Structures introduction to earth science

Geologic Structures

By Dr. S. A. Isiorho

Copyright © by Isiorho

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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
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Compressive- converging plate boundary
Tensional- diverging plate boundary
shearing- transform plate boundary

Strain- rocks response to stress through change in shape
and volume

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Types of Deformation (Strain)


Plastic strain- body does not return to its
original size/shape i.e. the rock is ductile
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Elastic strain- body recovers to its original
size/shape after stress is removed
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Stress is applied gradually to deep warm rocks

deformation not permanent (minor amount of
stress)

Fracture (Brittle)- body breaks or cracks


stress amount exceeds the yield point or elastic
limit
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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

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General Structural description
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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

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Folds
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Folds- rocks deform plastically (most occur at
convergent plates)
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Basin
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Syncline- trough-like
Anticline- arch-like
Types of folds: symmetrical, broad, open, isocline,
overturn,recumbent, plunging
rock deformation that is bowl shaped.

Domes


rock deformation that is oval-shaped bulges

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Faults
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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
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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)
 Oblique (combination of strike-slip & dip-slip)

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Unconformities
 Unconformities

represent a break
(missing time/period) in rock record
 Three types of unconformity
 Dis-conformity
 Angular

unconformity
 Non-conformity

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Mountain Creation
Most dramatic effect of stress
 Fold

and Thrust Mountains
 Fault Block Mountains- tensional
 Unwarped Mountains- broad regional
uplift

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