Sea-Floor

1. Sea-Floor Spreading

2. Mid-Ocean Ridge • The Mid Ocean Ridge is the longest chain of mountains in the world.

3. Mid-Ocean Ridge • The mid-ocean ridge is more than 50,000 kilometers long. • That’s longer the total circumference of the earth!

4. Mid-Ocean Ridge • A steep-sided valley splits the top of the mid-ocean ridge for most of its length.

5. Mid-Ocean Ridge • The valley is almost twice as deep as the Grand Canyon.

6. Mid-Ocean Ridge • The mid-ocean ridge curves like the seam of a baseball along the sea floor, extending into all of Earth's oceans.

7. Mid-Ocean Ridge

8. Mid-Ocean Ridge

9. Mid-Ocean Ridge • Iceland is a part of the mid-ocean ridge that rises above the surface in the North Atlantic Ocean.

10. Mapping the Mid-Ocean Ridge • Scientists use sonar to map the ocean floor.

11. Evidence for Sea-Floor Spreading • The ocean floors move like conveyor belts, carrying the continents along with them. • This movement begins at the mid-ocean ridge. • At the mid-ocean ridge, molten material rises from the mantle and erupts. • The molten material then spreads out, pushing older rock to both sides of the ridge.

12. Evidence for Sea-Floor Spreading • The process that continually adds new material to the ocean floor is sea-floor spreading. • The sea floor spreads apart along both sides of the mid-ocean ridge as new crust is added.

13. Evidence for Sea-Floor Spreading New material is indeed erupting along the mid-ocean ridge.

14. Evidence From Drilling Samples   • Samples from the sea floor have been brought up through drilling pipes. • Scientists have determined the age of the rocks in the samples. • They found that the farther away from the ridge the samples were taken, the older the rocks were. • The youngest rocks were always in the center of the ridges. • This showed that sea-floor spreading really has taken place.

15. Subduction at Deep-Ocean Trenches • The ocean floor plunges into deep underwater canyons called deep-ocean trenches. • A deep-ocean trench forms where the oceanic crust bends downward. • Where there are deep-ocean trenches, subduction takes place. • Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

16. Subduction

17. Subduction at Deep-Ocean Trenches • Convection currents under the lithosphere push new crust that forms at the mid-ocean ridge away from the ridge and toward a deep-ocean trench. • New oceanic crust is hot. But as it moves away from the mid-ocean ridge, it cools and becomes more dense. • Eventually, gravity pulls this older, denser oceanic crust down beneath the trench • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle

18. Subduction at Deep-Ocean Trenches

19. Subduction and Earth's Oceans • The processes of subduction and sea-floor spreading can change the size and shape of the oceans. • Because of these processes, Earth's ocean floor is renewed about every 200 million years. • 200 million years is the time it takes for new rock to form at the mid-ocean ridge, move across the ocean, and sink into a trench

20. Subduction in the Pacific Ocean   • The vast Pacific Ocean covers almost one third of the planet. And yet it is shrinking. • The deep ocean trench swallows more oceanic crust than the mid-ocean ridge can produce. • The ridge does not add new crust fast enough, so the width of the ocean is shrinking. • This is happening to the Pacific Ocean, because it is ringed by many trenches.

21. Subduction in the Atlantic Ocean • The Atlantic Ocean is expanding. • The Atlantic Ocean has only a few short trenches. • As a result, the spreading ocean floor has virtually nowhere to go. • In most places, the oceanic crust of the Atlantic Ocean is attached to the continental crust of the continents around the ocean. • So as the Atlantic's ocean floor spreads, the continents along its edges also move. • Over time, the whole ocean gets wide.r • Which means one day we could have another…? • Supercontinent!