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Sea-Floor Spreading
I. Mapping the Mid-Ocean Ridge
• Mid-ocean ridge –
longest chain of
mountains in the
world
• Curves along the sea
floor
• Extends into all of
Earth’s oceans
I. Mapping the Mid-Ocean Ridge
• Discovered by use of
sonar used to map ocean
floor
• Acronym
SOund
Navigation
And
Range
II. Evidence of Sea-Floor Spreading
• Harry Hess (Am.
geologist) studies
mid-ocean ridges.
• He thought ocean
floors moved,
carrying continents
with them (like
conveyor belts).
II. Evidence of Sea-Floor Spreading
• Molten material from the mantle rises at the mid0cean ridge and erupts.
• Molten material spreads out, pushing older rock to
both sides
• Cools and forms solid strip of rock
• Continually adds new material to ocean floor
II. Evidence of Sea-Floor Spreading
Illustrate it!
II. Evidence of Sea-Floor Spreading
• pillow lava along midocean ridge
• form only when molten
material erupts and
hardens quickly
• shows this material
erupted over and over
along mid-ocean ridge
II. Evidence of Sea-Floor Spreading
Evidence from magnetic
stripes:
1. Earths magnetic
poles have reversed
themselves
2. Ocean floor rock lies
in a pattern of
magnetized stripes
3. Pattern is same on
both sides of ridge
II. Evidence of Sea-Floor Spreading
Evidence from drilling
samples:
1. Samples from sea
floor were brought
up through pipes
2. Age of rock was
determined
3. Oldest rocks were
furthest from ridge
III. Subduction at
Deep-Ocean Trenches
•
Ocean floor bends downward into deepocean trenches
•
Subduction takes place--ocean floor sinks
into mantle
•
New oceanic crust forms at mid-ocean ridge
and pushed old crust to the side
•
Allows part of ocean floor to sink into mantle
over millions of years
III. Subduction at
Deep-Ocean Trenches
IV. Subduction and Earth’s Oceans
Atlantic Ocean
• is expanding
• has only a few short
trenches
• new material has
nowhere to go
• pushes continents
along with it
• ocean gets wider
Pacific Ocean
• is shrinking
• subduction happens
faster that new
material is produced
at mid-ocean ridge
• ridge does not add
new material fast
enough
• ocean gets smaller