1. Sea-Floor Spreading

2. I. Mapping the Mid-Ocean Ridge
Mid-ocean ridge – longest chain of mountains in the world
Curves along the sea floor
Extends into all of Earth’s oceans

3. I. Mapping the Mid-Ocean Ridge
Discovered by use of sonar used to map ocean floor
Acronym
SOund
Navigation
And
Range

4. II. Evidence of Sea-Floor Spreading
Harry Hess (Am. geologist) studies mid-ocean ridges.
He thought ocean floors moved, carrying continents with them (like conveyor belts).

5. II. Evidence of Sea-Floor Spreading
Molten material from the mantle rises at the mid-0cean ridge and erupts.
Molten material spreads out, pushing older rock to both sides
Cools and forms solid strip of rock
Continually adds new material to ocean floor

6. II. Evidence of Sea-Floor Spreading
Illustrate it!

7. II. Evidence of Sea-Floor Spreading
pillow lava along mid-ocean ridge
form only when molten material erupts and hardens quickly
shows this material erupted over and over along mid-ocean ridge

8. II. Evidence of Sea-Floor Spreading
Evidence from magnetic stripes:
Earths magnetic poles have reversed themselves
Ocean floor rock lies in a pattern of magnetized stripes
Pattern is same on both sides of ridge

9. II. Evidence of Sea-Floor Spreading
Evidence from drilling samples:
Samples from sea floor were brought up through pipes
Age of rock was determined
Oldest rocks were furthest from ridge

10. III. Subduction at Deep-Ocean Trenches
Ocean floor bends downward into deep-ocean trenches
Subduction takes place--ocean floor sinks into mantle
New oceanic crust forms at mid-ocean ridge and pushed old crust to the side
Allows part of ocean floor to sink into mantle over millions of years

11. III. Subduction at Deep-Ocean Trenches

12. IV. Subduction and Earth’s Oceans
Atlantic Ocean
is expanding
has only a few short trenches
new material has nowhere to go
pushes continents along with it
ocean gets wider
Pacific Ocean
is shrinking
subduction happens faster that new material is produced at mid-ocean ridge
ridge does not add new material fast enough
ocean gets smaller