IGNEOUS ROCKS

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Chapter 10
C R U S T A L D E F O R M A T I O N
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• Earths architectural features
• Folds, Faults, Fractures, Joints, etc.
• Structural Geology
• Study of these features

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Why is it important?

• Oil / Gas Deposits
• Precious Other Metal Deposits
• Hazards Analysis
• Earthquakes, Dams, Nuclear Plants, etc.

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What is Deformation?

• Change in a rock mass
• Size
• Shape
• Orientation or
• Position

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Where Does Deformation Occur?

• Anywhere tectonic forces act upon formations
• e.g., Plate margins

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Deformation in rocks near Lulworth, Doset, England
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Types of Stress

• Stress
• Confining Pressure (stress uniform in all
  directions)
• Differential Stress (nonuniform stress)
• Compressional Stress
• Tensional Stress
• Shear Stress

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Undeformed rocks
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Horizontal compression
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Horizontal tension
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Shearing
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Strain

• Deformation caused by stress
• - a strained body does not retain its original
  shape during/after deformation

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Agents of Deformation

• Deformation depends on
• Temperature Confining Pressure
• Brittle Ductile deformation
• Brittle the material breaks
• Ductile solid-state flow change in size and
  shape without breakage

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• -Rock Type
• Rock Salt, Gypsum, Shale flow ductilely
• Limestone, marble, schist - intermediate
• Granite, basalt strong and brittle

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• Fluids
• small amounts of water increase ductility
• Time
• effects of small amounts of stress over time
• (wooden bookshelf loaded with books)

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Mapping Geological Structures

• Formations
• Strike and dip
• Folds
• Faults

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Formations

• Mappable rock unit
• Has definite upper lower contacts (boundaries
  with other units)
• Obvious characteristics (i.e., rock type) which
  allows it to be traced from place to place
• Is different from other rock units
• Generally sedimentary rocks
• Named for nearby location/feature main rock (or
  Formation if no single rock type)

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Named formations in Grand Canyon
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• Strike
• The compass direction of the line produced by the
  intersection of an inclined rock layer or fault,
  with a horizontal plane
• In northern hemisphere, measured from north
• Example N75E the horizontal line is oriented
  75º east of north

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• Dip
• The angle of inclination of the surface of a rock
  unit or fault measured from a horizontal plane
• Dip direction direction water would flow if you
  poured some on the surface

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75
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Folds

• Wave-like undulations of strata
• Most form from compressional stresses
• Microscopic to Macroscopic in scale

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Parts of a Fold

• Limbs
• The two sides of a fold
• Axial Plane
• - The surface that divides a fold as
  symmetrically as possible
• Hinge
• The line drawn along maximum curvature
• Plunge
• - angle the hinge makes with the horizontal

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Types of Folds

• Anticlines Synclines
• Symmetrical Antisymmetrical
• Overturned
• Recumbent (the axial plane is horizontal)

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Monocline

• Rocks bent due to a fault in the underlying
  bedrock

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Plunging folds
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Domes Basins

• Doubly-plunging folds
• Domes
• Large scale, circular or elongated upwarped
  folded sedimentary strata
• Basins
• Same, but downwarped

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Dome - Black Hills, South Dakota
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Basin bedrock geology of Michigan
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Faults

• Faults are fractures in the crust along which
  appreciable displacement has taken place
• Sudden movement along faults are the cause of
  most earthquakes

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Some Georgia Faults

• Brevard Fault Zone (Chattahoochee)
• Towaliga Fault / Goat Rock Fault
• Pine Mountain, Warm Springs
• Cartersville Fault
• Rome Fault

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Categories of Faults

• Dip-slip Faults
• Movement parallel to the dip of the fault surface
• Strike-slip Faults
• Movement parallel to the strike of the fault
  surface

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Hanging wall footwall
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Types of Dip-Slip Faults

• Normal Faults
• Reverse Faults
• Thrust Faults

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Normal fault - Hanging wall moved down (or
footwall up)
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Horst graben structure
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Reverse fault - Hanging wall moved up (or
footwall down)
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Thrust fault - Reverse Fault with an angle of dip
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Strike-Slip Faults

• Displacement is horizontal and parallel to the
  strike of the fault surface
• Right-lateral strike slip fault
• Left-lateral strike slip fault

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Right-lateral strike-slip fault (San Andres
fault, Calif)
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Joints

• Fractures in rocks where no appreciable movement
  has occurred
• May have two or three intersecting sets
• Mineral deposits may be deposited within joints
• Joints may affect construction projects
  (highways, dams)

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Columnar jointing Devils Tower, Wyoming
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Horizontal vertical jointing