LPKW+1.4

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=1.4 Sea-floor spreading=

Vocabulary
mid-ocean ridge-longest chain of mountians in the world sonar-device that maps tha ocean floor sea-floor spreading-molten lava erupting from the mid-ocean ridge forming new oceanic crust deep ocean trenches-places where old oceanic crust sinks back in the mantle subduction-procces where old oceanic crust sinks back into the mantle

Outline
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Mapping the mid-ocean ridge

 * Using sonar to map the mid-ocean ridge

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Evidence for Sea-Floor Spreading

 * 1960 Harry Hess
 * ocean floor like a conveyor belts

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Evidence from Magnetic Stripes
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 * magnetic strips
 * some rocks formed when the Earths magnetic field switched

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Evidence from Drilling Samples
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 * rocks farther from the ridge were older
 * younger ones were closer to the ridge

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Evidence from Molten Material
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 * molten rocks
 * shaped like pillows

= = Subduction at deep ocean trenches

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 * deep-ocean trenches are places where old oceanic crust sinks
 * oceanic crust bends downward when subduction occurs
 * new oceanic crust is hot but eventtually cools down and becomes more dense

Subduction and Earth's Ocean

 * ocean floor renewed every 200 million years