Plate Tectonics

1. Plate Tectonics

2. Continental drift: An idea before its time • Alfred Wegener • Proposed hypothesis in 1915 • Published The Origin of Continents and Oceans • Continental drift hypothesis • Supercontinent Pangaea began breaking apart about 200 million years ago

3. Pangaea approximately 200 million years ago

4. What would the world have been like as a supercontinent?A) Different climate - continental interiors hotter and drier (like Mongolia today)B) Lower species diversity due to connectivity of continents and fewer shorelinesC) Differences in marine environments - less shelf areaD) All of the above

5. What would the world have been like as a supercontinent?A) Different climate - continental interiors hotter and drier (like Mongolia today)B) Lower species diversity due to connectivity of continents and fewer shorelinesC) Differences in marine environments - less shelf areaD) All of the above

6. Watch Pangaea Breakup 200 Million Years Ago to present

7. Which continent moves the farthest during the breakup of Pangaea? North America South America Africa India E) Australia

8. Which continent moves the farthest during the breakup of Pangaea? North America South America Africa India E) Australia

9. Ocean opens between rigid plates But oceans are parts of plates and not independent Of continents…

10. Continental drift hypothesis • Continents "drifted" to present positions • Evidence used in support of continental drift hypothesis • Fit of continents • Fossil evidence • Rock type and mountain belts • Paleoclimatic evidence

11. The great debate • Objections to drift hypothesis • Inability to provide a mechanism capable of moving continents across globe • Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice • Geophysics suggests this was unlikely

12. Matching of mtn ranges on continentsOnce connected inone belt, now separated…

13. Paleoclimatic evidence for Continental Drift Dismemberingof an ancient ice sheet centeredover the southpole

14. Why is there no evidence for a dismembered ice sheet at the north pole at this time also?A) Continents were not located over the north pole at this timeB) An ice sheet onlyformed over the southpole and not the northC) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet D) Solar energy in the northern hemisphere was higher then due to sunspot activity

15. Why is there no evidence for a dismembered ice sheet at the north pole at this time also?A) Continents were not located over the north pole at this timeB) An ice sheet onlyformed over the southpole and not the northC) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet D) Solar energy in the northern hemisphere was higher then due to sunspot activity

16. The great debate • Continental drift and the scientific method • Wegner’s hypothesis was correct in principle, but contained incorrect details • For any scientific viewpoint to gain wide acceptance, supporting evidence required

17. Continental drift and paleomagnetism • Renewed interest in continental drift came from rock magnetism • Magnetized minerals in rocks • Show direction to Earth’s magnetic poles • Provide a means of determining their original latitude

19. What is the orientation of the Earth’s current magnetic field in Boulder, CO? North, inclined upward at about 45 deg North, inclined down at about 45 deg C) North, inclined down at about 90 deg D) North, inclined upward at about 90 deg E) None of the above

20. What is the orientation of the Earth’s current magnetic field in Boulder, CO? North, inclined upward at about 45 deg North, inclined down at about 45 deg C) North, inclined down at about 90 deg D) North, inclined upward at about 90 deg E) None of the above

21. Continental drift and paleomagnetism • Polar wandering • Apparent movement of magnetic poles in volcanic rocks indicates continents move • Shows Europe was closer to equator when coal-producing swamps existed

22. Continental drift and paleomagnetism • Polar wandering • Polar wandering curves for North America and Europe have similar paths but are separated by about 24 of longitude • Differences between paths reconciled if continents next to one another

23. Apparent polar-wandering paths for Eurasia and North America

24. The scientific revolution begins • During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor • Seafloor spreading hypothesiswas proposed by Harry Hess in the early 1960s

25. The scientific revolution begins • Geomagnetic reversals • Earth's magnetic field periodically reverses polarity – north magnetic pole becomes south magnetic pole, vice versa • Dates when polarity of Earth’s magnetism changed were determined from lava flows

27. Geomagnetic reversals • Geomagnetic reversals are recorded in the ocean crust • In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading

28. Paleomagnetic reversals recorded by basalt at mid-ocean ridges

29. Maps of Magnetic Stripes in Oceanic Crust

30. Why are seafloor magnetic anomalies symmetrically oriented about ridges? Rates of plate motions on the two divergent plates are the same for both plates Magma is intruded into the centers of spreading ridges and then pulled apart in the middle Convective cells in the mantle transfer heat and move rock straight up beneath ridges and then move outward D) All of the above

31. Why are seafloor magnetic anomalies symmetrically oriented about ridges? Rates of plate motions on the two divergent plates are the same for both plates Magma is intruded into the centers of spreading ridges and then pulled apart in the middle Convective cells in the mantle transfer heat and move rock straight up beneath ridges and then move outward D) All of the above

32. Structure of Juan de Fuca Plate

33. Age of Oceanic Crust

34. Why is there no oceanic crust older that 180Ma? A) Plate tectonics didn’t occur prior to this B) Spreading ridges ran much more slowly in the past C) Older oceanic crust has all been subducted D) Older oceanic crust probably exists somewhere, we just haven’t found it under the ocean yet.

35. Why is there no oceanic crust older that 180Ma? A) Plate tectonics didn’t occur prior to this B) Spreading ridges ran much more slowly in the past C) Older oceanic crust has all been subducted D) Older oceanic crust probably exists somewhere, we just haven’t found it under the ocean yet.

36. Geomagnetic reversal • Paleomagnetism was the most convincing evidence to support concepts of continental drift and seafloor spreading

37. Plate tectonics: The new paradigm • More encompassing theory than continental drift • Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading

38. Plate tectonics: The new paradigm • Earth’s major plates • Associated with Earth's strong, rigid outer layer • Known as the lithosphere • Consists of uppermost mantle and overlying crust • Overlies a weaker region in the mantle called the asthenosphere

39. Basal tractions drive plate motions

40. Earth’s major plates • Seven major lithospheric plates • Plates are in motion and change in shape and size • Largest plate is the Pacific plate • Several plates include an entire continent plus a large area of seafloor

41. Why did Pangaea break up? A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup • The Earth was hit by a giant asteroid at the time, pushing the plates apart • Stresses caused by orbital forcing • Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left

42. Why did Pangaea break up? A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup • The Earth was hit by a giant asteroid at the time, pushing the plates apart • Stresses caused by orbital forcing • Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left

43. Earth’s Tectonic Plates

44. Earth’s major plates • Plates move relative to each other at a very slow but continuous rate • Average about 5 centimeters (2 inches) per year • Cooler, denser slabs of oceanic lithosphere descend into the mantle • Motion defined by rotation around a pole

45. Plate boundaries • Interactions among individual plates occur along their boundaries • Types of plate boundaries • Divergent plate boundaries • Convergent plate boundaries • Transform fault boundaries

46. Types of Plate Margins

47. Plate boundaries • Each plate bounded by combination of all three boundary types • New plate boundaries created in response to changes in forces acting on rigid slabs

48. Divergent plate boundaries • Most are located along the crests of oceanic ridges • Oceanic ridges and seafloor spreading • seafloor is elevated forming oceanic ridges

49. Oceanic ridges and seafloor spreading • Seafloor spreading occurs along the oceanic ridge system • Spreading rates and ridge topography • Ridge systems exhibit topographic differences • Topographic differences are controlled by spreading rates (see map of age of oceanic crust for width of ridges relative to their age)

50. Which ridge spreads the fastest? Mid Atlantic ridge B) East Pacific Rise C) Australia-Antarctic Ridge D) Arctic Ridge