Structures introduction to earth science

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Geologic Structures By Dr S A Isiorho Copyright © by 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 and volume Copyright © by Isior Types of Deformation (Strain)  Plastic strain- body does not return to its original size/shape i.e the rock is ductile   Elastic strain- body recovers to its original size/shape after stress is removed   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 Copyright © by Isior 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 Copyright © by Isior 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 Copyright © by Isior Folds  Folds- rocks deform plastically (most occur at convergent plates)     Basin   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 Copyright © by Isior Faults    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)  Oblique (combination of strike-slip & dip-slip) Copyright © by Isior Unconformities  Unconformities represent a break (missing time/period) in rock record  Three types of unconformity  Dis-conformity  Angular unconformity  Non-conformity Copyright © by Isior Mountain Creation Most dramatic effect of stress  Fold and Thrust Mountains  Fault Block Mountains- tensional  Unwarped Mountains- broad regional uplift Copyright © by Isior
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