plate tectonics: sea floor spreading; plate motions!

Transcription

1 plate tectonics: sea floor spreading; plate motions!

2 plate tectonics! Earth s surface composed of! lithospheric plates! Asia! float or ride on! top of asthenosphere! Arabia! move relative to interior! concentrate deformation! at their boundaries! Africa! plates can contain both! continental and oceanic! crust and the upper mantle! India!

3 major plates!

4 plate motion! absolute motion! motion relative to an external reference frame! from:

5 plate motion! relative motion! absolute motions of plates leads to their motion! relative to each other! orange: divergence "blue: convergence "green: transcurrent!

6 relative vs. absolute motion! think about two trains traveling down the track! absolute motions:! "train from Westford: "train from Eastford: "70 mph to east (+ direction)! "60 mph to west (- direction)! relative motion of two trains with respect to each other:! (70 mph east) - (-60 mph west) = 130 mph toward each other!

7 relative vs. absolute motion! same two trains traveling down the track from Westford! 50 mph! 70 mph! absolute motions:! "train 1 from Westford: "train 2 from Westford: "50 mph to east (+ direction)! "70 mph to east (+ direction)! relative motion of two trains with respect to each other:! (70 mph east) - (50 mph east) = 20 mph!

8 relative plate motions yield 3 different types of boundaries:! from:

9 plate tectonics! has its intellectual roots in continental drift! Alfred Wegener proposed idea in 1912! German meteorologist, astronomer,! "balloonist, explorer! noted fit of continental coastlines! detailed older evidence:! continuity of structures, formations,! "fossil flora and fauna! brought new information:! widespread Paleozoic glaciation! "in southern continent! argued for a supercontinent called Pangaea!

10 supercontinent Pangaea! composed of northern Laurentia and southern Gondwana! from:

11 break-up of supercontinent Pangaea!

12 motion of continents through time! Plate reconstruction shown here starts at 730 MyBP - Late Proterozoic!

13 continental drift: fossil evidence! Africa, South America, Antarctica, India, Australia! have nearly identical fossils! Triassic! Triassic! Permian! Permian-Pennsylvanian!

14 continental drift: rock evidence! matching rock types! and fit of continents! (fit not great, but ok)! from:

15 continental drift: evidence of glaciation! important new contribution by Wegener! "(other evidence was well known)! distribution of Paleozoic glaciers! and striations for glacial flow! continents assembled together! explains striation patterns!

16 glaciation: 2 possible alternatives to explain! 1) continents stayed in place and poles wandered! look at modern distribution of climate zones! 2) poles remained fixed and continents wandered! present-day climate! from:

17 Wegener used climate zones to reconstruct where! the South Pole had to be through time! apparent polar wander! pole seems to move! Wegener believed the! continents moved,! not the poles!

18 continental drift: initially rejected (1920 s)! Wegener proposed that continents plowed through oceans! violated what was known about the strength of rocks! (if you pushed with the force necessary, rocks would break, not move)! could not explain motion of continents by Earth s rotation! and/or tidal forces (too small)! continental drift rejected for lack of driving mechanism!

19 paleomagnetism: development of field in 1950 s! earth s magnetic field! and! magnetism of rocks! formed at various! latitudes differ! (remember bar magnet)! dip, or inclination, of! magnetic lines of force! change as field! exits South pole and! enters North pole! steeper inclinations! near poles!

20 paleomagnetism: measuring magnetism in rocks of! " " " "any age! measure rocks in! North America! inclinations are consistent! with forming in a! magnetic field! with a pole in Asia!

21 paleomagnetism: measurement in rocks of many ages! reveals magnetic north changes through time! -- older rocks have magnetic poles farther away! apparent polar wander paths! --apparent movement of poles--! --paths for continents are different--! Europe! North America! alternative: pole is (relatively) fixed and continents move! resulting in apparent (large) motion of pole!

22 reconstruction of continents: using paleomagnetic! data and fit of continental shelves worked well! lining up of rock units! match of glacial! striations! fit of continental shelves!

23 sea floor spreading: key, critical concept! have already discussed many of the components! reversals of Earth s magnetic field! acquisition of magnetism in rocks! presence of mid-ocean ridges! bathymetry of sea-floor! composition of oceanic crust! convection in mantle (heat loss from interior)!

24 sea floor spreading! diagram shows what Harry Hess proposed in 1950 s! sea floor moves away from ridge due to mantle convection! mantle convection reflects rising hot material/sinking cold! hot material rises under ridge -- high elevation, heat flow,! " " " " " "basaltic volcanism!

25 sea floor spreading and plate tectonics! bottom diagram is modern understanding! sea floor spreading from rising hot mantle rock at ridge! subduction of cooled oceanic crust at trench!

26 sea floor spreading: what happens?! divergent boundary -- plates move apart at ridge! basaltic volcanism occurs at ridge! to generate new oceanic crust! new oceanic crust forms sea floor! and occupies space from spreading! older sea floor moves away! from ridge and cools! basalts acquire magnetism of! field at time of eruption! pattern of normal and! reversed stripes of! magnetism is! recorded on sea floor!

27 sea floor spreading: magnetic stripes! ships measuring magnetic field discovered stripes! magnetic field strength! did not match that! of present field, but was! greater (positive anomaly)! or! lesser (negative anomaly)! in symmetric patterns! centered on ridge axis! corresponded to! normally magnetized! (+ add to modern field)! and! reversely magnetized! (- subtract from modern)! sea floor basalt!

28 sea floor spreading: magnetic stripes! sea floor pattern matches that seen in continental rocks! (Vine and Matthews)! Gauss Gilbert can determine age of seafloor! --seafloor is older farther from ridge--!

29 sea floor: age from magnetic anomalies! know age (time) and distance from ridge (length)! yields spreading velocity! 1 cm/yr spreading rate in this example!

30 magnetic anomalies! real examples! from: from:

31 mid-ocean ridge: revisit transform faults! offset of spreading ridge! --displacement along transform fault is result of! sea floor spreading on both sides!

32 mid-ocean ridge: transform faults in Atlantic!

33 age of sea floor! increases away from ridges with oldest farthest from ridge! --oldest is ~180 Ma (Jurassic)-- (note symmetry of ages)!

34 measuring plate motions: methods! marine magnetic anomalies (stripes)! --know age of seafloor and distance from ridge--! Global Positioning System (GPS)! --measures directly; more in a minute--! hot spot tracks, such as Hawaii! --more in two minutes--!

35 measuring plate motions: GPS! measure location of specific point! marked by a stainless steel pin! return to exact spot at some! later time and re-measure! calculate velocity from change! in position of point vs. time! determine plate velocity from! many points on same plate! rates are accurate to a few mm s/yr!

36 plate motions: GPS-derived velocities of plates! agree with velocities from sea floor stripes! --GPS records years; stripes record 10 6 years--!

37 measuring plate motions: hot spots! hot spots are points where a narrow plume of upwelling! " material from depth reaches the surface! characterized by basaltic volcanism! fixed spots over which plates move! Hawaiian Islands formed over a hot spot! Mauna Loa volcano! both from:

38 hot spot: global distribution!

39 hot spot: cooling and subsidence of volcanoes! as islands move off hot spot, volcanism ceases! and islands cool and subside! (results in guyots and seamounts)! forms a hot spot track!

40 hot spot: tracks! Hawaiian Island-Emperor Seamount chain (hot spot track)! --hot spot currently beneath Big Island (Hawaii at SE end)--! can determine rate and direction of plate motion! rate = age of volcano / distance from hot spot! direction = trend of island chain! direction!

41 hot spot: measuring plate motions from tracks! rate = age of volcano / distance from hot spot! direction = trend of island chain!

42 hot spot: Hawaii-Emperor chain! note change in orientation of chain at ~43 my ago!

43 plate tectonics! theory holds that interiors of plates are rigid deformation only occurs at boundaries! global seismicity correlates with plate boundaries!

44 plate tectonics! remember: relative motions yield 3 different boundaries! transform! divergent! convergent! from:

45 plate tectonics: divergent boundary! mid-ocean ridge! sea floor spreading! generation of new (basaltic) oceanic crust!

46 plate tectonics: divergent boundary! sea floor spreading: crust increases in age from ridge! crust cools and subsides as it moves away from ridge! --water depth increases--! --oceanic crustal thicknesss increases--!

47 plate tectonics: divergent boundary! how do divergent boundaries form?! breaking up and rifting of continents! continental crust stretches and thins! grabens form due to extension in rift zones! volcanism begins as magma rises through thin crust! uplift occurs from thermal expansion of crust!

48 plate tectonics: divergent boundary! passive upwelling! think of continent! as wood blocks! as pieces of wood! move apart,! water rises! to fill gap!

49 plate tectonics: divergent boundary! continental rifting leading to sea floor spreading! rifting and extension! grabens, normal faults! sea floor spreading! mid-ocean ridge! continued spreading! passive margins!

50 plate tectonics: divergent boundary! East African rift system! Africa/Arabia splitting apart! Red Sea rift! Gulf of Aden rift! East African rift! Arabia! Africa!

51 plate tectonics: passive, continental margin! end product of continental rifting! normal faults and grabens in continental crust! below shelf and slope! reflect early continental rifting!

52 plate tectonics: transform boundary! plates slide horizontally! past each other! marked by transform faults! 3 types:! offset two ridges! offset ridge and trench! offset two trenches! San Andreas! is transform! that offsets! two ridges!

53 plate tectonics: transform boundary! migration of transform boundary causes pieces! at edge of one plate to transfer to another! San Andreas jumped east! (blue line to red)! piece between lines! transferred from! North American plate! to Pacific plate! one idea has San Andreas! jumping farther east to! Walker Lane/Owens Valley! --recent earthquakes--!

54 plate tectonics:! transform boundary! San Andreas! boundary between! North America and Pacific! San Francisco Bay area earthquakes! (USGS) (yellow dots)! San Andreas! and! Hayward faults! bound low elevations! near bay!

55 plate tectonics: transform boundary! transform faults and ridges! oceanic plates! continental plates!

56 plate tectonics: convergent boundary! plates move toward one another: subduction! subduction: one plate descends below another;! " " "oceanic crust is consumed! understanding of subduction process completed! formation of theory of plate tectonics! provided mechanism for removing oceanic crust! generated at mid-ocean ridges!

57 plate tectonics: convergent boundary! how was subduction discovered?! Wadati-Benioff zones: zones of dipping earthquakes to! " " " 100 s kms depth (max: ~670 km)! deep! intermediate! shallow! from:

58 plate tectonics: convergent boundary! Wadati-Benioff zone! hypocenters! northern Japan! epicenters! red dots are deepest earthquakes so they! plot on map as farthest from trench! from:

59 plate tectonics: convergent boundary! variations in dips of Wadati-Benioff zones!

60 plate tectonics: convergent boundary! imaging the subducting plate with seismic velocities! --subducting plate is cooler than surrounding mantle--! fast: cooler (denser material) slow: hotter (less dense material)! fast! slow!

61 plate tectonics: convergent boundary! less buoyant plate dives below more buoyant plate! oceanic lithosphere density > continental lithosphere! 3 types of convergence! ocean-ocean convergence! ocean-continent convergence! continent-continent convergence (collision)!

62 plate tectonics: convergent boundary! ocean-ocean convergence! one oceanic plate subducts below another! earthquakes occur along interface between two plates! trench, accretionary wedge, forearc basin, volcanic arc! think about this as a dynamic process plates move!

63 plate tectonics: convergent boundary! ocean-ocean convergence! trench: deep, narrow valley where oceanic plate subducts!! accretionary wedge: sediments that accumulated on subducting! " " "plate as it traveled from ridge are scraped " " "off and accreted (added) to overriding plate!

64 plate tectonics: convergent boundary! well-developed trenches in Indonesia/Phillippines! from:

65 plate tectonics: convergent boundary! ocean-ocean convergence! forearc basin: between accretionary wedge and volcanic arc! volcanic arc: mantle is perturbed by subduction process and! " "melts at depths of km, creating magma! " "that rises to the surface to form island volcanoes!

66 plate tectonics: convergent boundary! ocean-continent convergence! oceanic plate subducts below less dense continental crust! features same as with ocean-ocean convergence except that! "volcanoes are built on continental crust and in some cases! " "a backarc thrust belt may form!

67 plate tectonics: convergent boundary! ocean-continent convergence! volcanoes (magmatic arc): more silicic from addition of continental! " " " "material; batholiths form at depth! backarc thrust belt: thrust faults form behind arc in response! " " "to convergence; stickiness between plates! Andes; Cascades!

68 northeast Pacific: Juan de Fuca; Pacific; North American plates! from:

69 plate tectonics: convergent boundary! arc-trench gap! distance between the! trench and volcanoes! because the depth at which! magmas are generated! in subduction zones! is about km,! this distance depends! on the dip of the! subducting plate! if the dip of the subducting plate! is flat enough, no volcanoes form! subducted plate doesn t go deep! know dip by looking at distance! between volcanoes and trench!

70 plate tectonics: convergent boundary! boundary can migrate through time! --response to forcing either by overriding or subducting plate! overriding plate! pushes! trench! subducting plate! steepens! and pulls! overriding plate! toward trench!

71 plate tectonics: convergent boundary! continent-continent! neither plate wants to subduct! (both are buoyant)! result is! continental collision! mountain belts! thrust faults! detached subducting plate! suture zone--plate boundary!

72 plate tectonics: convergent boundary! continent-continent! model for India and Asia collision! from: from:

73 plate tectonics: Himalayas (continent collision)! EURASIAN! PLATE! Himalayas! are apart of! long! mountain belt! that extends! to Alps! INDIAN! PLATE! AFRICAN! PLATE!

74 plate tectonics: continent-continent collision! deformation from collision extends far into Tibet/Asia!

75 plate tectonics: what causes plates to move?! one idea! ridge push: sea floor spreading and gravity! sliding of plate downhill from ridge to trench! while being pushed by sea floor spreading!

76 plate tectonics: what causes plates to move?! another idea! slab pull: weight of subducting slab! subducting slab sinks into mantle! from its own weight, pulling the! rest of the plate with it! as subducting slab descends! into mantle, the higher! pressures cause minerals to! transform to denser forms! (crystal structures compact)!

77 plate tectonics: what causes plates to move?! slab sinking causes roll back and trench suction! slab pull is more important than ridge push! how do we know this?! plates that have the greatest length of subduction boundary! have the fastest velocities!

78 plate tectonics: what is driving mechanism?! must explain: sea floor spreading and subduction;! " " heat flow, warm and elevated ridges;! " " cold and deep trenches! mantle convection is likely candidate! but is mantle the cause or an effect! "of ridge push and slab pull?! from:

79 plate tectonics: how do continents rift apart?! one idea: mantle plumes of hot material! hot plume material ponds at base of continental lid! eventually, enough heat is produced! to stretch and thin continent! --hot spot and flood basalts--!

80 plate tectonics: mantle plume the movie!

81 plate tectonics: mantle plumes! from core-mantle boundary, D?!

82 plate tectonics: continental rifting! doming and uplift above plume! rifting of continent! eventually, rifts enough to! separate continents and! start sea floor spreading!

83 plate tectonics: continental rifting example!

84 plate tectonics: multiple plumes!

85 plate tectonics: global distribution! note numerous below Africa!