Sea-Floor Spreading
Why Was Continental Drift
Not Accepted?
Wegener couldn’t explain what force
was strong enough to move the plates.
The Great Global Rift
In 1953, American physicists Ewing and Heezen
discovered that through an underwater mountain
range ran a deep canyon. In some places the
canyon, called the Great Global Rift, came very
close to land. The rift appeared to be breaks
in the earth's crust, but perfectly fitted
breaks, like joints made by a carpenter.
The Hypothesis of
Sea-Floor Spreading
Who proposed this hypothesis?
In 1960, Herman Hess
proposed that the movement of
the continents was a result of
sea-floor spreading. He
hypothesized that the seafloor was spreading from
vents in the Great Global
Rift, where hot magma oozed
up. As the magma cooled it
forced the existing seafloor away from the Rift on
either side.
What Goes On At Mid-Ocean Ridges?
1. Sea-floor spreading
What Goes On At Mid-Ocean Ridges?
2. The youngest rocks of the sea
floor are forming in the middle at
the rift between the ridges.
What Goes On At Mid-Ocean Ridges?
3. Magnetic striping has been observed.
The Earth’s magnetic field reverses itself
Every few thousand years.
What Goes On At Mid-Ocean Ridges?
Magnetic reversals have been recorded
on the ocean floor.
What Was The Proof Of Sea-Floor Spreading?
1. Ancient magnetism records flip-flops of
earth’s magnetic field.
What Was The Proof Of Sea-Floor Spreading?
2. The Age of the Sea Floor Oceans are youngest
at the Mid-Ocean ridges and oldest near the continents.
This diagram shows the Americas
being separated from the Europe
and Africa as the seafloor spreads.
Continents are grey in color.
Youngest seafloor is red and as
seafloor gets older it becomes
yellow, green and then blue.
What Was The Proof Of Sea-Floor Spreading?
created at
destroyed in
3. New ocean floor is
mid-ocean ridges.
And ocean floor is
subduction zones.
What happened?
Hess' ideas neatly explained why the Earth does not
get bigger with sea floor spreading and why oceanic
rocks are much younger than continental rocks.
Hess died in 1969. Unlike
Wegener, he was able to
see his seafloorspreading hypothesis
largely supported as
knowledge of the ocean
floor increased
dramatically during his
lifetime.
The End
BONUS: What does this picture show?