1. Sea-Floor Spreading
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2. The Mid-Ocean Ridge The longest chain of mountains in the world
Extends into all of Earth’s Oceans
Most of the mountains lie under the ocean
In the mid 1900’s, scientists used sonar to map it

3. Evidence for Sea-Floor Spreading
Harry Hess proposed the following in 1960:
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.

4. Evidence 1 Molten Material
Alvin (A submarine) and its crew found molten material that had quickly hardened after erupting under water

5. Evidence 2 Magnetic Stripes
Reversal of magnetic field causes iron in ocean floor to line up in the direction of Earth’s magnetic poles
When the poles are reversed, the iron faces the opposite direction causing “stripes” in the ocean floor

6. Evidence 3 Drilling Samples
The Glomar Challenger, a drilling ship in 1968 gathered samples of the ocean floor
The age of the rock from the samples were older the farther away they got from the Mid-ocean ridge.

7. Subduction at Deep-Ocean Trenches
A deep-ocean trench forms where the oceanic crust bends downwards.
Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle

8. Subduction-cntd.
Convection currents push new crust at the M.O.R. away from the ridge and towards
New crust is hot, but it cools and becomes more dense, getting bulled below the trench
At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years

9. Subduction and Earth’s Oceans
Pacific Ocean- it is shrinking because subduction is happening faster than new crust is being added
Atlantic Ocean- it expanding because new crust is being faster than subduction