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9-4 Sea Floor Spreading. What is the process of sea-floor spreading? What happens to the ocean floor at deep ocean trenches. Mapping the Mid-Ocean Ridge. During the mid 1900’s sonar made it possible to map the ocean floor.
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9-4 Sea Floor Spreading What is the process of sea-floor spreading? What happens to the ocean floor at deep ocean trenches.
Mapping the Mid-Ocean Ridge • During the mid 1900’s sonar made it possible to map the ocean floor. • Prior to mapping it was believed that the abyssal plain was smooth and featureless. • Mapping with sonar revealed the Mid Ocean Ridge MOR
Features of the MOR • Longest mountain chain on the earth • Rift valley at its center that is deeper than the Grand Canyon. • Volcanoes along the ridge at various locations Surtsey Iceland.
Sea floor Spreading • Harry Hess US Naval Officer • Observed Magnetic anomalies in the Ocean Crust • Proposed that the ocean floors move like conveyor belts carrying along the continents. • At the mid ocean ridge, molten material rises from the mantle and spreads out pushing older rock to the side.
Sea floor spreading is supported by • New molten material discovered by deep ocean submarines like “Alvin” at the MOR • Magnetic Banding running parallel to the sea floor caused by shifts in the earth’s magnetic field about every 780,000 years • Rock samples from deep ocean drilling show that rocks get older as you move outward in both directions from the ridge.
Subduction at deep ocean trenched • Subduction is the process by which crustal material is drawn deep into the earth and recycled to become new crust at divergent boundaries. • Subduction occurs at deep ocean trenches found where the crust is being pushed together. (Convergent boundaries) • The ocean floor is renewed in this process about every 200 million years • Most subduction zones (deep ocean trenches) are found n the pacific
Summary • What is the role of the mid ocean ridge? • At the mid ocean ridge, molten material rises from the mantle and erupts. This material then spreads out pushing older rocks to both sides of the ridge • Describe the process of subduction at deep ocean trenches. • In the process of subduction, oceanic crust sinks down beneath the trench into the mantle. • What is the evidence for sea floor spreading? • Evidence of sea floor spreading includes molten material, magnetic stripes, and evidence from drilling samples • Where would the oldest rocks in the ocean floor be found? • The oldest rocks would be found the farthest away from the MOR.