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Date Created: 10/02/15
quot art 239 number of pieces called plates g p A jrijhese plat s g gid lithosphere ride over the softeg a lctile asthenosphere It Motion of these plates cause nearly all geologic features we observe on the Plate Tectonics Plate teetonie theory is powerful ltprouides auniliedmeehanism explaining Igneous sedimentary and metamorphic roelrs The distribution of earthquakes and volcanoes The origin oleontinents and oeeanhasins The distribution ollossil plants and animals The genesis and destmetion of mountain chains Continental drift Layers of the Earth Revisited The tectonic plates of the Earth lie Within the rigid or elastic lithosphere and rest upon the ductile asthenosphere BrittleDuctile Transition Mantle Asthenoshpere Elevations and Lithospheric Loading The weight of the lithosphere causes depression of the asthenosphere and bending in the lithosphere m Lithosphere not to scale Buoyancy First described by Archimedes more than 22 ka Floating solids displace water equal to their mass An iceberg sinks until the mass of water it displaces is equal to the total mass of the iceberg This concept applies to lithospheric plates Continental Floats higher Oceanic Sinks lower Mass of water N displaced quot X A Why Are Continents Higher Continental Crust granite density 26 gcm3 Oceanic Crust basalt density 30 gcm3 Continental Oceanic lithosphere lithosphere Water uid Pressure is m o n O 1 394 A E b c Review Hypsograph Exercise The Major Plates of Eart s L39thosphere Eurasian Plan ran 1 th oceanic and continental crust A few contain only oceanic crust Thus a plate 3 a continent Not all boundaries between continents and oceans are plate Most plates co tain boundaries Plate boundaries are zones of many earthquakes K quoti J K r V I 39 i quot 394 31 TV H 39 at 7 Tc 3 V v I J 39 7 l w xquot y i N l k v W North I y i l YAmenca v T I quot ZAritarctic i I Earthquakes clearly outline major plate boundaries Thus active continental margins ARE plate margins Passive continental margins are NOT plate margins So what happens at passive margins Active amp Passive Margins Margin boundary between a cont1nent and an ocean Active Margin A continental shoreline that IS a plate boundary Passive Margin A W continental shoreline that thliospheric IS NOT a plate boundary mantle Continental Shelf Thick sediment cover gentle slope shallow to moderate depth Abyssal Plain nearly at in slope deep depth crust CfOSSSGCthIl Of a passwe margm ASthenosphere The major plates of Eart s lithosphere North American Plate Euraslan Plate Geologists types of plate boundaries 1 Divergent 2 Convergent 3 Transform Type 1 Divergent Plate Boundary Divergent Two plates Midocean that pull away or separate from each other Produce new crust Examples Mid Atlantic Ridge East Paci c Rise Effect of movement Sea Floor Spreading a Divergent boundary Divergent Animation Type 2 Convergent Plate Boundary Convergent Two plates Overidmg F Volcamc arc thatmove towards or plate a Trench coll1de W1th each other W Consume old crust Example India into Asia NW Coast of US Effect of movement 1 Subduction or Collision b Convergent boundary Convergent Animation Type 3 Transform Plate Boundary Transform Two plates that slide horizontally past each other Do NOT consume or create crust Example North Anatolian Fault Turkey Dead Sea Fault Israel Jordan San Transform fault L Andreas Fault EffCCt Of movement c Transform boundary Transform Faulting Transform Animation Divergent Boundaries Sea oor spreading causes plates to move apart Magma wells up to ll the gap Magma cools adding material to each plate Divergent Plate Boundaries Sea Floor Spreading Time 1 Midocean ridge 11 New ocean oor Midocean ridge Oldest Older Younger Older Oldest ocean ocean ocean ocean ocean oor floor oor floor oor New Oceanic Crust at MidOcean Ridges Intrudes from Dikes Fault scarps Midocean ridge axis Sediment Pillow basalt Lithospheric mantle Asthenosphere MidOcean Ridges Linear mountain ranges in Earth s ocean basins Example The MidAtlantic Ridge Snakes N S through the entire Atlantic Ocean Elevated ridge 1500 km Wide 2 km above abyssal plains I Abyssal ynentali plain Why Are Midmean Ridges Elevated Lithospheric mantle Cooler e more dense 39 em e the rm the ewe e Qtfmgs eg Elim elk L iihfpyheweeejide M1 me WMEe ilheepfmewe dew Mm l e egtlhegmhere MidOcean Ridge Activity Black Smokers amp Pillow Basalts n quot n 3 Magma heats water and it jets out of these 0 When lava erupts underwater chimneylike vents The Water is black instead of making coherent because of dark colored mmerals Strange flows It forms rounded blob cr1tters 11ve here llke pillows Commonly called pillow basalts SJ u 253 BE 2 a SEE 4 North America 1 W2 39 A W 6 g TE 1Q 5 I Australia 3 1E 3 w E NW7 f Antarctica Ma 0 5 5 21 21 38 38 52 52 65 65 145 145 160 to Pliocene Convergent Plate Boundaries Subduction When two plates collide if one is oceanic one oceanic plate bends and sinks downward into the asthenosphere This is called subduction Why does the oceanic plate sink Because oceanic lithosphere is more dense than the asthenosphere it wants to sink Why is it more dense Its colder Slab sinking rates range 1015 cmyr gt l Floating Time 1 v Ime New trench farms f gt r lt2 Time 2 l a Future arc position quotquotquot quot Sinking anchor Futu re Features of Convergent Plate Boundaries Subduction Accretionnry prism Thrust belt due to com ression continental p volcanic arc Forearc Accretiona basin ry prism ma a gm Partial melting Subduction animation 3 Convergent Boundaries Accretionary prisms Deformed sediment wedges Sediments scraped off subducting plates are smeared and welded onto the overriding plates These contorted sediments can be pushed above sea Washington s Olympic Peninsula Taiwan Accretionary prism Convergent Boundaries Volcanic arc Volcanic belt on an overriding plate The descending plate contains uids squeezed out at 150 km depth Causes the overlying plate to partially melt Magmas burn upward fueling volcanic eruptions A curved Earth dictates that volcanic belts are curved Arc type depends on overriding plate Continental crust Continental arc Oceanic Island arc The WadatiBenioff Zone How do we know where the subducting plate is Mega Thrust Earthquakes occur along the interface between the upper and lower plate The pattern of earthquakes outlines the location and shape of the lower plate The deepest and largest earthquakes in the world occur at subduction zones What is the fate of the sinking plate Depth km 0 Area of Fig 415a 4 Volcanic arc Lithosphere 200 400 600 Upper mantle Downgo Intermediate Plae a t 1311a1lt 5 I Wadat1 Ben10ff Transmon zone zone Deep earthquakes 00 O 0 1000 1200 1400 Lower mantle 2 Deepest earthquakes Sinking lithosphere may go to the coremantle boundary 1600 Subducted Plate Graveyard Hot spot island The current hypothesis about the fate of subducted plates suggests that they may sink all the 0 way to the lower mantle near the coremantle boundary Geophysical techniques and models may help figure this out Volcanic arc Oceanic L39 osphere Midocean Continental4 670 Subduction of an Island Arc I Mar C glnal s Ontlnent backarc basal due to eXtension Volcanic island arc Island Arc Chain of volcanic islands that form behind a subduction zone 0 Island arcs may eventually get swallowed by the subduction zone 0 What happens when a continent collides into another continent collision animation Transform Boundaries Lithosphere slides past not created or destroyed Most transforms offset spreading ridge segments Some transforms cut through continental crust Characterized by Earthquakes Absence of volcanism Oceanic Transforms The MOR axis is offset by transform faults Offset of linear MOR is geometric necessity on a sphere Transforms provide strong evidence of sea oor spreading Abundant earthquakes common between ridge segments Earthquakes vanish past ridge segment overlaps Midocean ridge Younger plate Transform Plate Boundaries No Net Loss or Gain of Lithosphere Transforms indicate where plates slide past each other Most are fairly short Most occur as fracture zones adjacent to midocean ridges First properly described recognized by JT Wilson animation t l l l l l l l Old idea Time gt b bwmda es an the VI o Thc Amdmag Tramsfcmrm Wm a Tigh talatcm smsc mctmn papa dcmm 4 0 Acwmmcd rcs mwmm A 1 r A A N 39 l x A I 39 r My 44 s I bc NFLcm mg admg r m ngmZ 9 San Franc1sco v 4 1 n 11 ALL mum de m Mac mrm ccl u w mBajaiT 163 c Q L 7 Pacific Plate x 4 LL 3 H 41 4 i a r quot A m 107 me SA cmds am a air 400 km 250 mi Ridge segment Transform Trench SAF Aeri alV 16W 0 Tr p Jummmm W wtfmm T 1 o L rim ylt f zy u L Nam f l by Tina 3g Wt 1 s r01 bwmdm g A t that at jmct mm RE39 m5 9 m T m ridg ar dg9 m tramsfmrmmfur msitmrmm mneh Plate C Francisc Sea level Sea loco Hotspot volcano 1 Hot Spots Not all volcanoes on Earth are related to plate boundaries Some occur at hot spots locations where hot molten vo ggfggl 7 rock rises through tectonic plates Hot spots arise from hot mantle E plumes that are stationary F through time and buoyantly rise to through the cmst e g Hawaii and Yellowstone Hotspot volcano 2 remnant of Extinct r J volcano 1 volcano 2 Hompot volcano 3 hot spot animation Hawaiian Hot Spot The islands of Hawaii exist because of a hot spot The hot spot is currently under the big island of Hawaii Lithosphere The paci c plate 1s movrng to the NW So Oahu where Honolulu is is older not currently active Highest volcanoes are on Hawaii and the islands get more eroded because they are older to the NW So the orientation of the chain of islands can tell us what the plate motion is b The orientation of the Hawaiian archipelago changed at d x 39 g 40Ma 39 Emperor HSeamountsquot This tells 39 39 39 If nuimn geologists that 39 39 the motion of the paci c plate changed at 40Ma East Paci c Rise 1 3 Easter Island JV Iii 3939 quot i39ltluj39l xMacdonald seam d m 39 Other Hot Spot Tracks There are lots of hot spots on Earth Iceland is unique A hot spot and a divergent plate boundary all above sea level The Birth and Death of Plate Boundaries 0 A O i 47 if ARR Kquot F 3 f 153 4 T 53 1 40 is HE D IIItin st LizHi tt in MI C How ans diveigsnt i 7 M 397 i i 39 nonindnnss fonnnsd a wa if cnntinsntni inntsiiai granite is nosing iiftsda tinsn it is sninstiniss caiisoi continsntai tiiiting S T Ci i g cansss thinning the crust s iiiy puny Eastern Africa is currently being rifted apart Area 5 39 in Triple Called the East African Rift unmon 1 Associated Earthquakes Volcanism and Faulting Ri ing 11mm USO 0 The westem US most of Nevada part of Utah Califomilaz h g Death of a Subduction Zone Eventually a subduction zone may swallow all of the dense oceanic crust Two plates of continental material C cannot subduct because they are not dense enough collision animation Collisional mountain belt Trench Volcanic Detached sinking oceanic lithosphere Time 2 What Driws Manta MQ tfmm The old hypothesis wasthat Subducting Midocean Volcanic arc the asthenbsphere contais slab 7 4 ridge co quot 39 39 Old Convection Model twolayer RidgePush and SlabPull forces Abyssal plain Mid cean ridge SI ope Divergent boundaries are driven by a ridgepush force RidgePush is driven by gravity SlabPull force drives also from gravity Trench Dense lithosphere sinks and once it starts to sink the densitv difference pulls the rest of the plate with the sinking slab Ve eeities ef the Plates 0 Thanks to GPS and ether geodetic techniques We can measure the mettehs of the Earth s pllattes me up t0 the minute hatsis 120 GPS Motions in Los Angeles C 3 34 0039 39 NOPK WRHS r LASC I 5000 I PMHS CC DSHS Iasm 5amp5 7 CO CSDH 1 CRHS 361 TORP 39 241 3039 242 00 Relative Plate Velocities aquot vr i 139 AAA Convergent boundary Ridge Transform Absolute plate motions 0 Relative plate motions 55 cm per year Ignore red arrows They are absolute motions Black arrows show relative motions between plates