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Plate Boundaries Present day movement, accretion, reformation, segregation.

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Presentation on theme: "Plate Boundaries Present day movement, accretion, reformation, segregation."— Presentation transcript:

1 Plate Boundaries Present day movement, accretion, reformation, segregation

2 Fig. 4-4 Three types of plate boundaries 1.Divergent 2.Convergent 3.Transform

3 Figure 4.22 Schematic diagram showing the major features of a plate. Near the spreading center, where the temperature is high because of rising magma, the lithosphere is thin. Away from the spreading center, the lithosphere cools, becomes denser and also thicker, and so the lithosphere-asthenosphere boundary is deeper. When the lithosphere sinks into the asthenosphere at the subduction zone, it is reheated. At a depth of about 100 km, the oceanic crust starts to melt, and the magma rises and forms an arcuate belt of volcanoes parallel to the subduction zone.

4 F 1 Friction between convecting asthenosphere and rigid lithosphere F 2 Gravitational push from mid-ocean ridge (high topography) F 3 Pull from increasing density of slab as it cools F 4 Elastic resistance of oceanic plate being pulled into subduction zone F 5 Pull of overriding plate toward subduction zone as subducting plate bends F 6 Friction between subducting slab and overlying lithosphere F 7 Sinking of oceanic slab as it cools and becomes denser

5 Plate boundaries Divergent – new oceanic crust –Some mantle melts – new basaltic oceanic crust –Symmetrical geomagnetic record and aging –F 2 force – pushing –Cools, becomes more dense and subsides away from ridge Convergent – plates collide –Subduction of oceanic crust Basalt at the base of the lithosphere converted to eclogite Forms at depths > 50 km in the upper mantle –F 3 force - pulling Transform – plates slide past each other

6 Plate boundaries

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8 1.Divergent 2.Convergent 3.Transform

9 Rifting Buckling Shear 1.Divergent 2.Convergent 3.Transform

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11 Diverging Boundaries

12 Lava fountains (10 m high) spouting from eruptive fissures during the October 1980 eruption of Krafla Volcano. (Photograph by Gudmundur E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland.)

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17 Other evidence Hydrothermal vents (discovered in 1977) –Thermal anomalies (unusually warm water) found in 1972 over a ridge near Galapagos Islands –Sent Alvin down in 1977 Unusual organisms Pillow lavas from recently extruded ocean crust Hot water comes from seawater ventilating through the rocks Energy for life comes from geothermal energy and chemical energy comes from sea water – chemosynthetic bacteria are base of food chain

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19 Global Ridge System

20 Converging Boundaries Oceanic-oceanic convergence Continental-continental convergence Oceanic-continental convergence

21 Cooler, more dense slab sinks Melting of subducting slab + water and CO 2 + some mantle + some continental lithosphere

22 Converging plates 2 continental plates – e.g., Himalayas and Alps –Can get marine fossils in mountains (remember continental shelf is part of the continental lithosphere) Continental and oceanic plates – e.g., S America & N America –Mountains and island arcs 2 oceanic plates – Aleutian and Marianas trenches –Older slab (denser and cooler) sinks

23 Continental arc (oceanic plate- continental plate collision) Island arc (oceanic plate- oceanic plate collision) continental plate- continental plate collision Fig Three types of collisions

24 The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the Andes, as seen here in the Pachapaqui mining area in Peru. Continent-continent collision

25 Figure 4.24 Sediment at the edge of continental crust on the subducting plate is deformed and welded onto already deformed continental crust on overriding plate. Continental arc system Continent-oceanic collision Partial melting of slab (sinking plate), sediment cover and continental crust

26 Nazca Slab

27 Collision near continental edge Ocean crust sinks because its denser

28 Island arc systems (2 oceanic plates far from continental crust)

29 Island Arc Formation

30 Figure 4.20 Structure of tectonic plates at a convergent margin. Along the line of subduction, an oceanic trench is formed, and sediment deposited in the trench, as well as sediment from the sinking plate, is compressed and deformed to create a mélange of shattered and crushed rock shaped as a fore-arc ridge. The sinking oceanic crust eventually r eaches the temperature where melting commences and forms andesitic magma, which then rises to form an arc of volcanoes on the overriding plate. On the side of the island arc away from the trench, tensional forces lead to the development of a back-arc basin.

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33 Formation of new crust at convergent boundaries Melting and recycling of rock Segregation – Bowens reaction series Composition of new material depends on: –Temperature of magma formation –Sediment cover and water content of subducting plate

34 Exotic terranes Micro-plates –Small plates with plateaus or microcontinents –Get accreted onto larger continent (Pacific NW)

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36 Ophiolite suites Obducted ocean crust

37 Continent-oceanic collision

38 Transform faults –Plates moving past each other –Relieve pressure due to earth’s curvature, relative rates of spreading, etc

39 Axis of spreading is broken up by curvature of earth or unequal spreading, etc Spreading cannot proceed evenly on the surface of a sphere (this would necessarily require faster spreading at the equator and slower spreading at the poles

40 Transform Boundaries

41 Transform fault. No characteristic topographic expression, but margin is often marked by a long, thin valley. Earthquakes down to 100 km and often strong.

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43 N Andreas Fault

44 Transform faults and accretion Can cleave off bits of continental crust that are plastered onto other continental land masses

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46 Hotspots Mantle plume Direction of plate motion

47 Mantle plumes -P-wave velocity anomalies -High temperature (red and yellow) -East Africa rift valleys

48 Hot spots – surface expressions of plumes Stationary spot and plate moving over it.

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51 Now think of evolution of Earth Repeated recycling and collisions Continued fractionation/segregation Remnant landscapes

52 Plate tectonics in the Archaen Higher heat flow –More radioactivity in Earth’s interior –Vigorous mantle activity didn’t preserve crustal material Convection cells and plates likely smaller than today – more hot spots? Archean lithosphere may have been too hot and light to be subducted –Also thicker –Lighter material formed – more time to segregate At microplate boundaries plates may have been compressed and folded –Plate collisions accreted material – oldest rocks found in interior craton

53 Decreasing temperature of formation Repeated segregation

54 Fig Volcanic arc Time Forearc basin Oceanic crust Greenstone belt Continental crust Repeated accretion

55 Greenstone belts in the Pilbara Shield of Western Australia ~40 km (25 mi)

56 When did this happen Most models suggest much of the crust formed between 3 and 2.5 bybp Not all agree Later periods of episodic and rapid growth Wilson cycles of supercontinent formation and fragmentation

57 Fig The Wilson cycle of supercontinent assembly and fragmentation. (a) The continents are drifting toward a region of cold asthenosphere. The closing ocean is lined by subduction zones and is contracting. The other ocean is opening, and the oceanic lithosphere is connected to the continental lithosphere at both margins. (b) The continental fragments have collided, forming a supercontinent. Subduction has begun along the margins of the formerly opening ocean. The insulating effects of the thick continental lithosphere lead to the buildup of heat and the initiation of rifting. (c) What once was an opening ocean has become a closing ocean, with cool asthenosphere beneath. One Wilson cycle is now complete. (After P. Kearey and F.J. Vine, Global Tectonics, Oxford: Blackwell Scientific.)

58 Fig The ages of the components of the North American continent reveal that the continent has grown by the amalgamation of very old cratons, followed by accretion of younger material onto the periphery of the craton during plate collisions. (From J.P. Davidson, W.E. Reed, and P.M. Davis, Exploring Earth: An Introduction to Physical Geology, Reprinted by permission of Prentice Hall, Upper Saddle River, N.J.)

59 The Earth’s Early Crust Oceanic crustContinental crust First appearance~4.5 bybp~4.3 bybp Where formedocean ridge High temperature submarine plateaus Lower temperature CompositionbasaltTTG* Lateral extentwidespread, rapidly recycled local, rapidly recycled (?) How generatedpartial melting of ultramafic rock in upper mantle partial melting of wet mafic rocks *tonalite-trondhjemite-granodiorite Low K, high Si granitoids dominated by quartz and plagioclase feldspar

60 The Rock cycle

61 Evolution of modern plate tectonics Presence moderate temperatures – Venus is too hot so lithosphere never cool enough to subduct Heat removal from mantle through subduction of cool oceanic lithosphere and upwelling of new crust –Drives convection cells –Allows basalt eclogite transition to be shallow –Subduction leads to fractional melting of oceanic crust and segregation to form continental crust Presence of water –Needed for granite formation –Catalyzes fractional melting in subducting sediments

62 Period of major accretion (~ my) { Period of heavy bombardment Present-day plate tectonics “begins” period of rapid crustal growth Archaen-Proterozoic transition To modern plate tectonics 1. Early plates became bigger and thicker 2. Continued recycling of oceanic crust formed large amounts of buoyant continental crust Continued partial melting/distillation Separation of Si and other elements from Mg and Fe Conversion of mafic material to felsic material through rock cycle 3. Decrease in heat production slowed mantle convection Drove system to larger convection cells Allowed larger plates to travel farther on the Earth’s surface and cool more Led to subduction rather than collision of plates Modern plate tectonics

63 Period of major accretion (~ my) { Period of heavy bombardment Present-day plate tectonics “begins”

64 Alternative views Does life play a role? (Gaia) Earth is only planet with life AND plate tectonics Is there a connection? Cause-effect? See Lovelock work Life affects weathering and calcite deposition

65 Since the Archaean Intensity of plate tectonics has varied over time Wilson cycles – 500 my cycles –Evidence of supercontinent mybp –Pangea formed ~ 300 mybp –Causes not well understood Periods of rapid sea floor spreading (and vice versa) –Sea level rises because large amounts of shallow basalt form and don’t cool (and subside) much –High CO 2 release – released at spreading centers when new crust forms and subducting crust has sediment on it including calcite which releases CO 2 when it melts

66 Age of crustal material Continental crust is older because it doesn’t get subducted –Too buoyant –Becomes “core” for accretion –Collisions (closing of basins) mediate accretion –Losses only from weathering and subduction of sediment Oldest rocks are 4.3 – 4.4 by old Oceanic crust is young and constantly recycled (and fractionated) –Oldest oceanic crust is furthest from spreading centers near subduction zones

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68 Figure 8.18 Map of a closed Atlantic Ocean showing the rifts that formed when Pangaea was split by a spreading center. The rifts on today's continents are now filled with sediment. Some of them serve as the channelways for large rivers.

69 Net result Spreading rates at transform faults –Pacific plate moves NW at 8 cm/yr –N American plate moves W at 2 cm/yr –Indian plate moves NE at 12 cm/yr Pacific Ocean is shrinking and Atlantic is growing –Atlantic opened about 200 MY ago so there should be no rocks older than this in the Atlantic

70 Most recent episode of Seafloor spreading: Pangaea first broke into 2 pieces Sea opens between N and S continents and Between Africa and Antarctica India moves North

71 S Atlantic opens Antarctica moving S India moving N Australia separates and moves N

72 50 MY in the future: 1. Africa will move N and close Mediterranean Sea 2. E Africa will detach (Red Sea rift zone) and move to India 3. Atlantic Ocean will grow and Pacific will shrink as it is swallowed into Aleutian trench. 4. W California will travel NW with the Pacific Plate (LA will be swallowed into the Aleutian trench in 60 MY).


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