Sea-Floor Spreading

Mid-Ocean Ridges an undersea mountain chain where new ocean floor is produced Discovered by the use of sonar – device that determines the distance of an under water object by recording echoes of sound waves The time it takes for the echo to arrive tells the distance to the object Most mountains in mid-ocean ridges are hidden under hundreds of meters of water

Mid-Ocean Ridges

So, one may ask… What are these ridges? How do they form?

What is Sea-Floor Spreading? Harry Hess studied mid-ocean ridges and thought… Maybe Wegener was right! In 1960, introduced idea of 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 cont… 1. Begins at mid-ocean ridge that forms along a crack in the oceanic crust 2. Along ridge, molten material that forms beneath surface rises and erupts 3. Older rock moves outward on both sides of ridge

So…How can the ocean floor keep from NOT getting wider and wider?

Subduction at Trenches Ocean floor DOES NOT just keep spreading Ocean floor plunges into deep under water canyons called deep-ocean trenches At trench oceanic crust bends downward Part of the ocean floor sinks back into the mantle at these trenches Process takes millions of years

Process of Subduction Subduction – process in which the ocean floor sinks beneath deep ocean trenches and back into mantle As subduction occurs, crust closer to mid-ocean ridge moves away from ridges and toward trench Sea floor spreading and subduction work together Move ocean floor like it’s a conveyor belt

Process of Subduction cont… New oceanic crust is HOT, but as it moves away from the mid-ocean ridge, it cools becoming more dense Gravity pulls denser crust down beneath trench

Subduction and Earth’s oceans In the Pacific Ocean there are more trenches than mid-ocean ridges so the ocean is shrinking In the Atlantic Ocean there are more mid-ocean ridges than trenches so the ocean is getting bigger