Plate Tectonics: The Unifying Theory

1. Plate Tectonics:The Unifying Theory Peter W. Sloss, NOAA-NESDIS-NGDC

2. Fundamental concept of geoscience Integrates from many branches First suggested based on geology and paleontology Fully embraced after evidence from geophysics Plate Tectonics

3. • Distribution of earthquakes and volcanoes • Relationship of age and height of mountain belts • Age distribution of oceanic crust • Magnetic information in rocks What tectonics theory explains

4. Earth’s upper mantle (lithosphere) is broken into rigid plates which move with respect to each other Plates rest on and move in the asthenosphere There are 3 kinds of plate margins that are marked by earthquakes and volcanoes Divergent Convergent Transform The movement is driven by uneven distribution of heat within Earth and the mechanism that drives plate movement is not well known What is Plate Tectonics Theory?

5. Mosaic of Earth’s Plates Peter W. Sloss, NOAA-NESDIS-NGDC

6. Theory evolved from 2 concepts: • Continental Drift - by Alfred Wegener • Fit of continents - Pangaea • Ancient climatic evidence - glacial deposits • Fossil evidence - Glossopteris & Mesosaurus • Matched mineral zones & mountain chains • (Problem - no acceptable method of motion)

7. “Fit” of the Continents

8. Anomalous Distribution of Fossils(Mesosaurus)

9. ANDPALEOMAGNETIC EVIDENCE • Polar Wandering - either • the North Pole or the continents had moved • both Europe and North America had apparently moved as a single continent for several hundred million years

10. Apparent polar wandering curves

11. Seafloor Spreading • “Geopoetry” of Harry Hess & Robert Dietz • New seafloor forms by upwelling at the center of MOR and moves laterally • Older crust is destroyed in the subduction zones at the trenches • Seafloor is younger than 200 MY • Solved Continental Drift problem

12. Modern Proof of Seafloor Spreading • Deep Sea Drilling Project - sampled sea floor sediment & crust • Age & thickness increased with distance from MOR • Ages were symmetrical about MOR • Magnetic Anomalies - found weak & strong signal • Alternating normal & reversed polarization • Stripes || to & symmetrical about MOR

14. Magnetic Anomalies

15. Ocean crust records magnetic reversals

16. Age of Seafloor Crust Fig. 20.11 R. Dietmar Muller, 1997

17. More Proofs • Hot Spots - deep, long-lived, stationary mantle magma sources • Expressed at the surface by: • linear chain of volcanoes • aged with distance from hot spot • Over 100 identified • Used as fixed points against which plate motion is measured

19. Hawaiian-Emperor chain

20. Long-lived global hot spots

21. PUTTING IT ALL TOGETHER

22. Group of rocks all moving in the same direction Can have both oceanic and continental crust or just one kind Plate interior are relatively quiet Most activity is located at plate margins I. Plates

23. • divergent: mid-ocean ridges • convergent: collision zones volcanic arcs • strike-slip: San Andreas Fault Alpine Fault, N.Z. II. Types of plate boundaries

24. Usually begin within continents as hot spot burns thru crust and eventually grows to become ocean basin Two kinds of divergent margins Mid Ocean Ridges (MOR) Continental Rift Valleys A. Divergent plate boundaries

25. • Central rift valley (width is inversely proportional to the rate of spreading) • Region of high heat flow Shallow-focus earthquakes • Almost exclusively basalt volcanism Features of Mid Ocean Ridges

26. Earthquakes Associated with Divergent Margins

27. Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge Peter W. Sloss, NOAA-NESDIS-NGDC

28. Rifting and Seafloor Spreading

29. • East Africa, Rio Grande rift • Beginning of ocean formation (may not get that far) • Rifting begins at a triple junction (two spreading centers get together to form ocean basin, one left behind). • Rock types: basalt and sandstone Features of Continental Rifts

30. Hot Spot induced rifting • Hot spot burns thru crust -> 3 branched rift • Divergence begins • 2 branches are active • 1 branch is not active • Linear sea develops

32. Inception of Rifting Along theEast African Rift System Peter W. Sloss, NOAA-NESDIS-NGDC

33. Inception of Rifting Within a Continent

34. Nile Delta Gulf of ‘Aqaba Gulf of Suez Red Sea Earth Satellite Corp.

35. The Gulf of California Formed by Rifting of Baja California from Mainland Mexico Worldsat International/Photo Researchers

36. New crust created at MOR—old crust destroyed (recycled) at subduction zones 2 kinds: subduction & collision Relative important densities: continental crust ≈ 2.8 g/cm3 oceanic crust ≈ 3.2 g/cm3 asthenosphere ≈ 3.3 g/cm3 B. Convergent boundaries

37. Three types: ocean–ocean Philippines ocean–continent Andes continent–continent Himalaya Convergent boundaries

38. Island arcs: Chain of volcanic islands • Highly seismic tectonic belt of shallow to deep earthquakes • High heat flow arc of active andesitic volcanoes • Bordered by a submarine trench Ocean–Ocean

39. Ocean–Ocean Subduction Zone

40. Continental arcs: • Magmatic belt of active volcanoes (andesite to rhyolite) • Often accompanied by compression of upper crust which builds mountains Bordered by a submarine trench Ocean–Continent

41. Ocean-ContinentSubduction Zone

42. Continent–continent boundaries, convergence is accommodated by • Folding (shortening and thickening) • Strike-slip faulting • Underthrusting (intracontinental subduction) Continent–Continent

43. Continent–Continent Convergent Boundary

44. Continent-Continent Collision

45. • Product of the collision between India and Asia. • Collision began about 45 M yr. ago, continues today. • Before collision, southern Asia looked something like the Andes do today. Himalayas and Tibetan Plateau

46. Stages in the collision of India with Asia

47. C. Transform Boundary Offsetting Spreading Centers

48. Mostly obtained from magnetic anomalies on seafloor Slow spreading: 3 cm/year Fast spreading: 10 cm/year Very fast spreading: 17cm/year III. Rates of plate motion

49. Relative Velocity and Direction of Plate Movement Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

50. Each plate tectonic environment produces a distinctive group of rocks. By studying the rock record of an area, we can understand the tectonic history of the region. IV. Rock assemblages and plate tectonics