Sea Floor Spreading
EQ: What is the process
of sea-floor spreading?
Mid-Ocean Ridge
• Mid-Ocean Ridge:
The undersea
mountain chain
where new ocean
floor is made; a
divergent plate
boundary.
Mid Ocean Ridge
Sonar
• Sonar: A device that
uses sound wave
echo to determine
distance.
The sonar is used to map the
ocean floor.
Sound waves
bounce off
underwater
objects.
The time it takes
for the echo to
arrive indicates
the distance to
the object.
Evidence for Sea-Floor
Spreading
In the 1960s, Harry Hess
examined maps of the mid
ocean ridge. He proposed that
the ocean floors move like
conveyor belts, carrying the
continents with them.
Sea-Floor Spreading
The process
by which
molten
material
adds new
oceanic
crust to
the ocean
floor.
What evidence did scientists
find for sea-floor spreading in
the 1960s?
• Evidence from molten
material
• Evidence from magnetic
stripes
• Evidence from drilling
samples
Evidence From Molten Material
• Strange rocks shaped like pillows or
like toothpaste squeezed from a tube.
• These shapes can form only when hot
rock hardens quickly!
• They show that molten material has
erupted again and again from midocean ridges.
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html
The rock that makes
up the ocean floor
lies in a pattern of
magnetized “stripes”.
The rock in the ocean
is made of magnetic
iron.
Evidence From
Magnetic Stripes
Evidence From Drilling Samples
• The further away from the ridge the
rocks were, the older they were.
• The younger rocks were always in the
center of the ridges.
Subduction at Deep-Ocean
Trenches
• A deep
valley in
the ocean
floor;
oceanic
crust
slowly
sinks
towards
the mantle.
Deep-Ocean Trenches
Subduction
• Subduction: The
process where
oceanic crust sinks
down into a trench
and back into the
mantle; a
convergent plate
boundary.
Guide For Reading: What
happens to the ocean floor at
deep ocean trenches?
• At trenches, 2 plates collide causing
the denser of the 2 plates to dive
back to the mantle. This process is
known as subduction.
• Over millions of years, this material
may rise again as new oceanic crust.
Subduction and Earth’s
Oceans
Subduction in the Pacific
Ocean
• Subduction is occurring faster than seafloor spreading.
• Pacific Ocean is shrinking!
• This is because of many deep ocean
trenches.
Subduction in the Atlantic
• The Atlantic Ocean is expanding.
• Not as many trenches in the Atlantic.
• The ocean gets larger and pushes
against the continents.
Guide For Reading: What is the
process of sea-floor spreading?
• Molten rock erupts at the mid-ocean
ridges. It spreads out, pushing older
rock to both sides of the ridge.
• Oldest ocean floor collides with
continental crust.
• The more dense oceanic crust
subducts (sinks) back into the mantle
at a deep-ocean trench
Class Work and Homework
• Fill In
– What did I learn
– Confused
– Say
Carefully read pages F33 – F39, Answer
questions 1-5 on page F39