Chapter 8

1. Chapter 8 The Theory of Plate Tectonics

2. Objectives Given information about Plate Tectonics you will be able to describe: • The history & development of the theory • Supporting evidence • Plate tectonic processes • Phenomena explained by tectonic theory

3. Guiding Questions • What is the evidence for Gondwanaland? • How does paleomagnetism demonstrate that continents have moved in the course of time? • How was paleomagnetism used to show that lithosphere forms at mid-ocean ridges and then migrates away? • 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?

5. Plate Tectonics • Tectonics • Movement of Earth’s crust • Plate tectonics • Movement of discrete segments of Earth’s crust in relation to one another

6. Continental Drift • Idea that continents move horizontally over Earth’s surface • Alfred Wegener • Alexander Logiedu Toit Wegener du Toit

7. Continental Drift • Early scientists recognized relationship between fossils on continents separated by sea • Proposed land bridges

8. Continental Drift • Glossopteris flora present only in southern hemisphere continents

9. Continental Drift • Early recognition that Mid-Atlantic Ridge was site of landmass rupture and Atlantic Ocean formation

10. Continental Drift • Wegener’s Evidence • Continents fit together • Pangaea • Geologic similarities • Floral and faunal similarities

11. Continental Drift • du Toit’s evidence • Expanded Wegener’s ideas • Proposed Pangaea split into Laurasia & Gondwanaland • Mesosaurus fossils • Found on Gondwana continents • Freshwater • Could not swim across Atlantic

12. Continental Drift • Geologic Similarities • Brazil and South Africa have nearly identical geologic sequences • Similar in Antarctica and India • Glacial sediments • Coal

13. Continental Drift • Glaciers • Orientation of glacial markings on all continents suggests they were linked • Must reconfigure continents back to Pangaea for this to make sense

14. Additional Evidence • Lystrosaurus • Heavyset herbivore • Pig-like tetrapod • Found in Antarctica • 1969

15. Paleomagnetism • 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 in this orientation at formation

16. Paleomagnetism • Apparent Polar Wander • First studies indicated poles had moved • Instead, plates had moved • North American and European paths met

17. 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 billion years of accumulation • Driven by convective cells

18. 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

19. 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

20. Rise of Plate Tectonics • Crust is destroyed at subduction zones

21. 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

22. Processes at Plate Boundaries • Normal faults • Extensional motion • Thrust faults • Compressional motion • Strike-slip faults • San Andreas fault

23. Processes at Plate Boundaries • Mid-Ocean Ridges (MOR) • Graben • Valley bounded by normal faults along which a central block has slipped downward • Pillow basalt

24. Processes at Plate Boundaries • Transform faults • Offset MORs • Enormous strike-slip faults • Seismically active

25. Processes at Plate Boundaries • Subduction • Descending slab undergoes dehydration which causes partial melting of the overlying mantle • Molten material is less dense, rises • Common around Pacific • Ring of Fire • Location of most of the world’s trenches

26. 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

27. 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

28. Plate Motion • Why plates move: • Drag on the base of the plate – “Slab Pull” • Elevation at ridge pushes plate ahead of it – “Ridge Push”, “Gravity Sliding” • Plate is pulled into subduction zone by preceding parent plate – “Slab Pull” • Broken plate segments create additional forces

29. Plate Motion • Eight large plates • Several small plates • Not all plates move at the same rate • Relative motion

30. 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

31. 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

32. Plate Motion • GPS • Global Positioning System • Earth-orbiting satellites identify motion • Transmitter on satellite • Ground-based receiver • Average rate • 5 cm/year

34. Summary • Continental drift –proposed by Wegener, added to by du Toit • Plate Tectonic theory developed by late 1960’s • Evidence = fossils, rock types, shapes of continents, paleomagnetic data, gps • Driven by convection – heat in earth’s interior • 3 plate motions – convergent, divergent, transform • Phenomena explained on earth = mountains, trenches, ore deposits, volcanoes, earthquakes, tsunamis, etc.