Transcript Sea Floor Spreading: The Mid

Sea Floor Spreading:
The 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 1900’s, 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.
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.
Mid-Oceanic Ridge forming
This large fissure
observed on the crest of
the Endeavor Segment of
the Juan de Fuca Ridge
Evidence from Molten Material
• In the 1960’s, 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.
Evidence from Magnetic Strips
• Evidence shows that Earth’s poles have
been reversed throughout Earth’s 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
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.
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.
Sea Floor
spreading and
Subduction
Subduction and Earth’s 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