Seafloor spreading predicts that magnetic anomalies should be mirror images

Seafloor spreading predicts that magnetic anomalies

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Seafloor spreading predicts that magnetic anomalies should be mirror images across the mid- ocean ridge The Ocean Floor: - Reveals the location of: mid-ocean ridges, deep-ocean trenches, oceanic fracture zones The seafloor acts like a tape recorder that encodes, by magnetic imprinting, the history of the opening of an ocean basin in terms of the history of reversals of the geomagnetic field - Using the magnetic stratigraphy established on land (dated lava flows), geophysicists were able to determine how fast the ocean opened up
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Seafloor spreading: - Magnetic polarity reversals are imprinted as stripes in seafloor rock as the seafloor continues to spread - The width of the magnetic anomaly stripes is related to the spreading rate - Faster spreading = wide stripes - Slower spreading = narrow stripes - Isochrons: contours of equivalent age of the seafloor - They show the time that has elapsed and the amount of spreading that has occurred since the magnetized rocks were injected as lava into a mid-ocean rift Plate boundaries: - Lithosphere fragmented into 12 major tectonic plates - Plates move continuously at 1-15 cm/year - 3 types of boundaries: divergent, convergent, transform Divergent boundary: - Plates move apart - Lithosphere thickens away from the ridge axis - New lithosphere created at divergent boundary - Also called: mid-ocean ridge, ridge - Spreading edge: new and growing edge of a plate - Rifting leads to the formation of 3 spreading edges that meet at a triple junction - Typically, only 2 of the 3 spreading edges remain active on the site of seafloor spreading Divergent boundary seafloor spreading progression: - Early stage: rifting has progressed to mid-ocean ridge formation - Before substantial widening of the ocean, it forms a long, thin ocean basin with young oceanic crust - Mid-stage: ocean begins to widen - New seafloor is added at mid-ocean ridge - Continents move farther apart - Late stage: mature, wide ocean basin - Linear increase in age with distance from central ridge - Edge of ocean basin: oldest - Ridge proximal: youngest - eg. Atlantic Ocean Convergent boundary: - Plates move towards each other - Plate consumption = subduction - Also called: convergent margin, subduction zone, trench - 3 types of lithospheric destruction: oceanic-oceanic, oceanic-continental, c-c Oceanic-oceanic: subduction occurs because oceanic lithosphere, once it has aged more than 10 years, is denser than the asthenosphere and sinks under its own weight - Oceanic-continental: accretionary prism: wedge-shaped mass of sediment scraped off the subducting oceanic plate
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- Continental-continental: because continental crust is less dense than oceanic crust and cannot subduct, colliding lithospheric plates will ram each other - The rocks can’t be pushed down, so they have nowhere to go but up - Orogeny: mountain building, as continents ram into each other - eg. The Himalayas - Ophiolites: old oceanic lithosphere trapped between two colliding continental plates Transform boundary: - Plates slide sideways or past each other - Plate material is either created nor destroyed -
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