1 / 41

Chapter 8 The Theory of Plate Tectonics

Chapter 8 The Theory of Plate Tectonics. Guiding Questions. What is the evidence for Gondwanaland? How does paleomagnetism demonstrate that continents have moved? How was paleomagnetism used to show that lithosphere forms at and migrates away from mid-ocean ridges?

peri
Download Presentation

Chapter 8 The Theory of Plate Tectonics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 8 The Theory of Plate Tectonics

  2. Guiding Questions • What is the evidence for Gondwanaland? • How does paleomagnetism demonstrate that continents have moved? • How was paleomagnetism used to show that lithosphere forms at and migrates away from mid-ocean ridges? • How did features of the seafloor engender the concept of plate tectonics? • Why do faulting and volcanism occur along oceanic plate margins? • What causes lithospheric plates to move? • How can geologists measure rates of plate movement?

  3. Plate Tectonics • tectonics • movement of Earth’s crust • plate tectonics • movement of discrete segments of Earth’s cruise in relation to one another

  4. hypothesis theory law

  5. hypothesis A tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation. theory • Common usage-a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact. • Scientific usage-a more or less verified or established explanation accounting for known facts or phenomena: the theory of relativity. law paradigm-the generally accepted perspective of a particular discipline at a given time 

  6. Theory-a Skinnerism The best explanation we have (so far) that explains more of the facts than any other idea, and for which there is no serious evidence flaw. Plate tectonics explains volcanoe and earthquake belts, mountain ranges, etc.

  7. Continental Drift • the idea that continents move horizontally over Earth’s surface • scoffed at in the 40s • revolutionary in the 60s • proposed and defended by • Alfred Wegener • Alexander DuToit Puzzle-like fit of continents using 500 fathom coastline depth

  8. Evidence for Continental Drift Early scientists recognized the relationship between fossils on continents separated by sea Glossopteris flora present only in southern hemisphere continents • proposed land bridges • consistent with the distribution pattern • so is the idea of united continents

  9. Evidence for Continental Drift

  10. Evidence for Continental Drift Early recognition that Mid-Atlantic Ridge was site of landmass rupture and Atlantic Ocean formation

  11. Evidence for Continental Drift • Wegener’s Evidence • continents fit together • Pangaea • geologic similarities • floral and faunal similarities Wegener’s reconstruction of ancient world

  12. Evidence for Continental Drift • DuToit’s evidence • expanded Wegener’s ideas • Mesosaurus fossils • found on Gondwanaland (southern hemisphere) continents • a fresh/brackish water species • could not swim across Atlantic

  13. Evidence for Continental Drift • geologic similarities • Brazil and South Africa have nearly identical geologic sequences • similar in Antarctica and India • Glacial sediments • Coal

  14. Continuity of Geologic Structures

  15. Evidence for Continental Drift • glaciers • orientation of glacial markings on all continents suggest they were linked

  16. Additional Evidence • Lystrosaurus • heavyset herbivore • found in Africa and SE Asia • recently (1969) found in Antarctica

  17. Paleomagnetism-the smoking gun • magnetization of ancient rocks at the time of their formation • declination • angle that a compass needle makes with the line running to the geographic north pole • rocks lock this orientation in at formation

  18. Paleomagnetism • apparent polar wandering • first studies indicated poles had moved • instead, plates had moved • North American and European paths met

  19. Rise of Plate Tectonics • Harry Hess, 1962 • Geopoetry • continents didn’t plow through seafloor • entire crust moved • Crust must be created and destroyed • sedimentary cover too thin for four b.y. of accumulation • driven by convective cells

  20. Rise of Plate Tectonics • ridges • site of crustal formation • hot rising mantle material rises to top of lithosphere, cools • ocean crust is formed • bends away from center to form ridge

  21. Rise of Plate Tectonics • Guyots • had identified flat-topped seamounts in Pacific • realized they were volcanoes that had been eroded by waves at sea level • postulated as crust moved away from ridge it cools and sinks

  22. Rise of Plate Tectonics • crust is destroyed at subduction zones

  23. Test of Plate Tectonics • Paleomagnetism • Vine and Matthews, 1963 • measured magnetization of rocks across the Indian Ocean central ridge • found normal and reversed “stripes” • mirror image

  24. Processes at Plate Boundaries • Normal faults-extensional motion • Thrust faults-compressional motion • Strike-slip faults-e.g., San Andreas fault

  25. Processes at Plate Boundaries • Mid-Ocean Ridges (MOR) • graben • valley bounded by normal faults along which a central block has slipped downward • pillow basalt

  26. Processes at Plate Boundaries • transform faults • offset MOR’s • enormous strike-slip faults • seismically active

  27. Processes at Plate Boundaries • Subduction • descending slab undergoes partial melting • molten material is less dense, rises • common around Pacific Rim • Ring of Fire • location of most of the world’s trenches

  28. Processes at Plate Boundaries • associated with: • volcanoes-island arc • deep-focus earthquakes >300 km depth • forearc basin • zone of intensely deformed rocks in belt between island arc and deep-sea trench

  29. Processes at Plate Boundaries • forearc Basin • deep-ocean dark muds and graywackes with ocean crust mixed in • mélange • chaotic deformed mixture of rocks • accretionary Wedge • body of rock that accumulates as plate is subducted

  30. Plate Motion • Why plates move: • drag on the base of the plate • elevation at ridge pushes plate ahead of it • plate is pulled into subduction zone by preceding parent plate • broken plate segments create additional forces

  31. Plate Motion • eight large plates • several small plates • not all plates move at the same rate • relative motion

  32. Plate Motion • absolute plate motion • establish using fixed point • hot spot • small geographic area where heating and igneous activity occur within the crust • Yellowstone • Hawaii

  33. Plate Motion • Hawaiian hot spot • thermal plume creates volcano • plate moves away from plume • stranded volcano cools, leaves a chain • chain indicates direction and rate

  34. Plate Motion • GPS-Global Positioning System • Earth-orbiting satellites identify motion • transmitter on satellite with ground-based receiver • Average rate of movement: 5 cm/year

  35. Effect on Global Climate

More Related