Sea Floor Spreading: The Mid-Oceanic Ridge

1
Sea Floor SpreadingThe Mid-Oceanic Ridge

• The East Pacific Rise and the Mid-Atlantic Ridge
  are parts of the mid-ocean ridge, the longest
  mountain chain in the world.
• During the mid 1900s, scientists mapped the
  mid-ocean ridge using sonar.
• Most of the mountains are below the surface of
  the water, but there are places where the
  mountains poke above the surface.
• Iceland is an example of this.

2
Evidence of Sea Floor Spreading

• In 1960, Harry Hess, an American geologist,
  proposed the idea of sea floor spreading.
• At the mid-ocean ridge, molten material rises
  from the mantle adding new crust. The older
  material gets pushed to the sides. This is
  termed sea floor spreading.

3
Mid-Oceanic Ridge forming
This large fissure observed on the crest of the
Endeavor Segment of the Juan de Fuca Ridge
4
(No Transcript)
5
Evidence from Molten Material

• In the 1960s, scientists traveled within a
  submersible called the Alvin to an ocean depth of
  4 km along the ridge
• Scientists observed pillow shaped rocks that can
  only form from magma eruptions that harden
  quickly in water.

6
Evidence from Magnetic Strips

• Evidence shows that Earths poles have been
  reversed throughout Earths history. As iron in
  the molten material lines up toward the poles,
  new rock forming records the current magnetic
  position.
• This creates bands of magnetized rock that match
  up from each side of the ridge

7
(No Transcript)
8
(No Transcript)
9
Evidence from Drilling Samples

• In 1968, the Glomar Challenger, a drilling ship,
  obtained samples from the ocean bottom to
  determine the age of the rocks. The drill reached
  6km to the ocean floor.
• The scientist found that the samples further away
  from the ridge were older than those close to the
  ridge.

10
(No Transcript)
11
Subduction at Deep Ocean Trenches

• Rather than the ocean getting wider, the ocean
  floor plunges into deep ocean trenches
• Subduction is the process by which the ocean
  floor sinks back into the mantle
• Older oceanic crust is denser and so gravity
  pulls it down beneath the trench.
• This movement of the ocean floor is similar to a
  conveyor belt.

12
Sea Floor spreading and Subduction
13
Subduction and Earths Oceans

• Due to the processes of sea floor spreading and
  Subduction, the ocean floor is renewed every 200
  million years.
• The Pacific Ocean is the largest ocean, yet it is
  shrinking. This is due to a new number of
  trenches that may take in a greater amount of
  ocean floor that the mid-ocean ridge can restore.
• The Atlantic Ocean is expanding with only a few
  trenches