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Sea-Floor Spreading Introduction • Tube Worms - live in the Pacific Ocean about one mile deep near the hydrothermal vents. Mapping the mid-ocean ridge • The East Pacific Rise • • has a mid-ocean ridge. Curves around like a baseball. Lies hidden under hundreds of meters of water. Sonar • A device that bounces • sound waves off underwater objects and then records the echoes of these sound waves. Sonar mapped midocean ridges. Harry Hess • An American • geologist who studied mid-ocean ridges. He suggested that the ocean floors move like conveyor belts, carrying the continents along with them. • At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. As the molten material cools, it forms a strip of solid rock in the center of the ridge. Then more molten material flows into the crack. Sea-floor spreading • The process that • continually adds new material to the ocean floor. Evidence molten material, magnetic stripes, and drilling samples. Evidence #1 - Molten Material • The submersible, Alvin, found strange rocks shaped like pillows or like toothpaste squeezed from a tube. Such rocks can form only when molten material hardens quickly after erupting under water. Evidence #2 - Magnetic Stripes • Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field. Evidence #3 - Drilling Samples • The Glomar Challenger did a drilling sample and found rocks that the farther away from the ridge the older the rocks were. The younger ones were in the center of the ridge. Subduction at Deep-Ocean Trenches • Wider & wider? Deep• • ocean trenches Ocean floor plunges into deep underwater canyons are deep-ocean trenches. Subduction is the process by which the ocean floor sinks beneath a deepocean trench and back into the mantle. Subduction • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into mantle, over tens of millions of years. • Subduction and Earth’s Oceans • Earth’s ocean floor is renewed about every 200 million years. Subduction in the Pacific & Atlantic • Deep ocean trenches are swallowing more oceanic crust than the mid-ocean ridge can produce. Thus, the width of the Pacific will shrink. • The Atlantic is expanding. It has short trenches. In some places, the oceanic crust is attached to the continental crust which moves the continents.