Folds....Faults
Anticlines and Synclines-Class Notes
Jump to: Types of Folds Faults Types of Faults
Guided Student Notes - (Microsoft Word Document)

Folds

When rocks are deformed plastically,
they are bent into folds:

Stress and Strain

Stress is the applied force (The pushing and pulling on the rock layers).

Strain is the bending & twisting that happens to the rock also known as deformation.

Stress can be compressional, tensional or shear.
Compressional stress pushes matter (rock layers) together.
Tensional stress pulls matter (rock and dirt layers) apart.
Shear stress is rotational.the stress is parallel to a face of the material,
All applied stresses cause rock (or any other solid) to deform (strain). Strain can be elastic or plastic.
  • Elastic strain disappears on the release of the stress. (like a rubber band)
  • Plastic strain is permanent on release of stress. (like clay)
If a material undergoes continuous plastic deformation, it is said to be ductile. If it fractures, it is said to be brittle.
There are Three Main Types of Folds:

Anticlines: This is when layers are folded upwards in what looks like an arch. The layers are symmetrical (look alike) to either side of its center.

Rock layers in anticlines dip away from the center axis.

The oldest rocks are exposed on the center axis.


Synclines: This is when the rock layers are folded downward.

The youngest layers of rock are exposed on the center axis.


Monocline: This is when the rock layer has a gently dipping bend in the horizontal rock layer.

Faults:
When rocks are deformed (broken) brittly, they are displaced along fractures called FAULTS.
Breaks in rock are put into two categories (groups).

Fractures:
When there is no movement along either side of the rock break.
Fault:
When either side of the rock break moves in opposite directions.

FAULT TERMINILOGY

    1) "Hanging Wall"- The surface of block that is ontop of the plane of the fault.

    2) "Footwall"- The surface or block that lies below the plane of the fault.

    3) "Strike"- The direction in which the fault runs.

    4) "Dip"- The dip direction is perpendicular to the strike direction.
Types of faults:
Dip-Slip Faults:
Movement along dip-slip faults is vertical; one side moves up and the other side moves down.

Imagine that you are in a tunnel that was dug through the fault (break) plane. The wall hanging over you is called the HANGING WALL. On the other side of the fault, the wall you are standing on is called the FOOT WALL.

The two types of Dip-Slip Faults are Normal Faults and Reverse Faults:

Normal Fault: The hanging wall has slipped down in comparison to the foot wall.

Gravity causes the hanging wall to slip down. Normal Faults are from layers being pulled apart.

Also known as a GRAVITY FAULT.

ReverseFault: The hanging wall has slipped up in comparison to the foot wall.

When layers are pushed together this is the kind of fault that occurs.

Also known as a THRUST FAULT.


Strike Slip Fault: Two layers of rock are shifted horizontally or parallel to the fault plane.

 

 

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