Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
History of geology wikipedia , lookup
Physical oceanography wikipedia , lookup
Anoxic event wikipedia , lookup
History of geomagnetism wikipedia , lookup
Geomagnetic reversal wikipedia , lookup
Abyssal plain wikipedia , lookup
Oceanic trench wikipedia , lookup
Chapter Twelve Plate Tectonics: Creating Oceans and Continents Key elements of Plate Tectonics Earth’s lithosphere consists of rigid plates Plates move relative to one another by divergence, convergence, or transform motion Oceanic lithosphere forms at divergent plate boundaries and are consumed at subduction zones Most earthquakes, volcanism, faulting and mountain building take place at plate boundaries. Plate centers tend to be geologically stableKey Basic Assumptions of Plate Tectonic Theory Lithosphere consists of rigid plates (100 km average; 70 km for ocean & 150 km for continents) Plates move relative to one another by Divergence, Convergence, or Transform motion Formation of Oceanic lithosphere at divergent plate boundaries and is consumed at subduction zone Most earthquake activity, volcanism, faulting, and mountain building take place at plate boundaries Centers of plates are stable Plate velocity - determination Satellite based studies- plate motion and geomagnetic research Velocity of plates measures directly by ground-based laser which beams off reflectors on selected satellites Hot Spots- can serve as fixed reference point to measure absolute plate velocities Structures formed as plates move over hot spots also reveal speed and direction of plates Marine Magnetic Anomalies are used to estimate rates of divergence, and therefore plate motion Eco-sounder sonar- topography of sea floor and seismic profiling- study underlying layers Relative Plate velocities Relative Plate velocities-contd. Satellite Global system Mid-Pacific Hot spot Tracking Magnetic Field Reversals DISCOVERY OF MARINE MAGNETIC ANOMALIES (WW-II MAGNETOMETER OBSERVATIONS IN THE SHIP) VINE & MATHEWS’ HYPOTHESIS OF MAGNETIC REVERSALS (EVIDENCE OF SEA-FLOOR SPREADING AT DIVERGENT PLATE BOUNDARIES) MEASURING PLATE MOVEMENT BY MAGNETIC ANOMALIES (ANOMALY’S DISTANCE FROM SPREADING RIDGE) PLATE SPEED: 1 TO 10 CM PER YEAR Marine magnetic anomalies Marine magnetic anomaly-contd. Marine magnetic anomaly-contd. Directions and Rates of Plate movement Nature and Origin of the Ocean Floor Rifting rising mantle over hot spot- 3 radiating valleys. One fails to open and later becomes filled with sediments- aulacogen. When rifting stops- the rift edge becomes inactive tectonically and therefore have passive continental margin Divergent Plate Boundaries as divergence continues, full seaway forms and new oceanic lithosphere forms at the mid-oceanic ridge as up-welling ultramafic melt produce basaltic magma Transform Boundaries/offset mid-oceanic ridges transform fault becomes divided into short offset segments by ocean Information about the Ocean Floor – Echo-Sounding sonar, Seismic Profiling, Deep-Sea Drilling Project & Submersible vessels Active Rifting Rifting and Origin of Ocean Basins Active Arms marked by: – High heat flow – Normal faulting – Frequent shallow earthquakes – Widespread basaltic volcanism Rift valley widening leads to the development of new seaways and evaporite belts In Ocean, it leads to the formation of sediment deposition and development of Passive continental margins The East Africa Rift Zone The growth of oceanic basin Growth of ocean basin – contd. Growth of ocean basin – contd. Nature and Origin of the Ocean floor – contd. Oceanic trench forms where dense oceanic plate plunges (subducts) under less dense plates forming a depression in the earth’s surface Melange mixture of oceanic sediments and ophiolite rocks- form massive accretionary wedge that may be attached to the edge of the overriding plate Convergent collision of two plates resulting in suture zones Volcanic arc chain of volcanoes formed from subduction Structure of Oceanic Lithosphere Upper surface – 200 M: sediment of siliceous or carbonate ooze and/or reddish clay 200 m- 2 km: Oceanic basalt with top layer of pillow structures 3-6 km: Gabbro Below Gabbro is Peridodite Serpentinite formed by alteration of rocks by water – Entire sequence of ocean-floor rock may be altered its faults & fissures Structure of Oceanic Lithosphere-contd. H2O + Pyroxene (in basalt, gabbro) – chlorite H2O +Mg olivine (Umafic peridotite)Serpentine (Mg-Si mineral) Layer of Ophiolite suite Transform Boundaries & Offset Mid-Ocean Ridges Occur where plates slide past one another in opposite direction 15% total length of plate margins Faulted blocks move in opposite directions --produce stress --- leads to earthquakes Direction of motion of a plate Subduction-zone feature Breadth of arc-trench gap Breadth of arc-trench gap-contd Anatomy of a continent Continental shield Origin of a Supercontinent Origin of the supercontinent Pangaea Earth’s plates before Pangaea Gondwana: ~500 million yrs BP: Near South Pole; 4 Northern landmasses Laurasia: 3 Landmasses in the Northern Hemisphere; North America, Northern Europe, Southern Europe + Parts of Africa & Siberia Pangaea: Laurasia and Gondwana colloided to form Pangaea Future Events 50-100 Million yrs from now: Australia will collide with Southeast Asia Western part of California will separate from North America along the San Andreas fault and become a separate microcontinent Mediterranean will close as the African and Eurasian Plates collide Subduction may occur along the East coast of North America All of the Earth’s landmasses may reunite into another Pangaea-like supercontinent Driving Forces of Plate Motion Uncertainty on what drives Plate Motion Slow Asthenosphere convection currents Evidence against plate pushing by rising magma wedges at divergent boundaries Evidence in favor of plate pulling by descending slabs at subduction zones Gravity forces plates away from uplifted midocean ridge Convection Cells Hypothesis Deep convection cell hypothesis: Lower mantle to the surface of the earth Shallow convection cell hypothesis: Cells are driven within the Asthenosphere Three factors that may drive plate tectonics Convection cell hypothesis Convection cell hypothesis-contd. Configuration cell hypothesis Seismic Topography Seismic topography-contd. Thermal plumes Two patterns of Marine anomalies