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Objectives

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  1. Objectives • Identify the major types of rock deformation • Understand the movements associated with fault deformation

  2. Rock Deformation • When rocks are subjected to forces greater than their own strength causes them to deform • Types of Deformation • Folding • Faulting • Joints

  3. Elastic Deformation • When stress is first applied, rocks respond by deforming elastically • Reversal changes( changes back to original position) • Like a rubber band – the rock will return to nearly its original size and shape when the stress is removed

  4. Plastic Deformation • Once elastic limit is reached, rock deforms plastically or they fracture • Results in permanent changes to the size and shape of the rock • High temperatures and pressures cause deformation

  5. Simulations • Scientists have simulated how rocks respond to stress • Most rocks fracture when their elastic limit is reached • One thing that scientists cannot simulate is geologic time

  6. Geologic Time • Geologic time can cause rocks to deform rather than fracture rapidly like in the laboratory

  7. Folds • A bent rock layer or series of layers that were originally horizontal and subsequently deformed • Often occurs with flat-lying sedimentary and volcanic rocks • Types of folds • Anticlines • Synclines • Domes • Basins

  8. Anticline • Common type of fold formed from compression • The upfolding or arching of rock layers • Displayed when highways have been cut through deformed strata http://www.members.aol.com/rhaberlin/images/mbfold.gif http://www.exw6sxq.com/sparky/images/anticline_and_syncline.jpg

  9. Syncline • The downfolding or troughs of rocks http://www.tulane.edu/~sanelson/images/syncline.gif

  10. Domes • Vertical displacement • The upwarping that produces a circular or elongated structure • Erosions strip way upwarped sedimentary beds, exposing older rock at center http://en.wikipedia.org/wiki/Black_hills

  11. Basins • Downwarped structures • Contain sedimentary beds sloping at low angles • Younger rocks are at center, oldest rocks are at the flanks

  12. Faults • Fractures in the crust along which movement occurs • Fault zones – formed from large faults consisting of multiple interconnecting fault surfaces

  13. Dip-Slip Faults • Primarily vertical movement • Movement is along the inclination (dip) of the fault plane • Movement can be up or down fault plane • Two types • Normal faults • Reverse faults

  14. Normal faults • Hanging wall moves downward relative to the footwall http://www.tulane.edu/~sanelson/geol111/deform.htm

  15. Tensional Forces • Normal faults indicate tensional stress pulling the crust apart • Graben – a valley formed by the downward displacement of a fault-bounded rock • Horst – an elongate, uplifted block of crust bounded by faults

  16. Tensional forces http://www.tulane.edu/~sanelson/geol111/deform.htm

  17. Reverse Faults • Hanging wall moves upward relative to footwall http://www.tulane.edu/~sanelson/geol111/deform.htm

  18. Reverse Faults • Thrust faults – reverse faults with dips less than 45 degrees • Result from strong compressional stresses http://www.tulane.edu/~sanelson/geol111/deform.htm

  19. Compressional forces • When faultings are displaced toward one another • Primarily occurs at convergent boundaries • Generally produce folds as well as faults

  20. Strike-Slip Faults • Faults where the dominant displacement is along a strike, or trend • Associated with transform plate boundaries • Oblique-slip faults = Faults with horizontal and vertical movement http://www.tulane.edu/~sanelson/geol111/deform.htm

  21. Joints • Fractures in rock structures where no movement occurs • Two types • Columnar joints • Sheeting

  22. Columnar Joints • Form when igneous rocks cool and develop shrinkage fractures producing elongated, pillar-like columns

  23. Sheeting • Produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies • Results from gradual expansion that occurs as erosion removes the pressure of the overlying load

  24. Joints • Most produced when rocks are deformed by stresses associated with crustal movements during mountain building • Can also develop in response to relatively subtle and barely perceptible upwarping and downwarping of the crust

  25. Joints • Most rocks are broken by two or three sets of intersecting joints that slide the rock in numerous regularly shaped blocks • Strong influence on other geologic process • Chemical weathering • Groundwater movement

  26. What did you learn? • What is the difference between elastic and plastic deformation? • Describe how folds and faults differ in their formation. • What type of stress causes normal, reverse, and strike-slip faults? • Briefly describe how grabens and horsts are formed? ( * Identify type of fault and movement up, down) • What direction do normal faults move? Reverse faults? Strike-slip faults? (Up/Down/Lateral)

  27. In-class Assignment/Homework • Chapter Review Questions page 278 #1, 2, 3, 4, 5, 6, 7 Draw and Label a Normal, Reverse, and Thrust Fault. * Make sure you label the Footwall and Hanging Wall. Due Thursday when you walk into class