1. Sea-Floor Spreading

2. Sea-Floor Spreading
Sea-Floor Spreading –the process that continually adds new material to the ocean floor while pushing older rocks away from the ridge

3. Sea-Floor Spreading Deep in the ocean the water is near freezing.
There is no light
Life is rare
BUT near cracks in the ocean floor, tube worms exist.

4. Sea-Floor Spreading
Sonar - a device that bounces sound waves off under-water objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object.

5. Sea-Floor Spreading
Mid-Ocean Ridge – the longest chain of mountains in the world---these are divergent plate boundaries.

7. Sea-Floor Spreading
In 1960, Hess made his single most important contribution. In a widely circulated report Hess stated that the Earth's crust moved laterally away from long, volcanically active oceanic ridges

8. Sea-Floor Spreading

9. Ocean floor moves like a conveyor belt carrying continents with it.
New ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle spreading out and pushing older rocks to the sides of the crack. New ocean floor is continually added by the process of sea-floor spreading.

10. Sea-Floor Spreading
Evidence from Molten Material – Rocks shaped like pillows - show that molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly

11. Sea-Floor Spreading Evidence
In 1960, scientists used a submersible named Alvin, to see the ocean floor.

12. Sea-Floor Spreading Evidence

13. Sea-Floor Spreading
Evidence from Magnetic Stripes – Rocks that make up the ocean floor lie in a pattern of magnetized stripes which hold a record of the reversals in Earth’s magnetic field

14. Sea-Floor Spreading

15. Sea-Floor Spreading

16. Sea-Floor Spreading
Evidence from Drilling Samples – Core samples from the ocean floor show that older rocks are found farther from the ridge; youngest rocks are in the center of the ridge

17. Sea-Floor Spreading
Subduction – Process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle; allows part of the ocean floor to sink back into the mantle

19. Sea-Floor Spreading: Subduction zone
Deep-Ocean Trench – Occurs at subduction zones. Deep underwater canyons form where oceanic crust bends downward

20. Sea-Floor Spreading: Subduction zone - Pacific
Subduction in the Pacific Ocean
It is shrinking…how can this be?
A deep ocean trench swallows more oceanic crust
No new crust is being added

21. Sea-Floor Spreading: Subduction zone - Atlantic
Is expanding
Only a few short trenches
As a result, the spreading ocean floor has nowhere to go
So…as long as the Atlantic Ocean spreads so does the continents along its borders

22. Sea-Floor Spreading