9-4 Sea Floor Spreading
1.
2.
What is the process of sea-floor
spreading?
What happens to the ocean floor at
deep ocean trenches.
Mapping the Mid-Ocean Ridge
During the mid 1900’s sonar made it possible to
map the ocean floor.
Prior to mapping it was believed that the abyssal
plain was smooth and featureless.
Mapping with sonar revealed the Mid Ocean
Ridge MOR
Features of the MOR
Longest mountain chain on the earth
Rift valley at its center that is deeper than the
Grand Canyon.
Volcanoes along the ridge at various locations
Surtsey Iceland.
Sea floor Spreading
Harry Hess US Naval Officer
Observed Magnetic anomalies
in the Ocean Crust
Proposed that the ocean floors
move like conveyor belts
carrying along the continents.
At the mid ocean ridge, molten
material rises from the mantle
and spreads out pushing older
rock to the side.
Sea Floor Spreading Video
Sea floor spreading is supported by
1.
2.
3.
New molten material discovered by
deep ocean submarines like “Alvin”
at the MOR
Magnetic Banding running parallel
to the sea floor caused by shifts in
the earth’s magnetic field about
every 780,000 years
Rock samples from deep ocean
drilling show that rocks get older as
you move outward in both
directions from the ridge.
Subduction Video
Subduction at deep ocean trenched
Subduction is the process by which crustal material is
drawn deep into the earth and recycled to become new
crust at divergent boundaries.
Subduction occurs at deep ocean trenches found where
the crust is being pushed together. (Convergent
boundaries)
The ocean floor is renewed in this process about every
200 million years
Most subduction zones (deep ocean trenches) are
found n the pacific
Deep ocean trenches
Summary
What is the role of the mid ocean ridge?
At the mid ocean ridge, molten material rises from the mantle and erupts. This
material then spreads out pushing older rocks to both sides of the ridge
Describe the process of subduction at deep ocean trenches.
In the process of subduction, oceanic crust sinks down beneath the trench into
the mantle.
What is the evidence for sea floor spreading?
Evidence of sea floor spreading includes molten material, magnetic stripes,
and evidence from drilling samples
Where would the oldest rocks in the ocean floor be found?
The oldest rocks would be found the farthest away from the MOR.