JLKR+1.4

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Vocabulary
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 * mid-ocean ridge-** the longest chain of mountains in the world
 * sonar-** sonar is a device that bounces sounds waves off underwater objects and records the echoes of the waves
 * sea-floor spreading-** the process that continually adds new material to the ocean floor
 * deep ocean trenches-** when the ocean floor plunges into deep underwater canyuns
 * subduction-** the process by which the ocean floor sinks beneath a deep ocean trenches and back into the mantle

Outline
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 * Introduction**


 * Mapping the Mid-Ocean Ridge**
 * use sonar devices
 * Evidence for Sea-Floor Spreading**
 * **Evidence from Molten Material**
 * The presence of rocks showed that molten material has erupted again and again from cracks along the central valley of themid-ocean ridge.
 * **Evidence from Magnetic Strips**
 * Long ago the poles have reversed themselves due to sea-floor spreading.
 * **Evidence from Drilling Samples**
 * They found that the farther away from the ridge the samples were taken, the older the rocks were. The youngest rocks were always in the the center of the ridges.


 * Subduction at Deep Ocean Trenches**
 * **Subduction in the Pacific Ocean**
 * The Pacific Ocean is shrinking because the deep-ocean trenches swallow more oceanic crust than the mid-ocean ridge can produce.
 * **Subduction in the Atlantic Ocean**
 * The Atlantic Ocean is expanding because it has has few short trences. So the spreading ocean floor is going nowhere.