1. Sea-Floor Spreading

2. Sea-Floor Spreading
Tubeworms have no mouth, eyes, or stomach. These bacteria convert the chemicals that shoot out of the deep sea vents into food for the worm.
What do they have to do with sea-floor spreading?

3. 1. Mid-Ocean Ridge – the longest chain of underwater mountains in the world
How did scientist discovered them?

4. Sonar
A device that bounces sound waves off underwater objects and then records the echoes of these sound waves.
Who mapped the ocean floor?

5. Harry Hess An American geologist who studied mid-ocean ridges.
He suggested that the ocean floors move like conveyor belts, carrying the continents along with them.

6. Conveyor belt
What is seafloor spreading?

7. Sea-floor spreading
The process that continually adds new material to the ocean floor.
What’s the evidence?
Evidence: molten material, magnetic stripes, and drilling samples.

8. How does it happen?
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.
As the molten material cools, it forms a strip of solid rock in the center of the ridge.
Then more molten material flows into the crack.

9. Let’s see it in action!

10. Evidence #1 - Molten Material
The submersible, Alvin, found strange rocks shaped like pillows or like toothpaste squeezed from a tube. Such rocks can form only when molten material hardens quickly after erupting under water.

11. Evidence from molten material
Rocks shaped like pillows (rock pillows) show that molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly

12. Evidence #2 - Magnetic Stripes
Scientists discovered that the rocks that make up the ocean floor lie in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field.
compass

13. Evidence #3 - Drilling Samples
The Glomar Challenger did a drilling sample and found that the older rocks were the ones farther away from the center. The younger ones were in the center of the ridge.

14. Stop Here
Summarize

15. Subduction at Deep-Ocean Trenches
Wider & wider? Deep-ocean trenches
Ocean floor plunges into deep underwater canyons are deep-ocean trenches.
Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

16. Subduction
At deep-ocean trenches, subduction allows part of the ocean floor to sink back into mantle, over tens of millions of years.
Subduction and Earth’s Oceans
Earth’s ocean floor is renewed about every 200 million years.

17. Subduction in the Pacific & Atlantic
Deep ocean trenches are swallowing more oceanic crust than the mid-ocean ridge can produce. Thus, the width of the Pacific will shrink.
The Atlantic is expanding. It has short trenches. In some places, the oceanic crust is attached to the continental crust which moves the continents.