Structures introduction to earth science

Structural Geology/Rock Deformation Structural Geology is that part of geology that looks at shape, arrangement, interrelationship between rock units and the forces behind them Rocks ar

Geologic Structures

By Dr S A Isiorho

Structural Geology/Rock

Deformation

Structural Geology is that part of geology that looks

at shape, arrangement, interrelationship between rock units and the forces behind them

Rocks are subjected to great forces- particularly at plate

margins

 Stress- force that tends to change shape or size of a rock Stress could be

 Compressive- converging plate boundary

 Tensional- diverging plate boundary

 shearing- transform plate boundary

 Strain- rocks response to stress through change in shape

Types of Deformation (Strain)

 Plastic strain- body does not return to its

original size/shape i.e the rock is ductile

 Stress is applied gradually to deep warm rocks

 Elastic strain- body recovers to its original size/shape after stress is removed

 deformation not permanent (minor amount of

stress)

 Fracture (Brittle)- body breaks or cracks

 stress amount exceeds the yield point or elastic limit

Factors affecting Rock

deformation

 Heat

 leads to stretching of rocks at near earth’s surface without breaking

 at depth- plastic deformation

 Time

 Composition

 different minerals have different strength

General Structural description

 Structures gives clues to geologic history of the past

 Principles of horizontality- what is that?

 How are structures depicted in a geologic map?

 In describing folds, need to know the orientation of the rock in space- STRIKE and the angle at which rock is inclined to the horizontal- DIP

 Sketch of a fold

 Folds- rocks deform plastically (most occur at convergent plates)

 Syncline- trough-like

 Anticline- arch-like

 Types of folds: symmetrical, broad, open, isocline, overturn,recumbent, plunging

 Basin

 rock deformation that is bowl shaped.

 Domes

 rock deformation that is oval-shaped bulges

 A fracture is a break in rock (could be a joint or a

fault)

 Fault is when there is a relative movement along the break

 Fault Types

 Strike-Slip faults

 horizontal movement (transform plate boundary)

 Left-lateral & right-lateral

 Dip-Slip faults

 Normal (tensional stress)

 Reserve (compressional stress)

 Thrust (low angle reserve fault)

 Unconformities represent a break (missing time/period) in rock record

 Three types of unconformity

 Dis-conformity

 Angular unconformity

 Non-conformity

Mountain Creation

Most dramatic effect of stress

 Fold and Thrust Mountains

 Fault Block Mountains- tensional

 Unwarped Mountains- broad regional uplift