Oceanic Lithosphere - 1

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  • Magnetic stripes not only tell us about the age of the oceans, they can also reveal the timing and location of initial continental break-up. The oldest oceanic crust that borders a continent must have formed after the continent broke apart initially, and just as sea-floor spreading began. In effect, it records the age when that continent separated from its neighbour. In the northern Atlantic, for example, oceanic crust older than 140 Ma is restricted to the eastern USA and western Saharan Africa, therefore separation of North America from this part of Africa must have commenced at this time. The oldest oceanic crust that borders South America and sub-equatorial Africa is only about 120 Ma old. Accordingly, it follows that the North Atlantic Ocean started to form before the South Atlantic Ocean.If new sea floor is being created at spreading centres, then old sea floor must be being destroyed somewhere else. The oldest sea floor lies adjacent to deep ocean trenches, which are major topographic features that partially surround the Pacific Ocean and are found in the peripheral regions of other major ocean basins. The best known example is the Marianas Trench where the sea floor plunges to more than 11 km depth. Importantly, ocean trenches cut across existing magnetic anomalies, showing that they mark the boundary between lithosphere of differing ages. Once this association had been recognised, the fate of old oceanic crust became clear – it is cycled back into the mantle, thus preserving the constant surface area of the Earth.http://openlearn.open.ac.uk/mod/resource/view.php?id=172175
  • uplift: due to continental collisions, former low-lying surfaces are forced up creating mountains and plateausAs soon as higher areas are formed, weathering and erosion beginsIn general, higher mountain ranges are younger, having experienced less erosionSubduction: sediments deposited in subduction zones are transported into the earth’s mantle; igneous rocks that were never eroded also enter subduction zonesMetamorphism: subducted material undergo intense heat and pressure, are transformed into magma in the upper mantle, eventually to re-surface through mid-ocean ridges
  • Oceanic Lithosphere - 1

    1. 1. Global Jeofiziğe Giriş Okyanusal Litosfer P RO F. D R . A L I O S M A N Ö N C E LMÜHENDISLIK BİLİMLERİ-GLOBAL JEOFIZIK-MUBİ7004
    2. 2. Ofis saatleri çizelgesiDr. Ali O. Oncel : Jeofizik, II. Kat (oncel@istanbul.edu.tr aliosman.oncel@gmail.com) Salı. 14.00-16:00Her zaman e-mail yoluyla randevu alabilirsiniz yada açıkkapı politikasına dayanarak beni görmeye gelebilirsiniz . Today’s class: Oceanic Lithosphere Reading: Fowler Chapter 9, pp.391-416
    3. 3. DERS ÖĞRETİM İZLENCESİ Global Jeofizik İçin ders içerikleria. Bölüm 1: Dünya’nın İç Yapısıb. Bölüm 2: Okyanusal Litosferc. Bölüm 3: İç yapıyla ilgili sismolojik ölçümlerd. Bölüm 4: Levha Hareketlerie. Bölüm 5: Isı Akısı http://geop503kfupm.pbwiki.com/CourseSyllabus
    4. 4. Ders kitabı: The Solid Earth by C.M.R. Fowler-2005 DerecelendirmeGörevler 10 %
    5. 5. OKYANUS ÇUKURLARIThe crust and lithospherethicken away from the rift.This is compensated byIsostasy and the crust upliftedin the rift-shoulders.Spreading rates are generallyquoted as half plate-separation rate. Forexample, Mid Atlantic Ridgeis spreading at a rate of 1 cmyr-1 but the North AmericanPlates and Eurasian platesare separating atapproximately 2 cm-1. Figure 9.1 of Fowler’s book
    6. 6. KITASAL SINIRLAR• Continental Shelf – part of the continental margin between the coast and continental slope; slopes 0.1 degree (1:500)• Continental Slope – part of the continental margin between the continental rise and the continental shelf; slopes about 3- 6 degree (1:10 to 1:20)• Continental Rise – Part of continental margin between the continental slope and abyssal plain; slopes generally 1:40 to 1:2000 Modified after Figure 9.2 of Fowler’s book• Abyssal Plains – deep, old ocean floor; well sedimented.
    7. 7. KITASAL DEPREMLER(1929) and Turbidity Currents
    8. 8. CONTINENTAL MARGIN: EXAMPLES Difference between the narrow Wider continental shelf on shelf on the west coast and wider shelf on the east coast the passive continental which is passive margin. margin around Britain South America Bathymetry map of the UK and Europe Passive Active Margin Margin Peru-Chile Trench
    9. 9. Europe Africa SouthAmerica
    10. 10. Question: The width of ocean floor between the spreading ridge in the South AtlanticOcean at 30 S and the edge of the continental shelves along the east coast of SouthAmerica and the west coast of southern Africa at 3 S is approximately 3100 and 2700 kmrespectively. Assuming that the spreading rate on this segment of the ridge is 38 mm y−1,estimate the maximum age of the sea floor on either side of the South Atlantic.
    11. 11. North America Europe Atlantic OceanSouth America basin Africa
    12. 12. Profile through the Mariana Trenchhttp://www.aquarius.geomar.de/omc/make_map.html
    13. 13. Age of the Ocean Floorhttp://www.ngdc.noaa.gov/mgg/image/crustageposter.jpg
    14. 14. CLASSIFICATION OF IGNEOUS ROCKS What is the Igneous rock? A rock or mineral that solidified from molten or partly molten material, i.e. from a magma Etymology: Latin ignis, fire” How do we classify the igneous rocks?Simple classification based on Silica content (SiO2) can bedone (see Table 9.1) (silica content)66 wt. % - Acid (eg rhyolite) (“felsic”)52-66 wt% - Intermediate (eg andesite)45-52 wt% - Basic (eg basalt) (“mafic”)< 45 wt % - Ultrabasic (eg peridotite) (“ultramafic”)pp.395-397 of Fowler’s book
    15. 15. The Crust and Lithosphere  Continental crust: thicker, less dense, older 20 km (weathered areas) to 75 km (younger areas) thick  Oceanic crust: thinner, more dense, more recent ~7km thick  Lithosphere: the crust and uppermost (rigid) mantle This system is dynamic, not static!http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Inside.shtml
    16. 16. Rock Cycle http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Inside.shtml Mountains 1 Sedimentary (all types) erosion, Basins 2 5 subduction uplift sedimentation Sea floor Frei system diagram Subduction Zone (igneous) (all types) 4 extrusion, Aesthenosphere metamorphism volcanism (metamorphic) 3From: Modified after Lecture from Professor Frei
    17. 17. FELSIC VS. MAFIC Felsic Mafic• Light in color, includes • Dark in color, is rich in minerals with a lot of magnesium and iron aluminum and silicon . (Fe). Mafic Felsic Granite Gabbro clearly visible crystals dark, visible minerals
    18. 18. Granite Felsic Rhyolite Acid clearly visible crystals few visible crystals IntermediateDiorite Intermediate Andesite visible felsic and mafic crystals mainly felsic crystals visible
    19. 19. Peridotite UltramaficGabbro Basalt Mafic Basic dark, visible minerals dark, with no visible crystals
    20. 20. BASALTIC ROCKSBasaltic rocks on Earth, Moon and Mars commonlycomprise three minerals: olivine (Mg-Fe silicates, pyroxene(Ca-Mg-Fe silicates) and plagioclase feldspars (Ca-Na-Alsilicates) Olivine in basaltOlivine - Mg2SiO4 to Fe2SiO4Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6Plagioclase - CaAlSi3O8 to NaAlSi3O8
    21. 21. BASALTIC MAGMA Most basalts on Earth erupted from Middle OceanicRidges (MOR’s) and basaltic magma is an indicator ofextensional environments. Large volumes of basalts also erupted (in past) fromcontinental rifts and formed a distinctive composition and arecalled as Continental Flood Basalts (CFB). Similar floodbasalts recognized on Moon, Venus and Mars MOR basalt (MORB) has distinctive composition(particularly trace elements) that in most cases suggest theyare sourced from mantle that has already been previouslymelted (“depleted”) and hence lost some of its lower meltingcomponents.
    22. 22. ANDESITIC MAGMA St Helens is part of the Cascades (magmatic arc)Andesitic magmas are most commonly generated by subduction of the Juan de Fucaassociated with arcs (island arcs or continental (oceanic) plate beneath the North Americamagmatic arcs). Note these are both continental plate Mt St Helens
    23. 23. ANDESITIC MAGMAArc of igneous rocks (plutonic andvolcanic) can be on continent (ashere) or an island arc in oceans.Magma (which is most commonlyandesite) is generated above thedescending slab due to loweringof mp of lithosphere by presenceof volatiles (H20) released fromslab (and sediments). The magmas generated have a composition more silicic than basalt mainly because they are contaminated by passage through the silica-rich continental crust (as shown)
    24. 24. ANDESITE LINE • Geographic boundary between the basalt/gabbro of the Pacific Ocean basin and the andesites at the subductive margins of the surrounding continentsFrom: http://www.eos.ncsu.edu/eos/info/mea/mea101_info/chapters_color/KimColor6_04.html

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