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Mid-Ocean Ridge – the longest chain of
mountains in the world---these are
divergent plate boundaries.
Mapping the mid-ocean ridge:
• A device that bounces sound waves off
underwater objects and then records the
echoes of these sound waves.
• Sonar mapped mid-ocean ridges.
Harry Hess:
•Studied the Mid-Ocean Ridges
•Proposed the seafloor spreading theory in 1962.
•The Sea floor spreading theory states that new ocean
crust is being created at mid-ocean ridges and
destroyed at deep-sea trenches..
Sea-Floor Spreading????
• The process that continually adds new
material to the ocean floor.
The force responsible for driving or moving the plates is
Convection Currents occur within the mantle of the earth
when hot magma rises and cool magma sinks
•Magma, because it is hotter and less
dense, is forced up towards the ocean
floor cooling off when it touches
•When the magma hardens, a small
amount of new ocean floor is added
to the Earth’s crust. As more magma
rises and cools it pushes the new sea
floor away from the ridge. This process
is called sea floor spreading.
Evidence # 1:Molten Material
Rocks shaped like
- molten material has
erupted again and again
from cracks along the
mid-ocean ridge and
cooled quickly
Evidence #2 - Magnetic Stripes
• Scientists discovered
that the rock that
makes up the ocean
floor lies in a pattern
of magnetized
• They hold a record of
reversals in Earth’s
magnetic field.
Evidence #3 - Drilling Samples
• The Glomar
Challenger did a
drilling sample and
found rocks that the
farther away from the
ridge the older the
rocks were. The
younger ones were in
the center of the
Subduction at Deep-Ocean
• Deep-ocean trenches
• Ocean floor plunges into
deep underwater
canyons are deep-ocean
• Subduction- is the
process by which the
ocean floor sinks beneath
a deep-ocean trench and
back into the mantle.
• At deep-ocean trenches, subduction
allows part of the ocean floor to sink back
into mantle, over tens of millions of years.
• Subduction and Earth’s Oceans
• Earth’s ocean floor is renewed about
every 200 million years.
Subduction in the Pacific &
• Deep ocean trenches are swallowing more
oceanic crust than the mid-ocean ridge
can produce. Thus, the width of the
Pacific will shrink.
• The Atlantic is expanding. It has short
trenches. In some places, the oceanic
crust is attached to the continental crust
which moves the continents.
Animation of Sea Floor Spreading and Ages…2.5