Folds and FaultsFolds and Faults

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Stress and StrainStress 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.

3 Types of Stress3 Types of Stress• 1. Compressional - pushes matter (rock

layers) together.

• 2. Tensional - pulls matter (rock and dirt layers) apart.

• 3. Shear is rotational, the stress is parallel to a face of the material,

Types of Strain:Types of Strain:Elastic or plastic?Elastic or plastic?

• 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)

DuctileDuctile or Brittle or Brittle

• If a material undergoes continuous plastic deformation, it is said to be ductile.

• If it fractures, it is said to be brittle.

The 3 parts of a fold: The 3 parts of a fold:

• fold axis

• axial plane

• limb

The 4 Fold shapesThe 4 Fold shapes

• 1. symmetrical • 2. asymmetrical • 3. overturned - tipped in one direction so

that one of the limbs is overturned • 4. recumbent - lying on its side

There are three types of folds:There are three types of folds:

• 1. Anticline

• 2. Syncline

• 3. Monocline

1. Anticline1. Anticline

1. Anticline 1. Anticline (continued)(continued)

• 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.

2. Synclines2. Synclines

2. Syncline 2. Syncline (continued)(continued)

• This is when the rock layers are folded downward.

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

• Which is which?

MonoclineMonocline

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

3. Overturned3. Overturned

•

• Overturned folds in the Baltimore GneissBaltimore County, Maryland

FaultsFaults• When rocks are deformed (broken)

brittly, they are displaced along fractures called FAULTS.

Breaks in rock are put Breaks in rock are put into two categoriesinto two categories

• Fractures/joint: 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 TERMINILOGYFAULT TERMINILOGY• 1) "Hanging Wall"- The surface of block

that is on top 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 faultsTypes of faults• 1. Dip-Slip Faults:

Movement along dip-slip faults is vertical; one side moves up and the other side moves down.

• The two types of Dip-Slip Faults are

A) Normal Faults

B) 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.

A. Normal FaultA. Normal Fault

• Reverse Fault: 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.

B Reverse FaultB Reverse Fault

Strike Slip FaultStrike Slip Fault

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

• Example – The San Andreas Fault

San Andreas Fault in California

• What are the three types of stresses?• What is the elastic limit?• What is a geological cross section?• What is metamorphic rock?• What is the hinge line?• What are: Plunging folds, open folds, isoclinal

folds, overturned folds, recumbent folds?• Look at the pictures or diagrams – they help to

clarify!!!