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Sea Floor Spreading
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Vocabulary
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 * Mid Ocean Ridge** - THe undersea mountain chain where new ocean floor is produced.
 * Sonar** - A system that determins the distance underwater by recording the echos.
 * Sea-floor Spreading** - THe process by which molten material adds new oceanic crust.
 * Subduction** - Process by which ocean floor sinks.

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 * ===Mapping the Mid-Ocean Ridge===
 * The East Pacific Rise is a part if the mid-ocean ridge
 * The mid-ocean ridge is the longest chain of mountains in the world.
 * A little bit of the mid-ocean ridge is on top of the water.
 * The island of Iceland is a part if the mid-ocean ridge that pokes on top of the water
 * Scientists use sonar to map the mid-ocean ridge
 * Sonar is a divice that bounces sound waves of underwater and records the echos of these sound waves.
 * ===Evidence for sea floor spreading===
 * Harry Hess was one of the scientists who studied the mid-ocean ridge.
 * He began to think about continental drift
 * He thought Wegener was right.
 * He thought the ocean floor moves like conveyor belts, taking the continents with them as they move.
 * This movement begins at the mid-ocean ridge.
 * The mid-ocean ridge forms along a crack in the oceanic crust.
 * Molten material rises from the mant;e and erupts
 * The molten material then spreads out, pushing the older rocks to the side.
 * As the molten material cools, it forms a strip of rock.
 * Harry Hess called this "sea-floor spreading"
 * ===Evidence from molten material===
 * In the 1960's scientists found evidence that new material is erupting along the mid-ocean ridge.
 * Scientists dived down to the bottom in a small submersible named "Alvin"
 * They saw strange rocks shaped like pillows or like toothpaste squeezed from a tube.
 * These rocks only can form when molten material hardens quickly after erupting underwater.
 * ===Evidence from magnetic stripes===
 * Scientists studied patterns in the rocks of the ocean floor, they found more evidence in sea-floor spreading.
 * Evidence shows that Earth's magnetic poles have reversed themselves 780,000 years ago.
 * The rocks that makes up the ocean floor lies in a pattern of magnetized "stripes"
 * These stripes hold a record if reversals in Earth's magnetic field.
 * Rock that hardens at the same time has the same "magnetic memory"
 * ===Evidence from drilling samples===
 * A ship called //The Glomar Challenge,// sent drilling pipes underwater to drill samples of the oceanic crust.
 * The farther away the rocks were, the older the rocks were.
 * ===Subduction at deep-ocean trenches===
 * A deep-ocean trench is a deep underwatercanyon.
 * Where there are deep-ocean trenches, subduction takes place.
 * Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.
 * ===Subduction and Earth's oceans===
 * Because of the process of subduction and sea-floor spreading, ocean floor is renewed about every 200 million years.
 * That's the time it takes to form at the mid-ocean ridge, move across the ocean, and sink into a trench.
 * ===Subduction in the Pacific Ocean===
 * The vast of the Pacific Ocean covers one third of the planet.
 * The pacific ocean is shrinking.
 * Sometimes a deep-ocean trench swallows more oceanic crust than the mid-ocean ridge can produce.
 * If the ridge doesn't add new crust fast enough, the width of the ocean will shrink.
 * ===Subduction in the Atlantic ocean===
 * The Atlantic Ocean is expanding.
 * The Atlantic Ocean only has a few short trenches.
 * The spreading ocean floor has nowhere to go.
 * Over time, the Atlantic Ocean gets wider.
 * Over time, the Atlantic Ocean gets wider.



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