9­19 Sea Floor Spreading.notebook
September 22, 2011
What observations
support the theory of
continental drift?
In other words, what
Wegener's
evidence supports
research on land
continental drift?
ce
n
e
i
Sc
er
z
i
g
Ener
features, fossils,
and evidence of
climate change.
• In 1925, Germany outfitted a boat and set out for two
years to systematically and scientifically look at the oceans.
• This expedition was the first use of closely spaced echo
sounders to map deep sea topography and the first to reveal
the extent of the sea floor's rugged terrain.
• The expedition also found that a continuous mountain-like
ridge runs through the Atlantic to the southwest of Africa.
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9­19 Sea Floor Spreading.notebook
September 22, 2011
In the mid-1900s, scientists
mapped mid-ocean ridges using
sonar.
What is sonar?
Sonar is a device that bounces
sound waves off underwater
objects and then records the
echoes of these sound waves.
The time it takes for the echo to
arrive indicates the distance to the
object.
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9­19 Sea Floor Spreading.notebook
mid-ocean ridge
an undersea
mountain chain
where new ocean
floor is produced,
it is a divergent
plate boundary
September 22, 2011
divergent boundary
a plate boundary
where two plates
move away from
each other
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9­19 Sea Floor Spreading.notebook
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• Harry Hess was a geologist and
Navy submarine commander during
World War II.
• Part of his mission had been to
study the deepest parts of the
ocean floor. In 1946 he had
discovered that hundreds of flattopped mountains shape the Pacific
floor.
• The discovery of the Great Global Rift in the 1950s
inspired him to look back at his data from years before.
• Harry Hess proposed in 1960 that the movement of the
continents was a result of sea-floor spreading.
• In 1962, he added a geologic mechanism (seafloor
spreading) to account for Wegener's moving continents.
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9­19 Sea Floor Spreading.notebook
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Sea-floor spreading
• The sea floor spreads apart along both sides of
a mid-ocean ridge as new crust is added
• As a result, the ocean floors move like conveyor
belts, carrying the continents along with them
1. Sea-floor spreading
begins at a mid-ocean
ridge, which forms
along a crack in the
ocean crust.
2. Along the ridge, molten material that
forms several kilometers beneath the
surface rises and erupts.
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9­19 Sea Floor Spreading.notebook
3. At the same
time, older rock
moves outward
on both sides of
the ridge.
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4. As the molten material cools,
it forms a strip of solid rock in
the center of the ridge.
5. When more molten material
flows into the crack, it forms
a new strip of rock.
Evidence for Sea-Floor Spreading
1. Molten Material
2. Magnetic Stripes
3. Drilling Samples
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9­19 Sea Floor Spreading.notebook
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Evidence from Molten Material
Scientists found rocks that were shaped like pillows or
like toothpaste squeezed from a tube.
These rocks only form when molten material hardens
quickly after erupting under water.
These rocks
showed that
molten material
has erupted
again and again
along the midocean ridge.
Evidence from Magnetic Stripes
Earth's magnetic poles have
reversed themselves many
times during Earth's history.
Scientists discovered that
the rock that makes up the
ocean floor lies in a pattern
of magnetized stripes.
The stripes hold a
record of reversals
in Earth's magnetic
field.
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Evidence from Drilling Samples
Rock samples were
collected in 1968 from a
drilling ship called the
Glomar Challenger.
Scientists determined the age of the rocks and
they found that the farther away from a ridge the
samples were taken, the older the rocks were.
The youngest rocks were always in the center of
the ridges.
Sea Floor Spreading
with Bill Nye
http://www.youtube.com/watch?v=GyMLlLxbfa4
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9­19 Sea Floor Spreading.notebook
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If new oceanic lithosphere is created at mid-ocean ridges, where does it go?
Subduction
when oceanic crust sinks
beneath a deep-ocean trench
and back into the mantle at a
convergent plate boundary
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