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Crustal Deformation

Crustal Deformation. Review of Chapter 11. Isostasy. Balance in possible vertical movement of the plates Gravity bears down Heated aesthenosphere is buoyant (heat=expand=less dense=rise & cool=shrink=more dense=fall) Usually equal or nearly so

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Crustal Deformation

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  1. Crustal Deformation Review of Chapter 11

  2. Isostasy • Balance in possible vertical movement of the plates • Gravity bears down • Heated aesthenosphere is buoyant (heat=expand=less dense=rise & cool=shrink=more dense=fall) • Usually equal or nearly so • Imbalance upsets isostasy and results in a rise or fall of plates • Fall somewhere results in rise elsewhere and vice versa

  3. Isostasy explains the vertical distribution of Earth's crust. George Bedell Airy proposed that the density of the crust is everywhere the same and the thickness of crustal material varies. Higher mountains are compensated by deeper roots. This explains the high elevations of most major mountain chains, such as the Himalayas. G H Pratt hypothesized that the density of the crust varies, allowing the base of the crust to be the same everywhere. Sections of crust with high mountains, therefore, would be less dense than sections of crust where there are lowlands. This applies to instances where density varies, such as the difference between continental and oceanic crust.

  4. Deformation • All changes in volume or shape of rock • Reaction to stress • From confining pressure • Greater with depth • Shallow = brittle • Deeper = ductile

  5. Stress • A force that acts on rocks to change shape and/or volume • Compression – squeeze • Tension – pull apart • Shear – an an angle

  6. Strain • Change in shape or volume of rock as a result of stress • Stress must exceed resistance of rock which is variable • Rock composition • Temperature • Pressure • Amount/type/speed of stress

  7. Types of Deformation Brittle

  8. Types of Deformation Elastic (reversible)

  9. Types of Deformation Plastic (permanent)

  10. Rocks deform elastically when exposed to stresses. Experiments have proven that most rocks at depth deform plastically once their elastic limit is surpassed. Surface rocks also deform elastically, but turn brittle and fracture when they exceed their elastic limit.

  11. Strike and Dip • Strike is the direction in which strata lie • Dip is the angle of inclination from the surface at which strata lie

  12. The structure of a slope is called its strike and dip. Geologists draw these symbols on maps to define the way beds of rock are at angles, or the way they are dipping. Strike and dip are at right angles to each other. The arrow points in the direction of dip.The angle of dip is the angle (in degrees) off of horizontal that the bed is dipping. It is usually measured with a Brunton compass and is given by a number like 28 degrees or 45 degrees.

  13. Folds • Flat rock bent into undulations, usually by compression • Sides are called limbs and come off the axis • May have 1 or 2 limbs • Axis may not be horizontal = plunging

  14. Anatomy of a Fold

  15. Types of Folds-Anticline

  16. Types of Folds-Syncline

  17. Types of Folds-Monocline

  18. Types of Folds-Recumbent

  19. Types of Folds

  20. Fold sizes • Small in hand-held sizes • Large; seen only from the air • Ridge • Valley

  21. Domes and Basins • Gentle upwarping or downwarping of crustal rock produce domes and basins • Erosion of these structures results in an outcrop pattern that is roughly circular or elongated

  22. Domes and Basins

  23. sandstone Crystalline Metamorphic core limestone

  24. Faults • Rock breaks in response to severe stress (fracture or joint) • Movement along fracture (horizontal and/or vertical • Surface of movement is fault plane

  25. Dip-slip faults • Movement is vertical (on the dip) • Footwall below and hanging wall above • Normal, reverse, thrust

  26. Normal Fault

  27. Fault block valleyFault block ridge

  28. Fault scarp

  29. Strike-slip Fault

  30. Oblique-slip fault

  31. Joint to Fault

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