Download Sea Floor Spreading Guided Notes

3/21/2016
MID-OCEAN RIDGES
SONAR
•A mid-ocean ridge is an
underwater mountain chain
where new ocean floor/crust is
produced.
•Scientists have discovered these
mid-ocean ridges using sonar; a
device that bounces sound
waves off underwater objects
and then records the echoes of
the sound waves.
SEA-FLOOR SPREADING
SEA FLOOR SPREADING
•Harry Hess was an American
geologist who studied mid-ocean
ridges. His studies showed that
Alfred Wegener was right about
the continents drifting apart!
•The sea floor spreads apart along
both sides of a mid-ocean ridge
as new crust is added. The
ocean floors move like conveyer
belts, carrying the continents with
them.
EVIDENCE FOR SEA FLOOR SPREADING
MOLTEN MATERIAL EVIDENCE
•Molten material
•Magnetic stripes
•Drilling samples
•Scientists found strange rocks
shaped like pillows or toothpaste
squeezed from a tube. Such
rocks only form when molten
material hardens quickly after
erupting underwater.
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3/21/2016
MAGNETIC STRIPES
DRILLING SAMPLE EVIDENCE
•Scientists have discovered that
the rock that makes up the
ocean floor lies in a pattern of
magnetized “stripes”. These
stripes hold a record of reversals
in the Earth’s magnetic field,
because the rock contains iron.
•Samples from the sea floor were
brought-up and dated. They
found that the farther away from
the ridge the samples were
taken, the older the rocks were.
The youngest rocks were always
in the center of the ridges.
SUBDUCTION AT TRENCHES
SUBDUCTION
•Just as sea-floor spreads, it also
plunges into deep underwater
canyons called deep-ocean
trenches. During this process, the
ocean floor bends downward
and sinks back into the mantle.
•At a deep-ocean trench,
subduction occurs.
Subduction is the process by
which ocean floor sinks back
into the mantle.
SUBDUCTION
•The processes of subduction and
sea-floor spreading can change
the size and shape of the
oceans. Because of this, the
ocean floor is renewed about
every 200 million years!
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