1. Sea-Floor Spreading

2. 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

3. Mid-Ocean Ridges

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

5. 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

7. 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

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

9. 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

11. 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

12. 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

13. 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