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Section 4: Sea-Floor Spreading What is the process of sea-floor spreading? What is the evidence for sea-floor spreading? What happens at deep-ocean trenches? Mid-Ocean Ridges Mid-ocean ridge is an undersea mountain chain that is part of a long system of mountains that wind beneath all of Earth’s oceans. Mid-Ocean Ridges Extends into all of Earth’s oceans Most of the mountains lie under the water There are a few places where the ridge pokes above the water Where does this occur? Iceland What Is Sea-Floor Spreading? In sea-floor spreading, the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added. As a result, the ocean floors move like conveyor belts, carrying the continents along with them. Sea-Floor Spreading Begins at the mid-ocean ridge Molten material that forms beneath the surface rises and erupts Older rock moves outward on both sides of the ridge As the molten material cools, it forms a strip of solid rock in the center of ridge Evidence for Sea-Floor Spreading Several types of evidence supported sea-floor spreading: •Eruptions of molten material Evidence for Sea-Floor Spreading •Magnetic stripes in the rock of the ocean floor Evidence for Sea-Floor Spreading •The ages of the rocks themselves from drilling Deep-Ocean Trenches If the ocean floor is spreading, is our Earth getting larger? NO! The ocean floor sinks beneath deep under water canyons called deep-ocean trenches Subduction at Trenches In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches. The Process of Subduction Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle. Ocean floor closer to a mid-ocean ridge moves away from the ridge and toward a trench. Sea-floor spreading and subduction work together. They move the ocean floor as if it were on a giant conveyor belt. What is happening to the Atlantic Ocean? Growing an Ocean Because of sea-floor spreading, the distance between Europe and North America is increasing by a few centimeters per year.