R I N G
diverse rock types undergo differential weathering and erosion; easily
eroded rocks exhibit more extensive effects of weathering and erosion than the
resistant rocks.
A rock that is strong under certain
environmental conditions may be easily weathered and eroded in a different environmental setting. Rocks that
are resistant in a climate dominated
by chemical weathering may be weak
where physical weathering processes
dominate, and vice versa. Quartzite is
a good example. It is chemically nearly
inert and harder than steel, but it is
brittle and can be fractured by physical weathering. Shale is chemically inert but mechanically weak. In humid
regions, limestone is highly susceptible
to carbonation and solution, but under
arid conditions, limestone is much more
resistant. Granitic outcrops in an arid or
semiarid region resist weathering. However, the minerals in granite are susceptible to alteration by oxidation, hydration,
and hydrolysis, particularly in regions
with warm, humid conditions. Accordingly, granitic areas are often covered by
a deeply weathered regolith when they
have been exposed to a tropical humid
environment.
(a)
(b)
● FIGURE
15.17
(a) Due to the dominance of slow physical weathering and the sparseness of vegetation, slopes in arid
and semiarid environments tend to be bare and angular. Slope angles reflect differences in component
rock resistance to weathering and erosion. (b) Because humid region hillsides are affected by the more
rapid chemical weathering processes and regolith is held longer on the slope by vegetation, slopes in
wetter climates are much more rounded with a deeper weathered mantle than arid region slopes.
already occurred in some regions to historic cultural artifacts
made of limestone and marble (metamorphosed limestone), both
containing calcium carbonate ( ● Fig. 15.18). Because many of the
world’s great monuments and sculptures are made of limestone or
marble, there is a growing concern about weathering damage to
these treasures. The Parthenon in Greece, the Taj Mahal in India,
and the Great Sphinx in Egypt are examples of structures made
of rock where pollution-induced chemical solution, and related
growth and hydration of salts, are damaging and rotting away
monument surfaces.
Rock Type
Wherever many different rock types occupy a landscape, some will
be more resistant and others will be less resistant to the weathering processes operating there. Because erosion removes weathered
rock fragments more easily than large, intact rock masses, areas of
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Structural Weaknesses
In addition to rock type, the relative
resistance of a rock to weathering depends on other characteristics, such as
the presence of joints, faults, folds, and
bedding planes that make rocks susceptible to enhanced weathering. Sandstone is only as strong as its
cement, which varies from soluble calcium carbonate to inert and
resistant silica. In general, the more massive the rock, that is, the
fewer the joints and bedding planes it has, the more resistant it is
to weathering.
The processes of volcanism, tectonism, and rock formation
produce fractures in rocks that can be exploited by exogenic processes, including weathering. Joints can be found in any solid rock
that has been subjected to crustal stresses, and some rocks are intensely jointed ( ● Fig. 15.19). Joints and other fractures that commonly develop in igneous, sedimentary, and metamorphic rocks
represent zones of weakness that expose more surface area of rock,
provide space for flow or accumulation of water, collect salts and clay
minerals, and offer a foothold for plants ( ● Fig.15.20). Rock surfaces
along fractures tend to experience pronounced weathering. Chemical and physical weathering both proceed faster along any kind of
gap, crack, or fracture than in places without such voids.
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Because joints are sites of concentrated weathering, the spatial pattern of joints strongly influences the landforms and the
appearance of the landscapes that develop. Multiple joints that
parallel each other form a joint set, and two sets, each composed
● FIGURE
15.18
These limestone tombstones have undergone extensive chemical weathering, accelerated by air pollution.
What kind of chemical weathering has impacted the iron fence?
of multiple parallel joints, will cross each other at an angle
( ● Fig. 15.21). Joints divide rocks into many different configurations, most commonly resembling blocks or columns, which are
often visible in the topography. Over time, preferential weathering and erosion in crossing joint sets leave rock in the central area
between the fractures only slightly weathered while the rock near
the fractures acquires a more rounded appearance. This distinctive, rounded weathered form, known as spheroidal weathering, develops especially well on jointed crystalline rocks, such as
granite ( ● Fig. 15.22). Spheroidal weathering is a result that can
occur from the interaction of many weathering processes; it does
not refer to a specific weathering process. Once a rock becomes
rounded, weathering rates of spheroidal outcrops and boulders decrease because there are no more sharp, narrow corners or edges
for weathering to attack, and a sphere exposes the least amount of
surface area for a given volume of rock.
Topography Related to Differential
Weathering and Erosion
J. Petersen
In the previous chapter, we learned that structural upfolds (anticlines) do not always form topographic ridges and that structural
downfolds (synclines) do not always form topographic valleys.
The reason for this stems from the resistance of the folded rock
units to the weathering and erosion processes. If resistant rocks
are found in the center of a syncline, they will eventually create
a topographic high regardless of the structural downfold. Weak
rocks, even when forming an anticline, are too easily attacked by
weathering and erosion to exist in the landscape as a topographic
high for very long. Variation in rock resistance to weathering
● FIGURE
15.19
J. Petersen
Robert Simmon/ARIA/NASA Earth
(a) Massive jointing influences the location and shape of canyons in and around Zion National Park, Utah. To
get an idea of the scale of this satellite image, note the smoke from the wildfire at upper left. (b) Farther north,
in Bryce Canyon National Park, Utah, numerous closely spaced vertical joints in sedimentary rocks are sites of
preferential weathering and erosion, leaving narrow rock spires between joints.
(a)
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(b)
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V A R I A B I L I T Y I N W E AT H E R I N G
canyon base, ancient resistant metamorphic rocks have produced
a steep-walled inner gorge. The topographic effects of differential
weathering and erosion tend to be more prominent and obvious
in landscapes of arid and semiarid climates. In these dry environments, chemical weathering is minimal, so slopes and varying
rock units are not generally covered under a significant mantle of
soil or weathered rocks. In addition, vegetation typically does not
mask the topography in arid regions as it does in humid regions.
Another example of differential weathering and erosion
can be seen in the Appalachian Ridge and Valley region of the
eastern United States ( ● Fig. 15.24). The rock structure here
● FIGURE
15.21
Multiple cross-cutting joint sets are visible in this aerial view of part of the
Colorado Plateau.
D. Sack
With north at the top of this photo, what directions do the two most
apparent joint sets trend?
15.20
Vertical joints in rocks are sites of concentrated weathering and erosion
as water preferentially accumulates in and moves along them. Higher up
at a narrow spot, this joint has accumulated enough soil for the cactus to
grow, while enhanced weathering and erosion lower down has noticeably widened the joint.
D. Sack
● FIGURE
● FIGURE
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15.22
Cross-jointed granite east of the Sierra Nevada in California forms this hill of spheroidally weathering blocks of rock.
J. Petersen
exerts a strong and often highly visible influence on the
appearance of landforms and landscapes. Given sufficient
time, rocks that are resistant to weathering and erosion
tend to stand higher than less resistant rocks. Resistant
rocks stand out in the topography as cliffs, ridges, or
mountains, while weaker rocks undergo greater weathering and erosion to create gentler slopes, valleys, and
subdued hills.
An outstanding example of how differential weathering and erosion can expose rock structure and enhance
its expression in the landscape is the scenery at Arizona’s
Grand Canyon ( ● Fig. 15.23). In the arid climate of
that region, limestone is resistant, as are sandstones and
conglomerates, but shale is relatively weak. Strong and
resistant rocks are necessary to maintain steep or vertical
cliffs. Thus, the stair-stepped walls of the Grand Canyon
have cliffs composed of limestone, sandstone, or conglomerate, separated by gentler slopes of shale. At the
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National Park Service/Mark Lellouch
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● FIGURE
15.23
NASA
The Grand Canyon of the Colorado River in Arizona is a classic example of differential weathering and erosion in
an arid climate. Weathering, mass wasting, and erosion work together to make differences in rock structure and
strength visible in the landscape. Rock layers of varying thickness and resistance result in a distinctive array of
cliffs formed by strong rocks and slopes formed by less resistant rocks.
● FIGURE
15.24
A satellite image of Pennsylvania’s Ridge and Valley section of the Appalachians clearly shows the effects of
weathering and erosion on folded rock layers of different resistance. Resistant rocks form ridges, and weaker
rocks form valleys.
Can you see how the topography of the Ridge and Valley section influences human settlement patterns?
consists of sandstone, conglomerate, shale, and limestone folded
into anticlines and synclines. These folds have been eroded
so that the edges of steeply dipping rock layers are exposed as
prominent ridges. In this humid climate region, forested ridges
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composed of resistant sandstones and conglomerates stand up to
700 meters (2000 ft) above agricultural lowlands that have been
excavated by weathering and erosion out of weaker shales and
soluble limestones.
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