105_02-platetectonics_part_2 - Field normal Stripes...

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Unformatted text preview: Field normal Stripes explained Vine Vine and Matthews (1963); explained stripes by combination of sea-floor seaspreading and periodic magnetic reversals Field reversed The rock of oceanic crust preserves a record of the Earth’s magnetic polarity at the time the crust formed. Eventually, a symmetric pattern of polarity stripes develops. Field normal Age of the oceanic crust: • youngest along the mid-oceanic ridge mid• oldest along the continents Plate Plate Tectonics – A Unifying Theory Plate tectonics = Continental Drift + Geomagnetism + Seafloor Spreading Plate Tectonics – A Unifying Theory the the outer rigid lithosphere is divided into plates defined by ridges, trenches and transform faults the the lithosphere overlies a much weaker and hotter (plastic) asthenosphere the the material here is deformable and thus moves the overlying lithospheric plates The Supercontinent Cycle Continents Continents came together at end of Paleozoic Era (~250 Ma) to form Pangaea Pangaea Pangaea began to break up during Triassic Period (~200+ Ma) 1 Basic Plate Boundaries Boundaries Geologists define three types of plate boundaries, based simply on the relative motions of the plates on either side of the boundary. There are three basic types— divergent, convergent, and transform plate boundaries. Divergent plate boundary Diverging plate boundary Upwarping Rift Valley Linear Sea Ocean with MidMid-Ocean Ridge The Process of Rifting Rifting is the process by which a continent splits and separates to form a new divergent boundary. This animation shows the progressive formation and evolution of a continental rift, and the formation of a midmidocean ridge. The The Process of Rifting Red Sea East African Rift 2 Dots: Earthquake center Crosses: volcanic center Red Sea Rift Mediterranean Sea Jerusalem Nile delta Cairo Divergent: new ocean floor crust created as plates move apart. Mid ocean ridges. MidMid-Atlantic ridge: Divergent plate boundary Iceland: Iceland: rifting at a divergent plate boundary Rift zone at Þingvellir 3 Rift zone at Þingvellir Rift zone at Þingvellir – June 2007 Convergent Plate Boundaries At convergent plate boundaries or convergent margins, two plates move toward each other. One oceanic plate bends and begins to sink down into the asthenosphere beneath the other plate. OceanOcean-ocean convergent boundary: Island arc Japan OceanOcean-ocean convergent boundary: Island arc OceanOcean-ocean convergent boundary: Island arc Japan 4 Convergent Plate Boundaries Ocean - continent convergence: volcanic arc Continent - continent convergence: mountain range Himalayas Continent - continent convergence: mountain range Convergent Plate Boundaries 5 Formation of Himalayas at Convergent Plate Boundary Experiment with model clay Movement of the Indian plate to the North still goes on: 4.5 cm/yr Map shows direct GPS measurements Asia 2 cm/yr India Roof of the Earth: Mount Everest Nepal 6 Transform plate boundaries Plates Plates slide past one another, roughly parallel to the boundary Transform plate boundaries San San Andreas, California Tectonic plate movement How How do we find the speed with which the plates are moving? – Paleomagnetics Determine Determine plate position at different times 15 yrs of SLR data – Magnetic stripes Measures Measures spreading rate – Hot spots Stationary Stationary underneath plate: Hawaii – Direct measurement with satellite laser ranging techniques (SLR) 15 yrs of SLR data Tectonic plates Plate tectonic speed: 1-10 (20) cm/yr 1- 7 Lithosphere and Asthenosphere Heat Heat The driving force – Convection cells – Hot stuff rises – Cold stuff sinks Asthenosphere: 1% molten Deformable Moves lithosphere The driving force The Earth is a lava lamp! The driving force Plate Tectonics Chapter 2 8 ...
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