SEA-FLOOR SPREADING
TXT: PG. 97
CHAPTER 3, SECTION 4
Background
• Deep ocean parts don’t receive a lot of
sunlight
• Cold---temp near freezing
• Areas where there is space between the plates
allows water down into the crust, then brings
it back up.
• These warm areas provide a great area for life
to thrive, and support information given by
Wegener’s “continental drift” theory.
Mid-Ocean Ridges
• Ridge: Area of higher relief on the ocean floor
(underwater mountain range).
• Sonar: Device used to bounce sound waves off of
objects deep in the water.
• Time it takes for echo to return=distance to the object
• Ridges curve around the ocean floor, going into
all oceans
• They connect in a system that forms the world’s
longest mountain chain (60,000 km)
Sea-Floor Spreading
• Places where the sea floor spreads apart along
an ocean ridge.
• Convection currents push new magma up into
the crack.
• Magma cools at the surface, causing “new
crust” to form and making the older ocean
floor move.
Evidence
• Eruptions of molten material
• Scientists used “Alvin”—small sub used to dive into high
pressure areas of the ocean
• Found rocks that indicated molten material was present
and hardening quickly
• Magnetic stripes in the rock
• Last reversal of the earth 780,000 years ago.
• New layers of rock have formed above this reversal,
indicating new rock in the area.
• Older rock lines up one way
• New rock lines up opposite
• Ages of the rocks
• Scientists used the Glomar Challlenger to drill into the
ocean floor
• Scientists dated the rocks and determined that older
rocks were further away from the mid-ocean ridge.
• The youngest of the rock was in the center of the ridge.
Deep-Ocean Trenches
• This is what keeps the ocean floor from
getting wider and wider
• In this area, the ocean floor dips down
• After tens of millions of years, the trench
allows part of the ocean floor to sink back to
the mantle.
Subduction
• Definition: Process where the ocean floor
sinks under a deep-ocean trench and back into
the mantle
• As the new crust moves away from the ridge,
it cools…what does that do to it’s density?
• Right, it gets more dense…eventually so dense
that gravity pulls it down beneath the trench.
• This process changes the size and shape of the
ocean floor…
• Ocean floor is renewed every 200 million years
(time for new rock to become old, dense, and
sink)
• Pacific ocean: New crust isn’t being produced as
fast as it’s sinking-----the ocean is shrinking
• Atlantic Ocean: Old crust isn’t sinking as fast as
new is forming---the ocean is growing causing the
continents on it’s edges to move.