Download Sea Floor Spreading

Sea Floor Spreading
Chapter 1-4
Mid Ocean Ridges
• Mapped by sonar in mid
1900’s
• Curve like seams on a
baseball along the sea
floor
• Extend across all oceans
• Some are completely
under water
• Some poke through
– Iceland
How were these ridges formed?
• Studied by American
geologist Harry Hess in
1960’s
• Thought Wegner may
have been right
• Proposed idea of sea
floor spreading
Sea Floor Spreading
• Sea floor spreads apart at
mid ocean ridge as new
crust is added.
• Molten material erupts
from ridge, cools into a
strip of new rock
• Older rock moves
outward as it is replaced
by new molten rock
• Cause ocean floors to
move like conveyor belts
carrying continents with
them
Evidence from Molten Material
• Pillow shaped rocks found along mid ocean
ridge by the crew of the submarine Alvin.
• Rocks can only be formed by molten material
that cools quickly under water.
Evidence from Magnetic Stripes
• Scientists pattern of magnetized “stripes” of
rock on ocean floor
• Hold record of reversals of Earth’s magnetic
field
Evidence from Drilling Samples
• Drilling ship Glomar Challenger brought up
samples from ocean floor that showed rocks
further away from ridge were older and closer
rocks were younger
Why doesn’t the ocean floor keep on
getting wider
• Part of the ocean floor sinks back into the
mantle at deep ocean trenches by the process
of subduction
Subduction
• As new, hot oceanic crust moves away from
the mid ocean ridge, it cools down and
becomes more dense.
• Gravity causes denser crust to sink down into
trenches
Subduction and the Earth’s Oceans
• Ocean floor is renewed about every 200
million years because of seafloor spreading
and subduction.
• Pacific Ocean is shrinking because it’s
numerous trenches are swallowing more crust
than is being formed
• Atlantic ocean is growing because it has less
trenches