Plate Tectonics

1. Plate Tectonics GLY 2010 – Summer 2012 - Lecture 15 1

2. Subduction Zones • Plots of earthquake foci over time delineate the position of subducting plates • The plate which is subducted is always denser than the plate which remains on the surface • Earthquakes associated with convergent plate margins where one plate is subducting are of shallow, intermediate and deep focus • Deep focus earthquakes are not known exceptat subduction zones 2

3. Shallow Subduction Angle • Plates near the spreading center will be much warmer • They will be only slightly denser than surface plate, and the subduction angle will be shallow 3

4. Steep Subduction Angle • Plates far from the spreading center will be relatively cold, and therefore dense • They will subduct at a steep angle 4

5. Breakup of Subducting Slabs • Slabs break off, and are no longer attached to the subducting plate • Lack of attachment stops their movement, and they no longer generate earthquakes 5

6. Accumulation of Slabs • Broken slabs are now known to accumulate within the mantle, stacking up like pancakes 6

7. Oceanic Trenches • Subducting plate drags part of the surface with it • Creates large oceanic trenches, which also serve to mark the top of the subduction zones 7

8. Continental Volcanic Arcs • Plates subducted under continents partially melt, creating long chains of stratovolcanoes on the continents • Cascades and Andes are examples 8

9. Volcanic Island Arc • Plates subducted under oceanic plates create chains of oceanic islands • Japan, the Philippines, and Indonesia are examples 9

10. Plate Motions • Two plates move relative to each other • Convergent - Plates move toward each other, often a head-on collision • Divergent - Plates move away from each other • Sideways (transform) - Plates move past each other along transform faults 10

11. Convergent Movement

12. Divergent Movement

13. Transform Motion

14. Plate Types • At any given point, a plate is either oceanic or continental • Interactions between plates are thus: • Ocean-ocean (O-O) • Ocean-continent (O-C) • Continent-continent (C-C) 14

15. Plate Interactions 15

16. Summary of Plate Interactions

17. Hydrothermal Vents • Spreading centers are marked by vents which spew hydrothermal fluids as hot as 650C • Fluids contain dissolved metals which precipitate when they hit cold ocean water, encrusting basalt - vents are called “black smokers” for this reason 17

18. Hydrothermal Vent – Black Smoker • Black Smoker along the Juan de Fuca Ridge • Temperature: 648ºC • Courtesy PBS Station WGBH Click to play video 18

19. Alvin • With room enough for only one pilot, a cameraman, and a massive IMAX camera, Alvin dove to depths of 12,000 feet (3,700 meters) during an ambitious effort to film hydrothermal vents on the Mid-Atlantic Ridge Alvin photographed from research vessel Atlantis 19

20. Hydrothermal Vent – White Smoker • “White smokers” release water that is cooler than their cousins’ and often contains compounds of barium, calcium, and silicon, which are white Hydrothermal vents are believed to play an important role in the ocean’s temperature, chemistry, and circulation patterns 20

21. Vent Biology - Tubeworms • A tube worm colony near hydrothermal vents Click to play video 21

22. Vent Biology – Vent Crabs • The vent crab is typically found among dense clusters of tubeworms at an average depth of 1.7 miles and can tolerate a temperature gradient that ranges from 77°F in the tubeworm clumps, to 36°F, which is the temperature of the water surrounding the vent sites Click to play video 22

23. Vents of the World • Some known vent localities on the ocean floor – note association with mid-ocean ridges 23

24. Earth’s Magnetic Field • Earth has a strong magnetic field • It is dipolar, with the poles being called north and south 24

25. Earth’s Magnetic Polarity • Present north magnetic pole is located near the south geographic pole • South magnetic pole is located near the north geographic pole 25

26. Rock Magnetism • Rocks often become magnetized because magnetic mineral grains (usually magnetite) are aligned • Rock’s magnetic field is fixed at the time magma cools below the Curie point for igneous rocks, or at the time of lithification for sedimentary rocks • Magnetism of older rocks is called “paleomagnetism” 26

27. Magnetic Inclination and Declination

28. Magnetic Stripes • In the early 1960’s oceanographic research uncovered a curious phenomenon, called magnetic stripes • Measurements of the earth’s magnetic field show small variations from place to place 28

29. Magnetic Anomalies • Magnetic Anomaly = Average regional magnetic field of the earth - magnetic field at a point • Plotting magnetic anomalies lead to a curious pattern of “stripes”, first seen in the Atlantic, later in the Pacific 29

30. Explanation of Stripes • The first to propose an explanation that was scientifically accepted was L. Wilson Morley, a Canadian geoscientist • Morley sent his paper to the British journal Nature in January, 1963 • Nature rejected the article, and it was not published until more than a year later in another journal 30

31. Morley’s Idea • Sea-floor spreading - new magma emerging at a MOR and hardening into rock, which then spread away from the ridge with time (Hess-Dietz hypothesis) • Polarity reversals - the North and South magnetic poles changing position suddenly 31

32. Polarity Reversal

33. Vine-Matthews • Working completely independently of Morley, and unaware of his idea, D.H. Matthews and his graduate student, Frederick Vine, formulated an explanation often called the Vine-Mathews theory Frederick J. Vine 33

34. Vine-Matthews Publication • Published “Magnetic Anomalies Over Ocean Ridges (in Nature!) in September, 1963 • Matthews was a Research Fellow of King's College, Cambridge 34

35. Vine-Matthews-Morley Theory • If we assume sea-floor spreading is occurring, the magnetic field of the rock is fixed, in alignment with the earth’s field, at the time the rock cools • The measured field above such rocks equals the earth’s field plus the rock’s field (because they are aligned) 35

36. After Polarity Reversal • If the earth’s field reverses, the field of the previously created rock will be aligned against the earth’s field, slightly decreasing it • A second reversal will again align the field of the rock and earth 36

37. Magnetic Stripes • As magma rises, it hardens and its magnetic field matches the present field of the earth - after a polarity reversal, it will be aligned against the earth’s field 37

38. Sea Floor Magnetism Animation

39. Anomalies • When the rock’s field and the earth’s field are aligned, the field at a point will be greater than the regional average - a positive anomaly • When the rock’s field and earth’s field are in opposite directions, the field at a point will be less than the regional average - a negative anomaly 39

40. Anomaly Diagram • Anomalies are really just regions of high and low magnetic intensity 40

41. Magnetic Stripe Theory • By postulating a series of irregular (in time) polarity reversals, with continuous eruption of magma at the spreading center, Morley and later Vine-Mathews offered an explanation for magnetic stripes • After several years of discussion, this explanation was accepted by most earth scientists in a series of conferences in 1967-68 41

42. Magnetic Reversal Record • Recent magnetic field data from lava samples of known age 42

43. Rate of Plate Movement • Once Plate Tectonics was accepted, it became necessary to determine how fast plates move • Three methods have been used • Hot spots • Satellite Tracking • Magnetic reversal 43

44. Hot Spots • Hot spots, which generate magma in the asthenosphere, below the moving lithospheric plates, may be used as a reference since they are effectively stationary relative to lithospheric plate 44

45. Hot Spots “Movement” • Hot spots produce volcanoes, like the Hawaiian Islands or many seamounts or guyots (mountains that made it above sea-level, then were flattened by wave erosion) 45

46. Hot Spot Diagram • Diagram showing creation of several Hawaiian Islands • Age of islands should be progressively older as they move away, and this is observed 46

47. Hot Spot Animation

48. Pacific Ocean Near Hawaii 48

49. Satellite Tracking • Lasers bounce beams off satellite in geosynchronous orbit • Time necessary to make the round-trip journey can be used to measure the distance of the laser station from the satellite • The amount this changes from year to year can be used to determine plate motions 49

50. GPS Receiver • Receivers such as this receive signals from satellites…using a computer, the distance to each satellite is computed • The position on the earth’s surface is then calculated 50