NSSS1.4

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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 bounces sound waves off underwater objects and then record the echoes of these waves.
 * Sea-floor spreading**: The process that continually adds new material to the ocean floor.
 * Deep-ocean trenches**: Deep underwater canyons.
 * Subduction**:The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

Introduction

 * The Ocean Floor
 * Freezing temperatures
 * Crushing pressure
 * Most bizarre creatures ever found.
 * Ex. Giant red tube worms, Giant clams nearly a meter across and strange spider-like crabs

Mapping the Mid-Ocean Ridge

 * Mid-Ocean Ridge is the longest chain of mountains in the world.
 * Goes through all the oceans and around the world ( Only underwater )
 * Use sonar to locate it.
 * Sonar is a device that bounces sound waves off underwater objects and then records the echoes of these waves.

Evidence for Sea-floor Spreading

 * At the mid-ocean ridge, molten material rises from the mantle and erupts spreading molten material and pushing older rock to both sides of the ridge.
 * Harry Hess, an American geologist, was one of the scientists who studied the mid-ocean ridge.
 * Hess called the process that continually adds new material to the ocean floor is called sea-floor spreading.

Evidence from Molten Material
Went down in a submersible named Alvin.
 * In the 1960's, scientists found evidence of new material.
 * Scientists found pillow like lava ( forms when magma cools very quickly) on the ocean floor.

Evidence from Magnetic Stripes

 * Scientists have found patterns in the rocks on the ocean floor.
 * Evidence shows that the Earth's poles have reversed themselves.
 * This last happened 780,000 years ago.
 * ocean floor lies in a pattern of magnetic stripes.
 * These stripes hold the record of reversals in the Earth's magnetic field.

Evidence from Drilling Samples

 * //The Glomar Challenger//, a drilling ship built in 1968, sent drilling pipes through the water to dig deep holes in the ocean floor.
 * It sent the pipes 6 kilometers down.
 * Samples were brought up through the pipes.

Subduction at Deep-Ocean Trenches

 * A deep ocean trench forms where the oceanic crust bends downward.
 * Suduction occurs at these places.
 * At deep ocean trenches, subduction allows part of the ocean floor to sink back into the mantle.

Subduction and Earth's Oceans

 * The ocean floor is renewed every 200 million years.
 * This is the time it takes the rocks to form at the mid ocean ridge.

Subduction in the Pacific Ocean

 * The Pacific Ocean covers one-third of our planet and it is shrinking.
 * The Pacific Ocean has a lot of trenches.
 * A deep ocean trench swallows more oceanic than the mid-ocean ridge can produce.
 * Then if the mid-ocean ridge does not add new crust fast enough it will shrink.
 * This is what is happening to the Pacific Ocean.

Subduction in the Atlantic Ocean

 * The Atlantic Ocean is expanding.
 * Unlike the Pacific, the Atlantic only has a few short trenches.
 * As a result the new ocean floor has nowhere to go.
 * Therefore the Atlantic Ocean is expanding.