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Geology 12 -

Geology 12 -. Structural Geology. BIG IDEA The form, arrangement, and structure of rocks are affected by 3 dimensional forces over time. Rocks - Igneous, sedimentary, metamorphic. Forces - Pressure (think plate tectonics). Time - seconds to millions of years.

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Geology 12 -

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  1. Geology 12 -

  2. Structural Geology BIG IDEA The form, arrangement, and structure of rocks are affected by 3 dimensional forces over time. Rocks - Igneous, sedimentary, metamorphic Forces - Pressure (think plate tectonics) Time - seconds to millions of years

  3. Before we can understand how rocks have deformed, we must understand these laws of stratigraphy... • Law of Original Horizontality • We must assume that all sedimentary rock layers are always laid down horizontally • Law of Superposition • Oldest layers below younger layers

  4. STESS vs STRAIN Stress is the FORCE acting on a body of rock Stress is the cause How to DEFORM a rock layer... • Strain is the response of a rock to stress. • Strain is what happens

  5. Compression Tension Shear Types of Stress 1. CONFINING 2. DIRECTED

  6. ...Directed Pressure Divergent Boundaries: Convergent Boundaries: Transform Fault Boundaries:

  7. Strain - the response to stress It generally involves a change shape or volume of the rock. Strain = Deformation • There are two types of deformation; • Elastic and Plastic

  8. Elastic Deformation Changes in the shape of a rock are reversible. Rock returns to it’s original shape Like “Elastic Rebound Theory”

  9. Plastic Deformation Materials that undergo continuous plastic deformation are said to be ductile. Ductile = Folding Changes in rock are permanent • Materials that fracture are said to be brittle • Brittle = Fracture = Faulting

  10. Factors Effecting Rock Behaviour Temperature Low temperature = brittle, High temperature = ductile Confining Pressure Low pressure = brittle, high confining pressure = ductile Composition Some minerals are more ductile, some more brittle (due to chemical bonds) Water Wet rocks = ductile, dry rocks = brittle

  11. Part 2

  12. Strike and Dip Strike = the compass direction along which the bed of rock trends Dip = the angle at which the rock ‘dips’ with respect to the horizontal

  13. Here's The Diagram From Your Lab Book! Notice The Corresponding Strike / Dip Symbols How Would You Say This Verbally?

  14. How Would You Verbally Describe These Symbols?

  15. How Dip Affects The "Apparent Thickness" of a Bed on the Surface: Notice How Green“Appears” Wider Than Yellow! Yellow & Green Beds Are The Same Thickness!! The More Gentle The Dip…The Wider The Bed Appears @ The Surface

  16. Notice How The Angle of The SURFACE affects the “Apparent Thickness” of the beds! Gentle Slope All Yellow Beds Are The Same Thickness!! Flat Surface Steep Slope

  17. Faults vs Joints (Fractures) Joints (or Fractures) are breaks in rocks along which there is no movement Faults are breaks in rocks along which there IS movement

  18. Joint: Occurs when there is a fracture but NO MOVEMENT along it!! Fracture... No Movement Shown In Layers = JOINT!

  19. Faults: A Fracture in Rock (Fault Plane) Along Which there Is Movement

  20. #1 Dip-Slip Faults Movement occurs in direction of DIP = UP & DOWN

  21. Hanging-Wall Moves DOWN! Fault Plane

  22. Hanging-Wall Moves UP! Fault Plane

  23. A Special Type of Reverse Fault Where the Fault Plane is almost Horizontal!

  24. Relative Ages of Rocks On Either Side of Dip-Slip Faults

  25. #2 Strike-Slip Faults Movement occurs in direction of STRIKE = SIDE to SIDE

  26. Look DirectlyAcross The Fault…Which Way Has the Block on the Other Side Moved?? LOOK!! LOOK!!

  27. LOOK!! LOOK!!

  28. Can You Identify The Following Four Faults?

  29. Horsts & Grabens: A series of Normal-Faults (Dip-Slip) Produces this Structure Occur over large scale areas. Associated with Divergent Plate Boundaries. Result is alternating... Up Lifted Blocks=HORSTS & Down Shifted Blocks=GRABENS

  30. **Remember: Horst=High Graben=Ground HORSTS Divergent Boundary Tension GRABENS

  31. Stress = force Strain = change shape Elastic Plastic Compression (convergent) Tension (divergent) Shear (transform) Ductile Brittle Rebounds back to original shape Folding Reverse fault Stretching Normal fault Folding Faulting Strike/slip fault

  32. Folds: Occur When Rock Bends & Deforms, but DOES NOT break!

  33. Fold Types:

  34. A Symmetrical Fold...

  35. An Asymmetrical Fold...

  36. Overturned Folds:

  37. Chevron Folds:

  38. Important Structures To Know: Monocline Anticline (Symmetrical, Asymmetrical, Plunging) Dome Syncline (Symmetrical, Asymmetrical, Plunging) Basin

  39. Simply a gently dipping bend!

  40. A Monocline Does NOT Have Two Fold Limbs!

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