1. Sea-Floor Spreading Key Concepts
What is the process of sea-floor spreading?
What is the evidence for sea-floor spreading?
What happens at deep-ocean trenches?

2. Key Terms Mid-ocean ridge Sonar Sea-floor spreading Deep-ocean trench
subduction

3. Mapping the Ocean Floor
Sonar was used in the mid 1900’s to map the ocean floor
Sonar bounces sound waves off objects and records the echoes of these waves.
Time it takes for echo to come back indicates the distance from the object
Sonar discovered the mid-ocean ridges

5. Mid-Ocean Ridges
Mid-ocean ridge system is a chain of underwater mountains that extend into all of Earth’s oceans.
They are more than 50,000 km long
Most are under hundreds of meters of water
They reach the surface in a few places such as Iceland.
A steep sided valley splits the top of some mid-ocean ridges.

6. MID-OCEAN RIDGES

7. Mid- Ocean Ridges

8. Sea Floor Spreading
In 1960 Harry Hess, an American geologist, suggested that the sea floor spread 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 continents with them.
He called this sea-floor spreading
Molten material erupts through the valley in the center of the ridge. Hardens and cools to form new sea floor.

9. Sea Floor Spreading

10. Evidence for sea floor spreading
From Molten Material
Alvin, The world’s first deep ocean submersible used for deep sea exploration, found strange rocks shaped like pillows in the mid-ocean ridge. This was evidence that molten material came out of the ridge and hardened quickly

11. Bulbous Pillow Lava

12. Evidence from Magnetic Stripes
Scientists studied the rock patterns in the ocean floor .
They found that the rock on the ocean floor lies in a pattern of magnetized stripes
Magnetic stripes in ocean floor rocks are formed by the reversal of the Earth’s magnetic poles.
The last reversal happened 780,000 years ago.
The pattern of stripes matches on each side of the ocean ridge.

13. Magnetic stripes

14. Evidence from drilling samples
Glomar Challenger gathered samples of rocks by drilling into the ocean floor
Drilled through 6 km of water into ocean floor
Scientists then determined the age of the rocks
They found that the rocks next to ocean ridges are younger than rocks farther away.
As the ocean floor spreads the older rocks move farther away.

15. Glomar Challenger

16. Is the Earth’s surface getting larger from sea-floor spreading?

17. Subduction at Trenches
Deep ocean trenches are deep underwater canyons
In a process that takes tens of millions of years, the ocean floor sinks back into the mantle at deep ocean trenches.
Subduction is the process where the ocean floor sinks beneath a deep ocean trench.

18. Subduction zones Oceanic crust and Continental crust
Oceanic and Oceanic Crust

19. The Process of Subduction
Sea-floor spreading and subduction work together like a conveyor belt.
New oceanic crust is hot.
As it moves away from the ridge it cools, and becomes more dense
At a trench, the older, more dense crust is pulled by gravity and sinks down beneath the trench

20. Sea floor spreading and subduction

21. Subduction and the Earth’s Oceans
Sea-Floor spreading and subduction can change the size and shape of the oceans.
Ocean floor is renewed every 200 million years – the time it takes for the floor to travel from ridge to trench.
The Pacific Ocean is shrinking. More crust is being subducted than is being formed.
The Atlantic Ocean is expanding. More crust is being formed than is being subducted.

22. References http://www.whoi.edu/page.do?pid=8422