THE THEORY OF PLATE TECTONICS

1. THE THEORY OF PLATE TECTONICS

2. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size thru time • Activity at plate boundaries • Combines: • Continental drift • Sea-floor spreading • Paleomagnetism

4. INTRODUCTION • Ideas • Continental drift- Alfred Wegener • Sea-floor spreading • Paleomagnetism

5. Early Case for Continental Drift-Alfred Wegener (F.B. Taylor & H.H. Baker) • Continental coastlines fit together • 1620 Sir Francis Bacon: Africa and S. America • Rocks & structures indicated that continents joined • Pangea- supercontinent of the late Paleozoic • Separated into Laurasia & Gondwanaland • Fossil evidence- Glossopteris & Mesosaurus • Late Paleozoic glaciation • Skepticism about Continental Drift • Problem of driving mechanism

6. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks & structures • Paleoclimate

7. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks and structures • Paleoclimate

8. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks and structures • Paleoclimate

10. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size • Activity at plate boundaries • Combined: • Paleomagnetism • Sea-floor spreading

12. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size • Activity at plate boundaries • Combined: • Paleomagnetism • Sea-floor spreading

13. Paleomagnetism • Iron becomes magnetized below the Curie Point (600oC) • Magnetite and hematite aligns on existing magnetic field • Dip indicates old magnetic pole position • Apparent motion of north magnetic pole through time • Split in path • indicates continents split apart

15. Paleomagnetism • Magnetite aligns on existing magnetic field • Dip indicates old magnetic pole position • Apparent motion of north magnetic pole through time • Split in path • indicates continents split apart

17. SEA-FLOOR SPREADING • Magnetic anomalies • 1950’s detection of 10-50km wide strips symmetrical about ocean ridges • Vine and Matthews: magnetic reversals • Sea-floor moves away from mid-oceanic ridge • Plunges beneath continent or island arc- subduction Plate movement rate of 1 to 20 cm/year, 5 cm/yr average • Driving force • Mantle convection • Ridge Push- Slab Pull forces

19. SEA-FLOOR SPREADING • Explanations • Mid-oceanic ridge • Hot mantle rock beneath ridge • High heat flow • Basalt eruptions • Rift valley • Shallow-focus earthquakes

20. SEA-FLOOR SPREADING • Explanations • Oceanic trenches • Low heat flow • Negative gravity anomalies • Benioff zone earthquakes • Andesitic volcanism • Age of sea floor • Young age of sea floor rocks (oldest 160 my) • Implies youngest should be at ridges, oldest at trenches • Explains pattern of pelagic sediment

21. How do we know that plates move? • Marine magnetic anomalies • Vine-Matthews Hypothesis • Anomalies • Reversals • Normal and reverse polarity • Positive and negative anomalies • Measuring the rate of sea floor spreading • Predicting sea floor age

25. Plates and Plate Motion • Plate • Entirely sea floor or • continental and oceanic • Lithosphere • Crust & uppermost mantle • Thickness increases away from ridge • Asthenosphere • Low seismic velocity zone • behaves plastically

27. Plates and Plate Motion • Plate • Entirely sea floor or • continental and oceanic • Lithosphere • Crust & uppermost mantle • Thickness increases away from ridge • Asthenosphere • Low seismic velocity zone • behaves plastically

29. History of Continental Positions • Pangea split up 200 m.y. • Continents in motion for at least 2 billion years

31. How do we know that plates move? • Fracture Zones & Transform Faults • Pattern of earthquakes at ridges and fracture zones • Transform fault • Measuring plate motion directly • Use of satellites

33. Plates and Plate Motion • Interior of plates relatively inactive- Cratons • Activity along boundaries • Trenches (zone of subduction), melanges (complex of shear rock), accretionary prism (sedimentary and volcanic wedges separated by high angle faults) • e.g., earthquakes, volcanoes, young mountain belts • Plate tectonics a unifying theory for geology • Boundaries • Divergent • Convergent • Transform

34. DIVERGENT BOUNDARIES • During break up of a continent • Rifting, basaltic eruptions (Flood Basalts), uplifting • Extension- normal faults, rift valley (graben) forms • Shallow focus earthquakes • Continental crust separates • Fault blocks along edges • Oceanic crust created • Rock salt may develop in rift

37. East African Rift System • early stages of rifting • continental rifting

39. Red Sea Rift • Red Sea • Gulf of Eilat • Dead Sea • Linear Seas

42. TRANSFORM BOUNDARIES • Two plates slide past each other • Usually between mid-oceanic ridge segments • Can also connect ridge and trench • Or trench to trench • Origin of offset of ridges

44. CONVERGENT BOUNDARIES • Plates move toward each other • One plate overrides the other • Subduction zone

47. CONVERGENT BOUNDARIES • Continental-Continental convergence • Two continents approach each other and collide • Sea floor subducted on one side • Ocean becomes narrower and narrower • Continent wedged into subduction zone but not carried down it • Suture zone • Crust thickened • Two thrust belts • Mountain belt in interior of continent

48. Plate Size • 104 km2 to 108 km2 • New sea floor added to trailing edge of plate • e.g. North American plate growing at mid-Atlantic ridge • Oceanic plate might get smaller as continetal plate overrides it • e.g. Eastward moving Nazca plate subducted beneath westward moving South American plate

49. Intra-Plate Features • Thermal Plumes • Explains • Yellowstone volcanism • Hawaiian volcanism • Aseismic ridges