Chapter 9: How Rock Bends, Buckles,
and Breaks
How Is Rock Deformed?
ƒ Tectonics forces continuously squeeze, stretch,
bend, and break rock in the lithosphere.
ƒ The source of energy is the Earth’s heat, which is
transformed into to mechanical energy.
Stress
ƒ Uniform stress is a condition in which the stress is
equal in all directions.
ƒ In rocks it is also confining stress because any body of
rock in the lithosphere is confined by the rock around it.
ƒ Differential stress is stress that is not equal in all
directions.
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Differential Stress
ƒ The three kinds of differential stress are:
ƒ Tensional stress, which stretches rocks.
ƒ Compressional stress, which squeezes them.
ƒ Shear stress, which causes slippage and translation.
Figure 9.1
Stages of Deformation
ƒ Strain describes the deformation of a rock.
ƒ When a rock is subjected to increasing stress, it passes
through three stages of deformation in succession:
ƒ Elastic deformation is a reversible change in the volume or shape of
a stressed rock..
ƒ Ductile deformation is an irreversible change in shape and/or
volume of a rock that has been stressed beyond the elastic limit.
ƒ Fracture occurs in a solid when the limits of both elastic and ductile
deformation are exceeded.
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Figure 9.2
Figure 9.3
Ductile Deformation Versus Fracture
ƒ A brittle substance tends to deform by fracture.
ƒ A ductile substance deforms by a change of shape.
ƒ The higher the temperature, the more ductile and
less brittle a solid becomes.
ƒ Rocks are brittle at the Earth’s surface, but at depth,
where temperatures are high because of the
geothermal gradient, rocks become ductile.
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Figure 9.4
Confining Stress
ƒ Confining stress is a uniform squeezing of rock
owing to the weight of all of the overlying strata.
ƒ High confining stress hinders the formation of
fractures and so reduces brittle properties.
ƒ Reduction of brittleness by high confining stress is a
second reason why solid rock can be bent and
folded by ductile deformation.
Fracture
ƒ All the constituent atoms of a solid transmit stress
applied to a solid.
ƒ If the stress exceeds the strength of the bond
between atoms:
ƒ Either the atoms move to another place in the crystal
lattice in order to relieve the stress, or;
ƒ The bonds must break, and fracture occurs.
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Strain Rate
ƒ The term used for time-dependent deformation of a
rock is strain rate.
ƒ Strain rate is the rate at which a rock is forced to change
its shape or volume.
ƒ Strain rates in the Earth are about 10-14 to 10-15/s.
ƒ The lower the strain rate, the greater the tendency
for ductile deformation to occur.
Figure 9.6
Enhancing Ductility
ƒ High temperatures, high confining stress, and low
strain rates (characteristic of the deeper crust and
mantle):
ƒ Reduce brittle properties.
ƒ Enhance the ductile properties of rock.
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Composition Affects Ductility (1)
ƒ The composition of a rock has pronounced effects on its
properties.
ƒ Quartz, garnet, and olivine are very brittle.
ƒ Mica, clay, calcite,and gypsum are ductile.
ƒ The presence of water in a rock reduces brittleness and
enhances ductile properties.
ƒ Water affects properties by weakening the chemical bonds in
minerals and by forming films around minerals grains.
Composition Affects Ductility (2)
• Rocks that readily deform by ductile deformation
are limestone, marble, shale, phyllite, and schist.
ƒ Rocks that tend to be brittle rather than ductile are
sandstone and quartzite, granite, granodiorite, and
gneiss.
Rock Strength (1)
ƒ Rock strength in the Earth does not change uniformly
with depth.
ƒ There are two peaks in the plot of rock strength with
depth.
ƒ Strength is determined by composition, temperature, and pressure.
ƒ Rocks in the crust are quartz-rich, so the strength
properties of quartz play an important role in the
strength properties of the crust.
ƒ Rock strength increases down to a depth about 15 km.
Above 15 km rocks are strong (they fracture and fail by
brittle deformation).
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Rock Strength (2)
ƒ Below 15 km, fractures become less common
because quartz weakens and rocks become
increasingly ductile.
ƒ Rocks in the mantle are olivine-rich. Olivine is
stronger than quartz, and the brittle-ductile
transition of olivine-rich rock is reached only at a
depth about 40 km.
Figure 9.7
Rock Strength (3)
ƒ By about 1300oC, rock strength is very low.
ƒ Brittle deformation is no longer possible. The
disappearance of all brittle deformation properties
marks the lithosphere-asthenosphere boundary.
ƒ In the crust large movements happens so slowly
(low strain rates) that they can be measured only
over a hundred or more years.
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Abrupt Movement
ƒ Abrupt movement results from the fracture of brittle
rocks and movement along the fractures.
ƒ Stress builds up slowly until friction between the two
sides of the fault is overcome, when abrupt slippage
occurs.
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The largest abrupt vertical displacement ever observed occurred
in 1899 at Yakutat Bay, Alaska, during an earthquake. A stretch
of the Alaskan shore lifted as much as 15 m above the sea level.
Abrupt movements in the lithosphere are commonly
accompanied by earthquakes.
Gradual Movement
ƒ Gradual movement is the slow rising, sinking, or
horizontal displacement of land masses.
ƒ Tectonic movement is gradual.
ƒ Movement along faults is usually, but not always, abrupt.
Figure 9.9
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Evidence Of Former Deformation
ƒ Structural geology is the study of rock deformation.
ƒ The law of original horizontality tells us that
sedimentary strata and lava flows were initially
horizontal.
ƒ If such rocks are tilted, we can conclude that
deformation has occurred.
Dip and Strike
ƒ The dip is the angle in degrees between a horizontal
plane and the inclined plane, measured down from
horizontal.
ƒ The strike is the compass direction of the horizontal
line formed by the intersection of a horizontal plane
and an inclined plane.
Figure 9.10
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Figure 9.11
Deformation By Fracture
ƒ Rock in the crust tends to be brittle and to be cut by
innumerable fractures called either joints or faults.
ƒ Most faults are inclined.
ƒ To describe the inclination, geologists have adopted two
old mining terms:
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The hanging-wall block is the block of rock above an inclined
fault.
The block of rock below an inclined fault is the footwall block.
ƒ These terms, of course, do not apply to vertical
faults.
Figure 9.12
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Classification of Faults (1)
ƒ Faults are classified according to:
ƒ The dip of the fault.
ƒ The direction of relative movement.
ƒ Normal faults are caused by tensional stresses that
tend to pull the crust apart, as well as by stresses
created by a push from below that tend to stretch the
crust. The hanging-wall block moves down relative
to the footwall block.
Figure 9.13
Figure 9.13B
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Classification of Faults (2)
ƒ A down-dropped block is a graben, or a rift, if it is
bounded by two normal faults.
ƒ It is a half-graben if subsidence occurs along a single fault.
ƒ An upthrust block is a horst.
ƒ The world’s most famous system of grabens and half-grabens is the
African Rift Valley of East Africa.
ƒ The north-south valley of the Rio Grande in New Mexico is a graben.
ƒ The valley in which the Rhine River flows through western Europe
follows a series of grabens.
Figure 9.14
Classification of Faults (3)
ƒ Reverse faults arise from compressional stresses.
Movement on a reverse fault is such that a hangingwall block moves up relative to a footwall block.
ƒ Reverse fault movement shortens and thickens the crust.
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Classification of Faults (4)
ƒ Thrust faults are low-angle reverse faults with dip less
than 15o.
ƒ Such faults are common in great mountain chains.
ƒ Strike-slip faults are those in which the principal
movement is horizontal and therefore parallel to the
strike of the fault.
ƒ Strike-slip faults arise from shear stresses.
ƒ The San Andreas is a right-lateral strike-slip fault.
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Apparently, movement (more than 600 km) has been occurring along it
for at least 65 million years.
Figure 9.17
Figure 9.18
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Classification of Faults (5)
ƒ Where one plate margin terminates another
commences, their junction point is called a
transform.
ƒ J. T. Wilson proposed that the special class of strike-slip
faults that forms plate boundaries be called transformfaults.
Figure 9.19
Evidence of Movement Along Faults
ƒ Movement of one mass of rock past another can
cause the fault’s surfaces to be smoothed,
striated, and grooved.
ƒ Striated or highly polished surfaces on hard rocks,
abraded by movement along a fault, are called
slickensides.
ƒ In many instances, fault movement crushes rock adjacent
to the fault into a mass of irregular pieces, forming fault
breccia.
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Deformation by Bending
ƒ The bending of rock is referred to as folding.
ƒ Monocline: the simplest fold. The layers of rock are
tilted in one direction.
ƒ Anticline: an upfold in the form of an arch.
ƒ Syncline: a downfold with a trough-like form.
ƒ Anticlines and synclines are usually paired.
Figure 9.21
Box 9.1
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The Structure of Folds (1)
ƒ The sides of a fold are the limbs.
ƒ The median line between the limbs is the axis of the fold.
ƒ A fold with an inclined axis is said to be a plunging fold.
ƒ The angle between a fold axis and the horizontal is the
plunge of a fold.
ƒ An imaginary plane that divides a fold as symmetrically
as possible is the axial plane.
Figure 9.22 C,D,E
Figure 9.22 A, B
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The Structure of Folds (2)
ƒ An open fold is one in which the two limbs dip
gently and equally away from the axis.
ƒ When stress is very intense, the fold closes up and
the limbs become parallel to each other.
ƒ Such a fold is said to be isoclinal.
The Structure of Folds (3)
ƒ Eventually, an overturned fold may become
recumbent, meaning the two limbs are horizontal.
ƒ Common in mountainous regions,such as the Alps and
the Himalaya, that were produced by continental
collisions.
ƒ Anticlines do not necessarily make ridges, nor
synclines valleys.
Figure 9.23
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Figure 9.24
Figure 9.25
Figure 9.26
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Examples of Faults (1)
ƒ In the Valley and Ridge province of Pennsylvania, a
series of plunging anticlines and synclines were created
during the Paleozoic Era by a continental collision of
North America, Africa, and Europe.
ƒ Now the folded rocks determine the pattern of the topography
because soft, easily eroded strata (shales) underlie the valleys, while
resistant strata (sandstones) form the ridges.
ƒ The San Andreas Fault in California is a strike-slip fault.
Examples of Faults (2)
ƒ The Alpine Fault is part of the boundary between the
Pacific plate and the Australian-Indian plate, and slices
through the south island of New Zealand.
ƒ The North Anatolian Fault, also with right-lateral motion,
slices through Turkey in an east-west direction, and is the
cause of many dangerous earthquakes.
ƒ The Great Glen Fault of Scotland was active during the
Paleozoic Era.
ƒ Loch Ness lies in the valley that marks its trace.
Tectonism And its Effect On Climate
ƒ Temperature decreases with altitude.
ƒ The Sierra Nevada influences the local climate.
ƒ It imposes a topographic barrier to flow that forces the
winds upward, causing wind, rain, and snow on the
western slopes.
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