Chapter%209:%20How%20Rock%20Bends,%20Buckles,%20and%20Breaks

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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

• Uniform stress is a condition in which the stress
  is equal in all directions.
• In rocks it is also confining stress 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 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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Figure 9.1
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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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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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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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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 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.

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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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Figure 9.21
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Box 9.1
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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 9.22 C,D,E
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Figure 9.22 A, B
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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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Figure 9.23
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Figure 9.24
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Figure 9.25
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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 winds upward, causing wind, rain, and
  snow on the western slopes.