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Sea-Floor Spreading
Sea-Floor Spreading
Spreading –the
process that
adds new
material to the
ocean floor
while pushing
older rocks
away from the
Sea-Floor Spreading
Deep in the ocean
the water is near
There is no light
Life is rare
BUT near cracks in
the ocean floor, tube
worms exist.
Sea-Floor Spreading
Sonar - a device
that bounces
sound waves off
objects and then
records the
echoes of these
sound waves. The
time it takes for
the echo to arrive
indicates the
distance to the
Sea-Floor Spreading
Mid-Ocean Ridge – the longest chain of
mountains in the world---these are
divergent plate boundaries.
Sea-Floor Spreading
 In 1960, Hess made his single most
important contribution. In a widely circulated
report Hess stated that the Earth's crust
moved laterally away from long, volcanically
active oceanic ridges
Sea-Floor Spreading
 Ocean floor moves like a conveyor belt
carrying continents with it.
 New ocean floor forms along cracks in the
ocean crust as molten material erupts from the
mantle spreading out and pushing older rocks
to the sides of the crack. New ocean floor is
continually added by the process of sea-floor
Sea-Floor Spreading
Evidence from
Molten Material –
Rocks shaped like
pillows - show that
molten material
has erupted again
and again from
cracks along the
mid-ocean ridge
and cooled quickly
Sea-Floor Spreading
 In 1960, scientists used a submersible
named Alvin, to see the ocean floor.
Sea-Floor Spreading
Sea-Floor Spreading
Evidence from
Stripes – Rocks
that make up
the ocean floor
lie in a pattern
of magnetized
stripes which
hold a record of
the reversals in
Earth’s magnetic
Sea-Floor Spreading
Sea-Floor Spreading
Sea-Floor Spreading
Evidence from
Drilling Samples
– Core samples
from the ocean
floor show that
older rocks are
found farther
from the ridge;
youngest rocks are
in the center of
the ridge
Sea-Floor Spreading
Subduction –
Process by
which the ocean
floor sinks
beneath a deepocean trench
and back into
the mantle;
allows part of
the ocean floor
to sink back
into the mantle
Sea-Floor Spreading:
Subduction zone
Trench –
Occurs at
zones. Deep
canyons form
where oceanic
crust bends
Sea-Floor Spreading:
Subduction zone - Pacific
 Subduction in the Pacific Ocean
 It is shrinking…how can this be?
 A deep ocean trench swallows more oceanic
 No new crust is being added
Sea-Floor Spreading:
Subduction zone - Atlantic
 Is expanding
 Only a few short trenches
 As a result, the spreading ocean floor
has nowhere to go
 So…as long as the Atlantic Ocean
spreads so does the continents along its
Sea-Floor Spreading