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Sea-Floor Spreading I. Mapping the Mid-Ocean Ridge • Mid-ocean ridge – longest chain of mountains in the world • Curves along the sea floor • Extends into all of Earth’s oceans I. Mapping the Mid-Ocean Ridge • Discovered by use of sonar used to map ocean floor • Acronym SOund Navigation And Range II. Evidence of Sea-Floor Spreading • Harry Hess (Am. geologist) studies mid-ocean ridges. • He thought ocean floors moved, carrying continents with them (like conveyor belts). II. Evidence of Sea-Floor Spreading • Molten material from the mantle rises at the mid0cean ridge and erupts. • Molten material spreads out, pushing older rock to both sides • Cools and forms solid strip of rock • Continually adds new material to ocean floor II. Evidence of Sea-Floor Spreading Illustrate it! II. Evidence of Sea-Floor Spreading • pillow lava along midocean ridge • form only when molten material erupts and hardens quickly • shows this material erupted over and over along mid-ocean ridge II. Evidence of Sea-Floor Spreading Evidence from magnetic stripes: 1. Earths magnetic poles have reversed themselves 2. Ocean floor rock lies in a pattern of magnetized stripes 3. Pattern is same on both sides of ridge II. Evidence of Sea-Floor Spreading Evidence from drilling samples: 1. Samples from sea floor were brought up through pipes 2. Age of rock was determined 3. Oldest rocks were furthest from ridge III. Subduction at Deep-Ocean Trenches • Ocean floor bends downward into deepocean trenches • Subduction takes place--ocean floor sinks into mantle • New oceanic crust forms at mid-ocean ridge and pushed old crust to the side • Allows part of ocean floor to sink into mantle over millions of years III. Subduction at Deep-Ocean Trenches IV. Subduction and Earth’s Oceans Atlantic Ocean • is expanding • has only a few short trenches • new material has nowhere to go • pushes continents along with it • ocean gets wider Pacific Ocean • is shrinking • subduction happens faster that new material is produced at mid-ocean ridge • ridge does not add new material fast enough • ocean gets smaller