Chapter 9: How Rock Bends, Buckles, and Breaks

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Chapter 9 How Rock Bends, Buckles, and Breaks
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How Is Rock Deformed?

• Tectonics forces continuously squeeze, stretch,
  bend, and break rock in the lithosphere.
• The source of energy is the Earths heat, which
  is transformed into to mechanical energy.

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Stress ??

• Definition the force acted on per unit of
  surface area, SI unit Newton/m2Pascal.
• Uniform stress is a condition in which the stress
  is equal in all directions.
• It is confining stress or confining pressure (??)
  applied to rocks because any body of rock in the
  lithosphere is confined by the rock around it.
• Differential stress is stress that is not equal
  in all directions.

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Differential (Deviatoric) Stress

• The three kinds of differential stress are
• Tensional stress (???), which stretches rocks.
• Compressional stress(???), which squeezes them.
• Shear stress (???), which causes slippage (??)and
  translation (??).

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Stages of Deformation (??)

• Strain (??) describes the deformation of a rock.
• When a rock is subjected to increasing stress, it
  passes through three stages of deformation in
  succession
• Elastic deformation (????) is a reversible change
  in the volume or shape of a stressed rock..
• Ductile deformation (????) is an irreversible
  change in shape and/or volume of a rock that has
  been stressed beyond the elastic limit (????).
• Fracture (??) occurs in a solid when the limits
  of both elastic and ductile deformation are
  exceeded.

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Strain (??)

• Normal Strain (???)
• Shear Strain (???)

(1) Tensional strain
(2) Compressional strain
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Figure 9.2
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Figure 9.3
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Ductile Deformation Versus Fracture

• A brittle (??) substance tends to deform by
  fracture.
• A ductile substance deforms by a change of shape.
• The higher the temperature, the more ductile and
  less brittle a solid becomes.
• Rocks are brittle at the Earths surface, but at
  depth, where temperatures are high because of the
  geothermal gradient, rocks become ductile.

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A and B have the same elastic deformation, but B
experiences larger ductile deformation because
the temperature of A is lower than that of B or
the confining pressure applied to B is higher
than that to B.
Figure 9.4
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Confining Stress

• Confining stress is a uniform squeezing of rock
  owing to the weight of all of the overlying
  strata.
• High confining stress hinders the formation of
  fractures and so reduces brittle properties.
• Reduction of brittleness by high confining stress
  is a second reason why solid rock can be bent and
  folded by ductile deformation.

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Fracture

• All the constituent atoms of a solid transmit
  stress applied to a solid.
• If the stress exceeds the strength of the bond
  between atoms
• Either the atoms move to another place in the
  crystal lattice in order to relieve the stress,
  or
• The bonds must break, and fracture occurs.

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Strain Rate (????)

• The term used for time-dependent deformation of a
  rock is strain rate.
• Strain rate is the rate at which a rock is forced
  to change its shape or volume.
• Strain rates in the Earth are about 10-14 to
  10-15/s.
• The lower the strain rate, the greater the
  tendency for ductile deformation to occur.

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A has a high elastic limit and a low ductile
deformation, so the temperature is low and the
strain rate is also low. B has a lower elastic
limit, and high temperature, so the ductile
deformation is still high even B has a high
strain rate. C has the lowest elastic limit and
largest ductile deformation because its
temperature is high and strain rate is low.
Figure 9.6
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Enhancing Ductility

• High temperatures, high confining stress, and low
  strain rates (characteristic of the deeper crust
  and mantle)
• Reduce brittle properties.
• Enhance the ductile properties of rock.

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Composition Affects Ductility (1)

• The composition of a rock has pronounced effects
  on its properties.
• Quartz (??), garnet (???), and olivine (???) are
  very brittle.
• Mica (??), clay (??), calcite (???), and gypsum
  (??) are ductile.
• The presence of water in a rock reduces
  brittleness and enhances ductile properties.
• Water affects properties by weakening the
  chemical bonds in minerals and by forming films
  around minerals grains.

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Composition Affects Ductility (2)

• Rocks that readily deform by ductile deformation
  are limestone (???), marble (???), shale (??),
  phyllite (???) and schist (?? ).
• Rocks that tend to be brittle rather than ductile
  are sandstone (??) and quartzite (???), granite
  (???), granodiorite (?????) , and gneiss (???).

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Rock Strength (1)

• Rock strength (????) in the Earth does not change
  uniformly with depth.
• There are two peaks in the plot of rock strength
  with depth.
• Strength is determined by composition,
  temperature, and pressure.
• Rocks in the crust are quartz-rich, so the
  strength properties of quartz play an important
  role in the strength properties of the crust.
• 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)

• Below 15 km, fractures become less common because
  quartz weakens and rocks become increasingly
  ductile.
• 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.

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Figure 9.7
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Rock Strength (3)

• By about 1300oC, rock strength is very low.
• Brittle deformation is no longer possible. The
  disappearance of all brittle deformation
  properties marks the lithosphere-asthenosphere
  boundary.
• 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

• Abrupt movement results from the fracture of
  brittle rocks and movement along the fractures.
• Stress builds up slowly until friction between
  the two sides of the fault is overcome, when
  abrupt slippage occurs.
• 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.

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Gradual Movement

• Gradual movement is the slow rising, sinking, or
  horizontal displacement of land masses.
• Tectonic movement is gradual.
• Movement along faults is usually, but not always,
  abrupt.

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Figure 9.9
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Evidence Of Former Deformation

• Structural geology is the study of rock
  deformation.
• The law of original horizontality tells us that
  sedimentary strata and lava flows were initially
  horizontal.
• If such rocks are tilted, we can conclude that
  deformation has occurred.

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Dip (????) and Strike (??)

• The dip is the angle in degrees between a
  horizontal plane and the inclined plane, measured
  down from horizontal.
• The strike is the compass direction (North) of
  the horizontal line formed by the intersection of
  a horizontal plane and an inclined plane.

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Figure 9.10
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Figure 9.11
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Deformation By Fracture

• Rock in the crust tends to be brittle and to be
  cut by innumerable fractures called either joints
  (?? )or faults (??).
• Most faults are inclined.
• To describe the inclination, geologists have
  adopted two old mining terms
• 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.
• These terms, of course, do not apply to vertical
  faults.

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Figure 9.12
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Classification of Faults (1)

• Faults are classified according to
• The dip of the fault.
• The direction of relative movement.
• 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.

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Figure 9.13
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Figure 9.13B
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Classification of Faults (2)

• A down-dropped block is a graben (??), or a rift
  (??), if it is bounded by two normal faults.
• It is a half-graben (???) if subsidence occurs
  along a single fault.
• An upthrust block is a horst (??).
• The worlds most famous system of grabens and
  half-grabens is the African Rift Valley of East
  Africa.
• The north-south valley of the Rio Grande in New
  Mexico is a graben.
• The valley in which the Rhine River flows through
  western Europe follows a series of grabens.
• The Basin and Range Province in Utah, Nevada, and
  Idaho is a series of nearly parallel north-south
  striking normal faults which has formed horsts
  and half-grabens. The horsts are now mountain
  ranges and the grabens and half-grabens are
  sedimentary basins.

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Figure 9.14
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Classification of Faults (3)

• Reverse faults (???) arise from compressional
  stresses. Movement on a reverse fault is such
  that a hanging-wall block moves up relative to a
  footwall block.
• Reverse fault movement shortens and thickens the
  crust.

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Classification of Faults (4)

• Thrust faults (???) are low-angle reverse faults
  with dip less than 15o.
• Such faults are common in great mountain chains.
• Strike-slip (????) faults are those in which the
  principal movement is horizontal and therefore
  parallel to the strike of the fault.
• Strike-slip faults arise from shear stresses.
• The San Andreas is a right-lateral strike-slip
  fault.
• Apparently, movement (more than 600 km) has been
  occurring along it for at least 65 million years.

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Figure 9.17
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Figure 9.18
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Classification of Faults (5)

• Where one plate margin terminates another
  commences, their junction point is called a
  transform.
• J. T. Wilson proposed that the special class of
  strike-slip faults that forms plate boundaries be
  called transform-faults (????).

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Figure 9.19
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Evidence of Movement Along Faults

• Movement of one mass of rock past another can
  cause the faults surfaces to be smoothed,
  striated (?????) and grooved (???).
• Striated or highly polished surfaces on hard
  rocks, abraded (????) by movement along a fault,
  are called slickensides (???).
• 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

• The bending of rock is referred to as folding
  (??).
• Monocline (??) the simplest fold (??). The
  layers of rock are tilted in one direction.
• Anticline (??) an upfold in the form of an arch.
• Syncline (??) a downfold with a trough-like
  form.
• Anticlines and synclines are usually paired.

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The Structure of Folds (1)

• The sides of a fold are the limbs (????).
• The median line between the limbs is the axis of
  the fold.
• A fold with an inclined axis is said to be a
  plunging fold.
• The angle between a fold axis and the horizontal
  is the plunge (????) of a fold.
• An imaginary plane that divides a fold as
  symmetrically as possible is the axial plane.

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Figure 9.21
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??
??
Figure 9.22 A, B
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???
??
??
Figure 9.22 C,D,E
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The Structure of Folds (2)

• An open fold is one in which the two limbs dip
  gently and equally away from the axis.
• When stress is very intense, the fold closes up
  and the limbs become parallel to each other.
• Such a fold is said to be isoclinal.

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The Structure of Folds (3)

• Eventually, an overturned fold may become
  recumbent, meaning the two limbs are horizontal.
• Common in mountainous regions,such as the Alps
  and the Himalaya, that were produced by
  continental collisions.
• Anticlines do not necessarily make ridges, nor
  synclines valleys.

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Faulting causes a monocline to form.
Figure 9.23
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The evolution of a recumbent fold into a thrust
fault
Figure 9.24
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Topography resulting from different resistance to
erosion of rocks reveal the presence of the
plunging folds.
Figure 9.25
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Central Appalachians in Pennsylvanvia Strata are
folded into anticlines and synclines as a result
of collision during the assembly of Pangaea.
Figure 9.26
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Examples of Faults (1)

• 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.
• 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.
• The San Andreas Fault in California is a
  strike-slip fault.

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Examples of Faults (2)

• 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.
• 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.
• The Great Glen Fault of Scotland was active
  during the Paleozoic Era.
• Loch Ness lies in the valley that marks its
  trace.

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Tectonism And its Effect On Climate

• Temperature decreases with altitude.
• The Sierra Nevada influences the local climate.
• It imposes a topographic barrier to flow that
  forces the moisture-laden winds from the sea
  flowing upward, causing rain, and snow on the
  western slopes. When the winds flow down the
  eastern side of the mountains, most of the
  moisture has been dried out, causing the dry
  lands and deserts on Nevada and Utah.

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????????(1) ???? 03/01/2004 Assignments are
due at 5 pm on the designated date. The
deadlines are firm. If late assignments are
handed in within two weeks of the due date, the
grade will be penalized by 10 pts. After two
weeks of the due date, the grade will be
penalized by 20 pts. Copies of homeworks are not
allowed and will not be graded. 1. (30pt)
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Rate) ???,??????????????????? (Normal Stress)
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(Lithosphere) ????? (Rock Strength)
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