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Transcript
Ocean Floor - Plate Tectonics Lecture Notes • Early mapmakers noticed the apparent fit of continents on either side of ocean (matching coastlines) • Continental drift (Wegener)- Earth’s continents had once been joined as Pangaea • Similar fossils of plants and animals found on separated continents with different climates, proves that these continents were once closer to equator • Most scientists rejected Wegener's hypothesis– Could not explain how or why continents moved • Studies of ocean floor discovered vast, underwater mountain chains and ocean ridges • Rock samples taken from areas near ocean ridges were younger than samples taken from areas near deep sea trenches • Oldest seafloor is 180 million years old • Magnetic reversal- change in earth’s magnetic field – Same as present field- normal polarity – Opposite to present- reversed • Magnetic pattern on one side of a ridge is mirror image of pattern on other side • Seafloor spreading- new ocean crust is formed at ocean ridges and destroyed at deep sea trenches – Magma forced toward crust – Fills gaps and hardens – Forms new ocean floor • Theory of plate tectonics- earths crust and upper mantle are broken into slabs called plates –Plates interact at boundaries • Divergent boundary– Moving apart – Most are found on seafloor – Form ocean ridges – When on continent, form rift valley • Convergent boundary– – – – – Moving toward each other Classified according to type of crust involved Oceanic crust mostly basalt Continental crust granite and sedimentary rock Oceanic-oceanic- subduction occurs (one plate descends) creates deep sea trench, forms arc of volcanic islands: Mariana trench and islands • Oceanic-continental: oceanic is subducted, volcanoes erupt- PeruChile trench • Continental- continental: mountains form (Himalayas) • Transform boundary- plates sliding past each other – Crust is deformed or fractured – Long faults – Shallow earthquakes Late Paleocene 59 mya Early Eocene 52 mya SJVRocks!! CSUBakersfield Department of Late Miocene 10 myaGeological Sciences Pliocene 4 mya Early Miocene 21 mya Pleistocene 0.6 mya San Andreas Fault • Convection- transfer of energy between Earth’s hot interior and cooler exterior – Hot mantle less dense than cooler mantle – Hot mantle forced upward to crust – Cooler parts sink back down towards the core • Rising part of convection current spreads out as it reaches the upper mantle and causes both upward and side to side forces – Lift and split the lithosphere at divergent boundaries, Material rising from mantle hardens forms new crust – Downward part occurs where a sinking force pulls plates downward at convergent boundary Mantle Convection • Ridge push- forces in mantle cause asthenoshere to rise – Pushes oceanic plate toward trench – Could create drag on lithosphere • Slab pull- sinking region of mantle convection sucks oceanic plate downward