The Theory
of
Seafloor Spreading
Seafloor Bathymetry
Creating Maps of the Ocean Floor
• Scientists were able to
map the ocean floor
using sonar, an Echosounding devices.
• The time it takes for
sound waves to travel
through the ocean and
back is recorded to
calculate ocean depth
which is then converted
to an image of the
ocean floor.
Map of Ocean Floor
Mid-Ocean Ridges and Trenches form
the Boundaries of Earth’s Tectonic
Plates
HARRY H. HESS
• Scientist
(geologist) who
proposed the
Theory of
Seafloor
Spreading in
early the 1960 ‘s.
How Does Seafloor Spreading
rises through the
Work? 1. Magma
center of mid-ocean
2.
3.
4.
Subduction of
older, more dense
ocean crust into
the mantle where
it is recycled.
ridges.
It cools and forms new
rock on either side of the
ridge.
As more seafloor is
created, the older floor is
pushed away from the
ridge and spreads the
seafloor.
The oldest ocean floor
becomes more dense
because of the
accumulation of sediment
and forms trenches where
the seafloor is recycled
into the Asthenosphere in a
process called subduction.
The Theory Says:
• The seafloor spreads apart
along both sides of a midocean ridge as new crust is
being added.
• As a result the ocean floor
moves like conveyor belts,
carrying the continents with
them.
The Ocean Floor is NOT Flat; it contains a system of
mountains, valleys and trenches that extend through the
center of much of Earth’s oceans.
Mid-Ocean Ridges – Underwater volcanic
mountain range that is more than 56000
kilometers (35000 mi) long, where new
seafloor is being created.
Rift Valley – The valley between the ridges
where lava rises on the ocean floor and forms
new rock.
Trench – Deep, steep, narrow crack in the
ocean floor where old sinks back into the
mantle and is recycled.
Evidence to support
Sea floor Spreading
1. Evidence from Molten Material
Alvin
Submarine
• Hardened rocks
shaped like pillows
were discovered along
the ridge.
• These rocks only
form when lava cools
quickly underwater.
• Pillow Lava (rocks)
prove that new rock is
being formed at
ridges because it only
forms under water.
http://www.pmel.noaa.gov/v
ents/nemo/explorer/concepts
The Discovery of Magnetic
Stripes (of Rock)
• Using a
Magnetometer
hanging from a boat,
two English scientists,
Drummond Matthews
and Fred Vine
discovered magnetic
bands on either side
of the Mid-Atlantic
Ridge.
2. Evidence from Magnetic Striping
As new Basaltic seafloor
is created at the ridges,
the iron particles in it
magnetize differently
proving that new seafloor
was created at different
times in our history.
Normal Polarity = N
Reverse Polarity = S
The iron lines up in the
direction of the Earth’s
strongest magnetic pole of
the time and is locked into
place when the rocks form
proving that new rocks are
always forming at the ridges.
Magnetic Zebra Stripes
3. Age Evidence from Drilling
Samples
• The oldest ocean
rock is only 180
million years old
compared to the
continents which are
over 4 billion years
old.
• Young rocks are
found close to the
ridge
• Old rocks are found
further away from the
ridges.
Glomar Challenger
research ship used to drill in the
seafloor.
• Deep Sea Drilling
Ship
If New Seafloor is Always Created,
Then Why Isn’t
the Earth Blowing up Like A Balloon?
Old seafloor is
being
consumed or
recycled
at the
trenches!
• Places where seafloor is pushed back into the
Asthenosphere are called SUBDUCTION ZONES.
• Subduction occurs at the trenches.
So What is the Significance of
The Theory of Seafloor Spreading?
The Theory of
Seafloor Spreading
Answers the
Question:
HOW
the
Continents Moved!
As the seafloor spreads at midocean ridges, the ocean floor
moves carrying the continents with
it!