1. Sea-Floor Spreading

2. Mapping the Mid-Ocean Ridge
Mid-ocean ridge – the longest chain of mountains in the world; located along the sea floor
Mapped in the mid-1900s using sonar

3. Sonar
Sonar – device that bounces sound waves off underwater objects and then records the echoes of the sound waves
The time it takes for the echo to arrive indicates distance to object

4. Mid-Ocean Ridge
MOR curves like the seam of a baseball along the sea floor; it extends into all oceans
It is more than 50,000 km long; more than 31,000 miles

5. Mid-Ocean Ridge
Most mountains of MOR are hidden under hundreds of meters of water
MOR does rise above the surface in some places – Iceland, Surtsey

6. Mid Ocean Ridge
Most of the top of the MOR is split by a steep valley – twice as deep as the Grand Canyon

7. Evidence for Sea-Floor Spreading
Harry Hess – American geologist; studied MOR; reconsidered Wegener’s theory!

8. Harry Hess’s Theory of SFS
In 1960 Hess suggested ocean floors moved like conveyer belts, carrying continents with them.
Movement begins at the MOR
MOR forms along a crack in the oceanic crust

9. The Process of Sea-Floor Spreading
At the MOR molten material rises from the mantle and erupts.
The molten material spreads out to both sides of the MOR
The newly erupted material pushes the older material further out to both sides of the MOR
With each new eruption, molten material rises splitting apart the solid rock formed before, pushing it aside.
New material is continually being added to the ocean floor, while older material is pushed further away until it is subducted in a DOT and returns to the mantle where it is melted back into molten material.

10. 3 Types of Evidence for SFS
Molten Material
2. Magnetic Stripes
3. Drilling Samples

11. 1. Molten Material
In 1960s evidence is found of new material erupting again and again along the central valley of the MOR!
ALVIN-a small submersible that discovered “pillow lava” which forms when hot molten material hardens on the outside quickly in the cold ocean water. Each pillow expands until it bursts, allowing the molten material to flow out forming the next pillow.

12. 2. Magnetic Stripes
Scientists discover rock that makes up the ocean floor lies in a matching pattern of magnetized stripes on both sides of the MOR!
Ocean floor began as molten material; when it cooled and hardened the iron bits inside it lined up in the direction of Earth’s magnetic poles, creating a permanent “magnetic memory”
Earth’s magnetic field was reversed at one time!

13. 3. Drilling Samples
Drilling Samples – rock samples obtained by drilling into the ocean floor
Glomar Challenger – drilling ship built in 1968; samples from the ocean floor (6km underwater) were brought up through pipes
Age of rock samples were determined by scientists; discovered that the rocks farther away from the MOR were older and youngest rocks were always in the center of the ridge; matching pattern on both sides of MOR

14. Subduction at Deep-Ocean Trenches
Deep-Ocean Trench - a deep underwater canyons where the oceanic crust bends downward
Subduction – the process by which the ocean floor sinks beneath a DOT and back into the mantle
Convection Currents under the lithosphere push new crust formed at the MOR away from the MOR and towards the DOT
Hot, new oceanic crust cools and becomes more dense as it moves away from the MOR
Gravity pulls the older, denser oceanic crust down the DOT

15. SFS and Subduction
At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years
Sea-floor spreading and subduction work together as if the ocean floor were moving out from the MOR on a giant conveyer belt

16. Subduction and Earth’s Oceans
Processes of subduction and sea-floor spreading can change the size and shape of the oceans
It takes 200 million years for the ocean floor to be renewed – for new rock to form at the MOR, move across the ocean, and then sink into a DOT
Pacific Ocean is shrinking! Ringed by many DOT that swallow more oceanic crust than the MOR can make
Atlantic Ocean is expanding! Has only a few short DOT, most spreading oceanic crust is not being subducted

17. Sea-Floor Spreading
Define: mid-ocean ridge, sonar, sea-floor spreading, deep-ocean trench, subduction
List the evidence for sea-floor spreading
What is Alvin/ Glomar Challenger?
Describe the process of sea-floor spreading
Describe the process of subduction
Where are the oldest and youngest rocks on the ocean floor? Explain
Which process is a constructive force? Destructive force? Explain