Dynamic Earth

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Dynamic Earth

• Class 15
• 28 February 2005

2
The Flow of the Continents (Chapter 5)Building
MountainsRock Deformation and Earthquakes
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Rock Deformation

• Large scale deformation of the Earths crust
  Plate Tectonics
• Smaller scale deformation structural geology

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Deformation

• Changes in volume or shape of a rock body
• strain

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Deformation of rocks

• Folds and faults are geologic structures
• Structural geology is the study of the
  deformation of rocks and the effects of this
  movement

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Small-Scale Folds
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Stress

• The force that acts on a rock unit to change its
  shape and/or its volume
• Causes strain or deformation
• Stress
• Compression
• Tension
• Shear

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Compression
Action of coincident oppositely directed forces
acting towards each other
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Tension
Action of coincident oppositely directed forces
acting away from each other
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Shear
Action of coincident oppositely directed forces
acting parallel to each other across a surface in
a couple
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Differential stress
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Strength

• Ability of an object to resist deformation
• Compressive or tensile

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Strain
Any change in original shape or size of an object
in response to stress acting on the object
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Kinds of deformation

• Elastic vs Plastic
• Brittle vs Ductile

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Elastic Deformation
Temporary change in shape or size that is
recovered when the deforming force is removed
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Ductile (Plastic) Deformation

• Permanent change in shape or size that is not
  recovered when the stress is removed
• Occurs by the slippage of atoms or small groups
  of atoms past each other in the deforming
  material, without loss of cohesion

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Increasing stress
Increasing strain
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Brittle Deformation (Rupture)

• Loss of cohesion of a body under the influence of
  deforming stress
• Usually occurs along sub-planar surfaces that
  separate zones of coherent material

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Increasing stress
Increasing strain
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Factors that affect deformation

• Temperature
• Pressure
• Strain rate
• Rock type

The variation of these factors determines if a
rock will fault or fold.
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Rocks are

• Elastic and brittle near the earth's surface
• More plastic and ductile deeper in the crust
• Because of the increasing temperature and pressure

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Folds

• Most common ductile response to stress on rocks
  in the earth's crust

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Experimental Deformation of Marble
Brittle Deformation (low confining pressure)
Ductile Deformation (high confining pressure)
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Tectonic Forces and Resulting Deformation
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Geometry of Anticlines Synclines
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Types of Folds (bent planar structures)

• anticline older rocks on the inside
• syncline older rocks on the outside
• (scale - from mm to tens of km)

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Anticlines and Synclines
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Syncline
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Fold Terms

• axial plane the plane of mirror symmetry
  dividing the fold into two limbs
• axis line formed by the intersection of the
  axial plane and a bedding plane
• horizontal fold where the fold axis is
  horizontal
• plunging fold where the fold axis is not
  horizontal

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Fold Terminology
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Symmetrical, Asymmetrical and Overturned Folds
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Asymmetric Folds
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Overturned Folds
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Map View of Plunging Folds
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Oil Field at Crest of Plunging Anticline
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Fracture

• Most common brittle response to stress
• With No displacement Joint
• With displacement Fault

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Joints commonly form when the surface of a
volcanic rock cools and contracts
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Devils Tower, Wyoming
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Faults

• Occur when large stresses build up in the crust,
  often due to lava movement into shallow magma
  chambers
• Classified according to the kind of movement that
  has occurred along them

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Normal Fault Hanging wall drops down
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Tom Bean
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Rift Valley Formed by Extension
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Wildrose Graben, Southern California
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Reverse Fault Hanging wall moves up
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Reverse Fault
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Large-Scale Overthrust Sheet
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Keystone Thrust Fault, S. Nevada
Cambrian Limestone
Jurassic Sandstone
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Cross Section of the Keystone Thrust West of Las
Vegas
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French Thrust, Wyoming
Mississippian Limestone
Cretaceous Shale
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Strike-slip Fault
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• Faults may move several meters in a second or so
• This movement within the earth's crust usually
  generates an earthquake

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Global Locations of Earthquakes
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First-motion studies of earthquakes indicate
direction of movement on faults
Fig. 16.18
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Tuesday

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• for Chapter 5