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Dynamic Earth

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  1. Dynamic Earth Class 17 7 March 2006

  2. Homework, Chapter 5 • Why do some rock layers fold and others break into faults when they are subjected to crustal forces? All other things being equal, experimentation indicates that rocks subjected to low confining pressures and low temperatures, such as exist near Earth's surface, will tend to break under deforming pressure. Rocks subjected to conditions that simulate those deep below the surface will bend or fold when a similar force is applied.

  3. Experimental Deformation of Marble Brittle Deformation (low confining pressure) Ductile Deformation (high confining pressure)

  4. Factors that affect deformation • Temperature • Pressure • Strain rate • Rock type The variation of these factors determines if a rock will fault or fold.

  5. Homework, Chapter 5 • If you found tilted beds in the field, how would you tell if they were part of an anticline or a syncline? One could not usually make a determination at the site but would need to look for other outcrops of the same rocks in the surrounding area. If the outcrops define a strip of rocks that becomes older as one progresses toward its center, it is an eroded anticline. If the rocks become younger toward the center of the feature, it is an eroded syncline.

  6. Geometry of Anticlines & Synclines

  7. Homework, Chapter 5 • Evidence for vertical crustal movements is often found in the geologic record. Give some examples of such evidence. Areas that are lifted above the surrounding terrain are targets for increased erosion. Thus, the extensive erosion of such areas as the Black Hills is evidence that the area has been lifted. The tilt in adjacent sedimentary beds that were revealed by erosion indicates uplift as well.

  8. Homework, Chapter 5 • It has been suggested that the Himalaya Mountains and the Tibetan Plateau were uplifted ~2,000 m about 10 million years ago. What caused this sudden uplift? Mountains are pushed up when the continental lithosphere is compressed – in the process, both the crustal and mantle parts of the lithosphere are thickened, creating a deep root beneath the mountains. The mantle portion of the root is denser than the underlying asthenosphere and eventually drops off, allowing the mountains to rise higher.

  9. The drooling lithosphere

  10. Exam Review • Second Exam – Thursday March 9th • Exam will be “fill in the blank” (15 x 2 points) and short answer questions (9 x 5 points); 75 points total • Questions will come from Lectures (including videos), reading and homework

  11. How do we know about the Earth’s Interior? • By studying Meteorites • Direct observation (rocks originating from depth) • Experiments at high pressure • By studying earthquake waves (Seismology)

  12. Structure of the Earth • Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. • We can use the behavior of seismic waves to tell us about the interior of the Earth.

  13. Most common types of earthquake waves: • P-waves and S-waves – Body waves • Primary waves travel the fastest in the crust and usually are the first waves to arrive • Secondary (or Shear) waves are slower and therefore take longer to arrive

  14. Changes in P- and S- wave Velocity Reveal Earth’s Internal Layers Velocities generally increase in each layer

  15. Types of Seismic Waves

  16. P-wave Shadow Zone

  17. S-wave Shadow Zone

  18. S wave shadow zone P wave shadow zone

  19. Earth’s CORE • Outer Core - Liquid Fe, ~2200 km thick, No S-waves transmitted -> S-& P-wave Shadow Zones • Inner Core - solid Fe (some Ni, Co, S, C), ~2500 km thick • How do we know? Meteorites, Seismology, Magnetic field

  20. Isostasy:Another key to Earth’s Interior • Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts • First noted during a survey of India

  21. The less dense crust “floats” on the less buoyant, denser mantle Mohorovicic Discontinuity (Moho)

  22. Mantle Tomography • Uses numerous seismic data • Uses small changes in speed of seismic waves • Faster wave motion may correspond to denser or colder regions • Slower wave motion may correspond to buoyant or warmer regions

  23. Basics of Tomography

  24. Hotspots • Areas with volcanic activity NOT explained by plate tectonics • Mantle beneath may be hot, wet, or chemically different • Commonly active for long time

  25. Hotspot tracks Flood basalts Oceanic plateaus

  26. Stress • The force that acts on a rock unit to change its shape and/or its volume • Causes strain or deformation • Stress • Compression • Tension • Shear

  27. Strain Any change in original shape or size of an object in response to stress acting on the object

  28. Ductile (Plastic) Deformation • Permanent change in shape or size that is not recovered when the stress is removed • Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion

  29. Brittle Deformation (Rupture) • Loss of cohesion of a body under the influence of deforming stress • Usually occurs along sub-planar surfaces that separate zones of coherent material

  30. Factors that affect deformation • Temperature • Pressure • Strain rate • Rock type The variation of these factors determines if a rock will fault or fold.

  31. Folds • Most common ductile response to stress on rocks in the earth's crust

  32. Symmetrical, Asymmetrical and Overturned Folds

  33. Faults • Occur when large stresses build up in the crust • Most common brittle response to stress on rocks in the earth's crust • Classified according to the kind of movement that has occurred along them • Know the types of faults especially if they are important in mountain building

  34. Stacked Sheets of Continental Crust Due to Convergence of Continental Plates

  35. Overlapping Thrust Faults,e.g. the Himalayas

  36. Tilted Normal Fault Blocks,e.g. Basin and Range Province

  37. Structures of continents 1) Continents are made and deformed by plate motion. 2) Continents are older than oceanic crust. 3) Lithosphere floats on a viscous layer below (isostasy).

  38. Age of the Continental Crust Blue areas mark continental crust beneath the ocean

  39. Stages in the formation of the Southern Appalachians Fig. 17.30

  40. India has collided with Asia

  41. Next Tuesday and Thursday Coastal Processes (March 14th) Coastal Management (March 16th) Chip Fletcher (No reading or homework)