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08 - Why does the Earth have ocean basins San Andreas Fault...

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Unformatted text preview: Why does the Earth have ocean basins? San Andreas Fault, California boundary between North American and Pacific tectonic plates Hekla volcano, Iceland has erupted >20 times in last 1,000 yrs Structure of ocean basins South America Mid-ocean ridge Rift Atlantic Africa What do I mean by "structure"? structure" shape, depth, profile, configuration Why does the Earth have ocean basins? result of global tectonic configuration result of properties of Earth's interior Earth' constantly changing with active tectonic processes Oceanic crust Continental crust Fig. 3-19b, p. 78 Layering of Earth's interior density stratification what is density? mass/volume pure water = 1g/cm3 granite rock = 2.7g/cm3 Inner core separated during original Earth cooling Mantle Crust Fig. 3-1, p. 60 1 How do we know about the Earth's interior? Earth' Drilling holes? deepest we've drilled = 7.5 miles (473F, pressure squeezed we' (473 hole shut!) th of the Earth's radius That covered 1/500 Earth' Mantle ...no... no... Seismograph P S 0 10 20 Minutes 30 40 50 Earthquakes! ...yes! P S Surface waves seismic waves travel through earth technology over last 100 years recorded at seismograph stations (check out example in CIVL on 2nd floor) >4,000 seismograph stations world-wide world- Focus Core Fig. 3-2, p. 61 Forces that cause earthquakes generate energy low-frequency waves = seismic waves low waves reflect and bend as physical properties of medium they are traveling through changes Surface waves (shaking) Body waves P waves (slinky compression), travel slower S waves (side to side) Fig. 3-5, p. 63 Seismic P-waves like a slinky! Direction of particle motion Direction of wave propagation t1 t2 Seismic S-waves side-to-side Successive times unable to transmit through liquids Direction of particle motion Direction of wave propagation t1 t2 move through solids and liquids t3 t4 t5 t3 t4 t5 t6 Di la Co e pr om C Down t6 t io n m pr e ss io n Up Successive times n io ss 2 How seismic waves would travel if Earth was homogenous (the same) all the way through its interior How seismic waves would travel if Earth gradually became more dense with depth Fig. 3-4b, p. 62 P-waves travel through entire Earth, but are bent in transit S-waves cannot penetrate liquid outer core, creates "shadow" zone Solid inner core 103 Liquid outer core 103 Solid inner core Liquid outer core 103 P-wave shadow zone Mantle 143 P-wave shadow zone Mantl e Compositional classification Crust 1% of Earth volume Oceanic crust: basalt (2.9 g/cm3) (O, Si, Mg, Fe) Continental crust: granite (& other) less dense (O, Si, Al) 83% of Earth volume 4.5 g/cm (Si, O, Fe, Mg) 16% of Earth volume 13 g/cm3 (90% Fe) knowledge based on earthquake data, volcanic gases, variations in gravity, etc. Mantle Core 3 lithosphere Physical property classification cool, rigid, outer layer 100-200 km thick (varies spatially) 100 includes continental & oceanic crust + upper mantle hot, partially melted, slowly flowing includes upper mantle down to 350-350 km 350 same chemical composition as asthenosphere, but asthenosphere, does not melt because of high pressure = more dense, moves more slowly hot, dense, viscous liquid solid 360650 220400 Depth km mi 865 540 Ocean water Continental crust (1 g/cm3) (rigid) (2.7 g/cm3) Oceanic crust (rigid) (2.9 g/cm3) asthenosphere Lithosphere (rigid) 100200 60125 lower mantle (a.k.a. mesosphere) outer core inner core Upper Asthenosphere mantle (deformable, (3.3 g/cm3) capable of flow) Lower mantle (4.5 g/cm3) Fig. 3-6b, p. 65 Isostasy Earth's crust "floats" on denser but fluid mantle buoyancy - upward force on a submerged object from surrounding fluid - allows objects to float in fluid by displacing a volume of fluid equal in weight to the floating objects own weight - depends on mass and density (P = density x gravity x height) Icebergs sink into water so that the same proportion of their volume (~90%) is submerged. The more massive the iceberg, the greater this volume is. The large iceberg rides higher but also extends to a greater depth than the small one. Containership empty Containership loaded with cargo Displaced water ...the same thing happens with Earth's crust "floating" on the mantle. As size of mountain mass decreases with erosion, the amount of compensation ("root") into the mantle decreases as well. Deposition of sediment into a basin increases the load on that part of the crust. Fig. 3-8, p. 67 4 Continental drift theory that the continents are "floating" and moving over the floating" molten interior of the Earth first proposed in 1915 by Wegener Fit of continents around the Atlantic as calculated by Sir Edward Bullard at the University of Cambridge in the 1960s rock types and fossil collections match up at edges evidence accumulated... accumulated... fit of the shape of the continents around Atlantic Pacific "ring of fire" fire" Ocean floor profile symmetry Fig. 3-10, p. 69 Seismic events worldwide, January 1977 through December 1986 Seafloor spreading -new seafloor being created in middle of ocean floor! - so, is the Earth growing?? Fig. 3-14, p. 74 Plate tectonics first proposed: 1960 Harry Hess at Princeton unifying theory that combined theories of convection currents in mantle are driving "plate" movement plate" ridges University "continental drift" and "seafloor spreading" drift" spreading" The major tectonic plates new continental crust made at mid-ocean mid old crust destroyed at subduction zones Revolutionized geology!!! Fig. 3-15, p. 74 5 Plate boundaries new crust created at mid-ocean ridges mid volcanoes Co nv er (de gent str bo uc un tiv da ry e) divergent convergent crust subducted and re-melted re volcanoes and large earthquakes Trench Divergent boundary (constructive) 1 Transform boundary (conservative) transform plates sliding past each other earthquakes Rising magma Asthenosphere Fig. 3-16c, p. 75 Warping, stretching of East Africa Divergent plate boundaries seafloor spreading new ocean basins form ridges at spreading center small volcanoes / earthquakes examples at various stages of formation young: East African rift intermediate: Red sea mature: Atlantic ocean Continental crust Formation of rift valley Red Sea Fig. 3-17a-c, p. 76 Red sea Lake Bogoria, Kenya began ~25 million years ago pulling the Arabian plate (E) and African plate (W) apart Fig. 3-18, p. 77 Fig. 3-17d, p. 76 6 Atlantic ocean- mature divergent plate boundary oceanMid-ocean ridge Rift Africa Evolution of Atlantic basin over last ~210 million years breakup of supercontinent Pangea South America Atlantic Oceanic crust Continental crust http://www.classzone.com/books/earth_sci ence/terc/content/visualizations/es0806/es0 806page01.cfm?chapter_no=visualization Fig. 3-19b, p. 78 Fig. 3-20, p. 79 Geomagnetic polarity reversals direction of magnetic field switches polarity!! ~every 700,000 years (but range from <100,000 to 10s of millions of years) have been documented back ~330 million years (400 total) last reversal occurred 780,000 years ago what happens during reversal?? Paleomagnetic records key evidence of seafloor spreading every wonder why a compass points to the north? Fig. 3-26b, p. 84 magnetic particles align with Earth's magnetic field 9 Ma 6 Ma 3 Ma Today = Normal magnetic polarity = Reversed magnetic polarity Fig. 3-26a, p. 84 = Direction of plate movement Ma = mega-annum, millions of years ago Fig. 3-28b, p. 85 7 Matthews & Vine 1963 Discovered symmetric pattern in polarity "striping" around MidAtlantic ridge Pleistocene to Recent (01.6 Ma) Pliocene (1.65 Ma) Miocene (524 Ma) Oligocene (2437 Ma) Eocene (3758 Ma) Fig. 3-28a, p. 85 Paleocene (5866 Ma) Late Cretaceous (6688 Ma) Middle Cretaceous (88118 Ma) Early Cretaceous (118144 Ma) Late Jurassic (144161 Ma) Ma = mega-annum, millions of years ago Fig. 3-29, p. 86 Spreading center- hydrothermal vents Fig. 4-27, p. 119 Fig. 4-26, p. 119 Transform plate boundaries do not create or destroy crust plates slide past each other can be on continent or in ocean connect spreading centers (divergent boundaries) major earthquakes ex: San Andreas fault, California transform faults Fig. 3-25a, p. 82 8 Transform plate boundary ex: San Andreas Fault separates Pacific and North American plates horizontal movement (strike-slip) Pacific has been moving North for the last 10 million years grinding at a rate of 5cm/yr (avg) fault 1,300 km long, and extends ~15 km deep North American plate Pacific plate North American plate Pacific plate Convergent plate boundaries ocean-continent ocean ex: Andes Mountains, w South America Ocean-continent collision OceanPeruChile Trench Oceanic lithosphere Andes: Volcanic mountains South America: Continental lithosphere ocean-ocean ocean ex: Japanese volcanic islands continent-continent continent ex: Himalayas, India-Asia collision India- Ge ntl y de s ce nd Related to violent Earth processes! ing sla b Ex: Andes Mountains, west coast of South America 3-21, p. 80 Fig. Earthquakes around Japan 1975-1995 3D bathymetry of Japan subduction zone China Russia Japan Korea Tren ch Earthquakes: 1,000/year 1923- killed >100,000 1995- killed 6,000 (injured 415,000) tsunamis- because fault scarp is under ocean water Kobe 1995 Key Japan: example of subduction zone volcanic island arc Shallow Intermediate Deep Japa n 9 Ocean-ocean collision OceanMarginal sea Continental crust Oceanic crust Volcanic arc Trench Oceanic lithosphere South America Active volcano in Peru Pacific Ocean Magma Earthquake focus n io ct d u ne b o Su z Earthquake locations Ex: Japanese islands Andes mountains: example of oceanic-continent subduction zone CENTRAL AMERICA Trench W E Continent-continent collision Continentex: Himalayas Mountain range High plateau Coremantle boundary JAPAN W Trench E Continental crust Lithosphere Ancient oceanic crust Coremantle boundary Asthenosphere Fig. 3-22, p. 80 Terrane formation Him alay as continent or volcanic arc collisions with larger continent bits that scrape off and don't get subducted how west coast of North America has "grown" "exotic terranes" rare "ophiolites"- oceanic crust scraped off Fig. 3-34, p. 90 10 Hot spots mantle plumes not related to divergent plate boundaries / spreading centers maybe precursors to spreading centers? stationary relative to the plate motion plate moves over plume coming up Examples: Ophiolote, CA Fig. 3-36, p. 90 Yellowstone (Snake River Plain) Hawaii Global distribution of hot spots Yellowstone hotspot catastrophic eruptions 2Ma, 1.2 Ma, 600,000 "Supervolcano"!! Supervolcano" How does a hotspot form? 70 Ma ror Empe ounts Seam Aleutian Islands plate motion 70-40 Ma PACIFIC OCEAN 50 Ma 40 Ma 30 Ma 20 Ma plate motion 40 Ma - present bend = change in direction of plate movement (from N to NW) 10 Ma Kauai 5 Ma Haw a iian Ridg e Hawaii <1 Ma Loihi more evidence of plate motion over a liquid interior 11 Wilson cycles tectonic cycles over geologic time scales of ocean basin creation, growth, closure, and destruction... has happened several times over Earth's history Fig. 3-37, p. 91 Fig. 3-37a, p. 91 Fig. 3-37b, p. 91 Fig. 3-37c, p. 91 Fig. 3-37d, p. 91 12 Fig. 3-37e, p. 91 Fig. 3-37f, p. 91 Indonesia India Understanding of tectonic processes has important implications for life and $$: where are seismic hazards / risks the most intense how often to expect them? best methods of seismic retrofit California Fig. 3-38, p. 92 13 ...
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