Chapter 8 The Theory of Plate Tectonics

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

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

15. Continuity of Geologic Structures

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

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

18. 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

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

20. 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

22. 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

23. 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

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

25. 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

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

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

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

29. 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

30. 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

31. 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

32. 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

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

34. 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

35. 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

36. 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

37. Effect on Global Climate