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Sea-Floor Spreading
Mapping the mid-ocean ridge
• The East Pacific Rise
•
•
has a mid-ocean
ridge.
Curves around like a
baseball.
Lies hidden under
hundreds of meters of
water.
Sonar
• A device that bounces
•
sound waves off
underwater objects
and then records the
echoes of these
sound waves.
Sonar mapped midocean ridges.
Harry Hess
• An American
•
geologist who studied
mid-ocean ridges.
He suggested that the
ocean floors move
like conveyor belts,
carrying the
continents along with
them.
• At the mid-ocean ridge, molten material
rises from the mantle and erupts. The
molten material then spreads out, pushing
older rock to both sides of the ridge. As the
molten material cools, it forms a strip of
solid rock in the center of the ridge. Then
more molten material flows into the crack.
Sea-floor spreading
• The process that
•
continually adds new
material to the ocean
floor.
Evidence molten
material, magnetic
stripes, and drilling
samples.
Evidence #1 - Molten Material
• The submersible,
Alvin, found strange
rocks shaped like
pillows or like
toothpaste squeezed
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.
Evidence #2 - Magnetic Stripes
• Scientists discovered
that the rock that
makes up the ocean
floor lies in a pattern
of magnetized
“stripes”. 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
ridge.
Subduction at Deep-Ocean
Trenches
• Wider & wider? Deep•
•
ocean trenches
Ocean floor plunges into
deep underwater
canyons are deep-ocean
trenches.
Subduction is the process
by which the ocean floor
sinks beneath a deepocean trench and back
into the mantle.
Subduction
• 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 &
Atlantic
• 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.