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Sea-Floor Spreading
Mid-Ocean Ridges
• an undersea mountain chain where new
ocean floor is produced
– Discovered by the use of sonar – device
that determines the distance of an under
water object by recording echoes of sound
waves
• The time it takes for the echo to arrive tells the
distance to the object
• Most mountains in mid-ocean ridges are
hidden under hundreds of meters of
water
Mid-Ocean Ridges
So, one may ask…
• What are these ridges?
• How do they form?
What is Sea-Floor Spreading?
• Harry Hess studied midocean ridges and
thought…
– Maybe Wegener was right!
• In 1960, introduced idea
of sea-floor spreading
– the sea floor spreads
apart along both sides of
a mid-ocean ridge as new
crust is added. As a
result, the ocean floors
move like conveyor belts,
carrying the continents
along with them
Sea-Floor Spreading cont…
1. Begins at mid-ocean
ridge that forms along
a crack in the oceanic
crust
2. Along ridge, molten
material that forms
beneath surface rises
and erupts
3. Older rock moves
outward on both sides
of ridge
• So…How can the ocean floor keep from
NOT getting wider and wider?
Subduction at Trenches
• Ocean floor DOES NOT just keep
spreading
• Ocean floor plunges into deep under
water canyons called deep-ocean
trenches
– At trench oceanic crust bends downward
• Part of the ocean floor sinks back into the
mantle at these trenches
– Process takes millions of years
Process of Subduction
• Subduction – process in which the ocean
floor sinks beneath deep ocean trenches
and back into mantle
• As subduction occurs, crust closer to midocean ridge moves away from ridges and
toward trench
– Sea floor spreading and subduction work
together
• Move ocean floor like it’s a conveyor belt
Process of Subduction cont…
• New oceanic crust is HOT, but as it moves
away from the mid-ocean ridge, it cools
becoming more dense
• Gravity pulls denser crust down beneath
trench
Subduction and Earth’s oceans
•
In the Pacific Ocean there are more
trenches than mid-ocean ridges so the
ocean is shrinking
•
In the Atlantic Ocean there are more midocean ridges than trenches so the ocean is
getting bigger