SeaFloor Spreading MidOcean Ridges an undersea mountain chain

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 midocean 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 midocean 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 midocean ridges than trenches so the ocean is getting bigger