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MAGNETIC FIELD Some of our most important evidence of sea-floor spreading comes from magnetic reversals recorded on the ocean floor The magnetic poles can change place (called magnetic reversal) Magnetic mineral grains line up in opposite direction of magnetic field EARTH’S MAGNETIC FIELD If you could cover our entire planet with iron filings, they would form a similar pattern MID-OCEAN RIDGES • Mid-ocean ridge- an undersea mountain chain where new ocean floor is produced • Sonar is used to map the ocean floor • Most mountains in mid-ocean ridge are under hundreds of meters of water • Iceland is part of a mid-ocean ridge that rises above the surface Harry Hess (geologist) examined maps of mid-ocean ridges and thought: Maybe Wegener was right about continental drift! Process of Sea-floor Spreading Sea floor spreads apart along both sides of mid-ocean ridge Magma flows to the opening and then cools to form new sea floor. Sea floor continues to spread apart as new crust is added. Processes repeats like a conveyor belt, carrying continents along with them. EVIDENCE FOR SEA-FLOOR SPREADING Evidence supported Hess’s theory of sea-floor spreading: eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves. Because of sea-floor spreading, the distance between Europe and North America is increasing by a few centimeters per year. SUBDUCTION AT DEEP-OCEAN TRENCHES • Ocean floor doesn’t just keep spreading • part of the ocean floor sinks back into the mantle through deep-ocean trenches. Sea-floor Spreading https://app.discoveryeducation.com Key Terms DUE TOMORROW 1. Fault 2. Divergent boundary 3. Convergent boundary 4. Transform boundary 5. Rift Valley