PSNB+1.4

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=1.4 Sea-Floor Spreading=

Vocabulary

 * Mid-Ocean Ridge:** The longest chain of mountains in the world.
 * Sonar:** A device that records the echo, by bouncing sound waves of under water objects.
 * Sea-Floor Spreading:** The process that keeps adding new material to the ocean floor.
 * Deep-Ocean trenches:** Where the ocean's crust bends backward.
 * Subduction:** Where the ocean floor sinks beneath an underwater trench.

Outline

 * ===Mapping the Mid-ocean Ridge===
 * Longest chain of mountains in the world
 * Lies underwater, some parts poke above surface
 * Scientists map it with sonar
 * Iceland is a part of the ridge
 * Ridge's existence made scientists curious, they investigated further
 * ===Evidence for sea-floor spreading===
 * Harry Hess studied mid-ocean ridge
 * He thought the ocean floor moves like a conveyer belt, carrying continents along
 * Also thought that molten material comes out of the ridge, spreading out and pushing older rock to both sides of the ridge.
 * believed this molten material cools and forms a strip of rock in center of ridge
 * **Evidence From Molten Material**
 * Scientists went on a submersile named Alvin to see evidence
 * They saw pillow/toothpaste type rocks, they can only be formed if molten material has hardened quickly
 * **Evidence From Magnetic Stripes**
 * Earth's magnetic poles reversed themselves 780,000 years ago
 * New rock from molten material is always pointing north, so when poles reversed, they were pointing different ways.
 * Scientists saw this and started believing sea-floor spreading
 * **Evidence From Drilling Samples**
 * Scientists drilled holes in ocean floor using a ship, they found the youngest rocks were in the center, the oldest rocks were away from the ridge
 * ===Subduction at deep ocean trenches===
 * Old oceanic crust sinks into deep ocean trenches (underwater canyons)
 * **Subduction In the pacific ocean**
 * The pacific ocean is shrinking because the ridge can not produce as much crust as the big trenches are swallowing
 * **Subduction in the Atlantic Ocean**
 * The Atlantic Ocean is growing, because the trenches are too small, so they can not swallow as much as the ridge is creates

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