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Rock Structure, Weathering, and
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Mass Wasting
Chapter
14
Rock Structure as a Landform
Control
As denudation progresses, landscape features develop
according to patterns of bedrock composition and structure.
Under similar climatic and topographic conditions, rock
denudation rates are generally related to rock type (fastest
for sedimentary rock, and is progressively slower for
metamorphic and igneous types).
Denudation rates are related to rock hardness, but other
lithological factors, such as fracturing, jointing, and
permeability are also important.
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Rock Structure as a Landform
Control
Strike and Dip
Most rock layers are not flat, but rather are tilted at an angle.
The angle formed between the rock and the horizontal plane is
termed the dip (stated in degrees, ranging
from 0° to 90°).
The line of intersection between the inclined rock plane and a
horizontal plane gives the strike of the rock (always measured as
an acute angle to the east or west of north).
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Landforms of Horizontal Strata
Extensive areas of the ancient continental shields are
covered by thick sequences of sedimentary rock that were
deposited in shallow inland seas at various times over the
past 600 million years, since the Precambrian Era.
Uplifted with little disturbance other than minor crustal
warping or faulting, these areas became continental
surfaces underlain with horizontal strata.
Landforms of Horizontal Strata
In arid climates, the normal sequence of landform
development on horizontal strata is for a sheer rock wall, or
cliff, to form at the edge of a resistant rock layer.
p , which flattens
At the base of the cliff is an inclined slope,
out into a plain beyond. Erosion strips away successive
rock layers, leaving a broad platform capped by hard rock
layers. Such a platform is usually called a plateau.
Landforms of Horizontal Strata
Cliff retreat produces a mesa, essentially a small plateau
bordered on all sides by steep rock faces.
Co t ued retreat
Continued
et eat o
of tthe
e su
surrounding
ou d g ccliffs
s reduces
educes tthe
ea
area
ea
of a mesa, but it retains its flat top (eventually, it becomes a
small steep-sided feature known as a butte).
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Landforms of Folded Rock Strata
Areas underlain by gently upwarped or downwarped rock
layers form domes and basins.
More intense folding produces the wavelike strata of
y
anticlines and synclines.
A sedimentary dome is a circular or oval structure in which
strata have been forced upward at its centre.
Deep erosion of simple folds produces a ridge-and-valley
landscape—weaker rocks, such as shale and limestone,
erode away.
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Landforms of Folded Rock Strata
This leaves hard strata, such as sandstone or quartzite, to
stand in bold relief as long, narrow ridges.
The geometry of the folds will contribute to the differential
y
tend to be
resistance of the rock mass;; strata in a syncline
compressed, while in an anticline, extensional forces may
weaken and fracture the rock.
Where rock was weakened as it was bent upward, it can
erode away to form an anticlinal valley - a synclinal
mountain can form where the rock has been strengthened
by compression.
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Landforms Developed on other
Geological Structures
Erosion Forms on Fault Structures
Active normal faulting produces a sharp surface break — a
fault scarp — that can create a rock cliff hundreds of
metres high.
Erosion quickly modifies a fault scarp, but because the fault
plane extends down into the bedrock, its effect on erosional
landforms persists over a long geologic time span.
Landforms Developed on other
Geological Structures
Exposed Batholiths
Batholiths, huge intrusions of igneous rock that are formed
deep below the Earth’s surface, are sometimes uncovered
by erosion of the overlying rock materials.
Because batholiths are typically composed of resistant
rock, they erode into hilly or mountainous uplands.
Batholiths of granitic composition are a major component of
ancient shields.
Landforms Developed on other
Geological Structures
Exposed Batholiths
Small bodies of granite projecting from underlying
batholiths are often found surrounded by ancient
metamorphic rocks formed when the granite was intruded.
These prominent features are known as monadnocks or
inselbergs.
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Landforms Developed on other
Geological Structures
Deeply Eroded Volcanoes
With continued erosion, an extinct stratovolcano may
gradually be reduced to a remnant volcanic neck formed of
lava that solidified in the pipe of the volcano.
Radiating from it are wall-like dikes created from magma
that filled radial fractures around the ancient volcano.
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Landforms Developed on other
Geological Structures
Deeply Eroded Volcanoes
Shield volcanoes show erosion features that are quite
different from those of stratovolcanoes.
In the humid climate of Hawaii for example, streams have
cut deep, narrow valleys that open out into steep-walled
amphitheatres.
Over time, the original surface of the shield volcano will be
completely dissected, leaving a rugged mountain mass
composed of sharp-crested divides and canyons.
Landforms Developed on other
Geological Structures
Impact Structures
The typical impact speed of a meteorite, about 50,000 kph,
produces a crater that is 10 to 20 times larger than the
meteorite’s diameter.
Most craters are bowl-shaped depressions around which
the ejected material is deposited.
Rocks beneath a large crater are significantly altered, and
usually consist of a shallow layer of breccia made up of
coarse, angular fragments of the original rocks.
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Weathering
Weathering is the general term applied to the combined
action of all processes that cause rock to disintegrate
physically and decompose chemically due to conditions at
or near the Earth’s surface.
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Weathering
In physical weathering, rocks are fractured and broken
apart without chemical alteration due to processes such as
frost action expansion and contraction caused by changes
in temperature, and pressure from roots.
In chemical weathering, rock minerals are transformed from
those that were stable when the rocks were initially formed
into those that are stable under temperature, pressure, and
moisture conditions found at the Earth’s surface (oxidation,
hydrolysis, solution).
Weathering
Weathering leads to the production of regolith — a surface
layer of rock particles that lies above solid, unaltered rock.
Weathering, in conjunction with gravity, also creates a
number of distinctive landforms.
Physical Weathering
Physical weathering, also known as mechanical
weathering, produces regolith by the action of forces strong
enough to fracture the rock.
Fracturing
F
t i occurs when
h stress
t
is
i exerted
t d along
l
zones off
weakness in the rock.
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Physical Weathering
Frost Action
One of the most important physical weathering processes
in cold climates is frost action caused by the repeated
growth and melting of ice crystals in pore spaces and
cracks in a rock.
Water expands when it freezes, resulting in an increase in
volume of about 10 percent (over the course of many
freeze – thaw cycles, such expansion can fragment even
extremely hard rocks).
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Physical Weathering
Salt-Crystal Growth
The weathering of rock by salt crystals is similar to the
process of ice-crystal growth.
During long drought periods, capillary action moves
groundwater to the surface of the rock (water is drawn
upward through surface tension).
Physical Weathering
Salt-Crystal Growth
As water evaporates from the porous outer zone of the
sandstone, tiny crystals of salts, such as halite (NaCl),
calcite (CaCO3), or gypsum (CaSO4·2H20), are left behind.
Over time, the force created by the growth of these crystals
results in grain-by-grain breakup of the sandstone, which
crumbles into sand and is swept away by wind and rain.
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Physical Weathering
Unloading
Unloading is a process that relieves the confining pressure on a
rock mass (when the removal of overlying material brings a rock
mass nearer to the surface).
This causes the rock to crack in layers that are more or less
parallel to the surface, creating a type of jointing called sheeting
structure.
In massive rocks like granite or marble, thick curved layers or
shells of rock successively break free from the parent mass
below through the process of exfoliation.
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Physical Weathering
Other Physical Weathering Processes
Most rock-forming minerals expand when heated and contract
when cooled under normal diurnal temperature cycles - this can
create powerful disruptive forces on rocks in deserts and other
areas where the surface is subjected to intense daytime heating
alternating with cool nights.
Daily temperature changes can cause the breakup of a surface
layer of rock already weakened by other agents of weathering.
Physical Weathering
Other Physical Weathering Processes
Another mechanism of rock breakup is the growth of plant
roots, which can wedge joint blocks apart.
The primary role of plants in weathering is biochemical
rather than biophysical, and is associated mainly with
acidic exudates from mosses and lichens that colonize
bare rock surfaces.
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Chemical Weathering and its
Landforms
Chemical weathering is a process of mineral alteration.
The dominant processes of chemical change affecting
silicate minerals are oxidation, hydrolysis, and carbonation.
Chemical Weathering and its
Landforms
Hydrolysis and Oxidation
Oxidation and hydrolysis change the chemical structure of
primary minerals, turning them into secondary minerals that
are typically softer and bulkier and therefore more
susceptible to erosion and mass movement.
Decomposition by hydrolysis and oxidation changes rockforming minerals into clay minerals and oxides.
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Chemical Weathering and its
Landforms
Carbonation and Solution
Chemical weathering through acid action is mainly
associated with carbonic acid (H2CO3) formed when CO2
dissolves in water.
Rainwater, soil water, and stream water all normally
contain dissolved CO2.
Percolating water is primarily acidified as it moves through
the soil and reacts with CO2 generated by decomposition of
organic material.
Chemical Weathering and its
Landforms
Carbonation and Solution
Carbonic acid slowly interacts with feldspars and other
types of minerals to form carbonates; this process of
carbonation is an important intermediate stage in the
weathering of igneous rocks.
Limestone and marble, are particularly susceptible to acid
action. In this case, the mineral calcium carbonate (CaCO3)
dissolves and is carried away in solution in stream water.
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Chemical Weathering and its
Landforms
Limestone Solution by Groundwater
In moist climates, the slow flow of groundwater in the
saturated zone can dissolve limestone below the surface
surface,
producing large underground caverns.
Some caverns collapse, causing the ground above to sink
and a unique type of karst landscape to develop.
Chemical Weathering and its
Landforms
Limestone Solution by Groundwater
Limestone caverns are interconnected subterranean
cavities in bedrock formed by the corrosive action of
circulating groundwater on limestone.
A sinkhole is a surface depression in a region of cavernous
limestone.
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18
Mass Wasting
Gravity induces the spontaneous downslope movement of
rock fragments created by weathering.
This movement to lower elevations, which occurs without
the action of flowing water, wind, or moving ice, is called
mass wasting.
Movement of a mass of soil or weathered rock takes place
when the internal strength of the material declines to a
critical point where it can no longer resist the force of
gravity.
19
Mass Wasting
Slopes
On a typical hill, slope soil and regolith blanket the bedrock.
The thicknesses of these surficial materials are q
quite variable
and depend on the type and calibre of material, as well the
gradient of the slope.
Although soil rarely extends deeper than 1 or 2 m, residual
regolith overlying decayed and fragmented rock can be more
than 100 m thick. Soil and regolith may be absent in some
places, exposing outcrops of bedrock.
Mass Wasting
Slopes
Residual regolith derived directly from the rock beneath
moves slowly down the slope.
Accumulations of regolith at the foot of a slope are called
colluvium.
Beneath the valley floor are layers of regolith, called
alluvium, which is sediment transported and deposited by
streams.
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Mass Wasting
Soil Creep
On most slopes, soil and regolith move extremely slowly downhill
through the process of soil creep.
Soil creep occurs through the effect of some process of soil disturbance
acting under the influence of gravity.
Alternate wetting and drying of the soil, growth of ice needles and ice
lenses, heating and cooling of the soil, trampling and burrowing by
animals, and shaking by earthquakes all produce some disturbance of
the soil and regolith.
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Mass Wasting
Earth Flows and Rotational Slumps
In humid climates, a mass of moist soil and fine regolith may move
down a steep slope in the form of an earth flow.
The rate at which an earth flow develops can be slow or rapid,
depending on the texture of the material, moisture content, and angle of
slope.
A slump usually occurs when water percolates deep into a
mass of unconsolidated material, resulting in it dropping along a
concave slip plane (slow movement along a surface of weakness).
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Mass Wasting
Mudflow and Debris Flood
One of the most spectacular forms of mass wasting and a potentially
serious environmental hazard is the mudflow.
These streams
Th
t
off muddy
dd fl
fluid
id pour swiftly
iftl d
down canyons iin
mountainous regions.
Mudflows vary in consistency, from a viscous fluid similar to freshly
mixed concrete to fast-flowing, water-saturated mixtures of mud and
rock.
The more fluid type of mudflow is called a debris flow.
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Mass Wasting
Landslides and Rockfalls
A landslide is the rapid sliding of large masses of bedrock or regolith.
Landslides, unlike earth flows and mudflows, are triggered by earth
t
tremors
or sudden
dd rock
k ffailures
il
rather
th th
than b
by h
heavy rains
i or fl
floods;
d
they can also result when excavation or river erosion oversteepens a
slope.
Landslides can range from rockslides of jumbled bedrock fragments to
bedrock slumps in which most of the bedrock remains more or less
intact as it moves.
A Look Ahead
Flowing water, in the form of rivers and streams, is the
principle agent of erosion and is discussed in the next two
chapters.
The first deals with water in the hydrologic cycle, in soil,
and
d iin streams.
t
The second deals specifically with how flowing water
erodes rock and regolith and deposits transported sediment
to create a variety of distinctive fluvial landforms.
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