Transcript Seafloor Spreading

Seafloor Spreading
Mapping the Ocean Floor
• Before WWI in order to map the ocean
floor a rope was lowered from a boat to
the ocean floor and the depth was
recorded at different locations.
• The invention of sonar in which sound
waves bounced off the ocean floor to
determine depth made the mapping of the
ocean floor much faster and accurate.
• Researchers found underwater mountains and
valleys.
• Ridges containing long rift valleys where
volcanoes erupted were found.
• In the Atlantic, the Pacific, and in other oceans
around the world, a system of ridges called the
mid-ocean ridges, is present.
• Scientist became curious as to what caused
these ridges.
The Sea Floor Moves
• Harry Hess proposed a theory called
seafloor spreading.
• Hess proposed that hot, less dense
material flows beneath the crust. Rising
toward the surface at the mid-ocean ridges
then flowing sideways, carrying the
seafloor away from the ridges.
Hess’s Seafloor Spreading Theory
Cont.
• As the seafloor spreads apart the hot
magma fills the cracks. The magma cools
forming new seafloor. As the cooler more
dense material moves away from the midocean ridge it sinks helping to form a new
ridge.
Evidence of Spreading
• The youngest rocks are obtained closer to
the ridge and the farther away from the
ridge the older the rocks become.
• New seafloor features and life-forms also
were discovered there. Exotic life forms
are supported by the warm temps and
chemicals from the molten lava material.
Magnetic Clues
• When the Earth’s magnetic field reverses itself it
is recorded in the rocks forming the mid-ocean
ridges.
• The reversal of the magnetic field is recorded in
the rocks containing iron-bearing minerals such
as magnetite.
• Using a magnetometer scientists can detect
magnetic fields, the rocks on the ocean floor
show many periods of magnetic reversal.
Magnetic Time Scale
• The magnetic alignment in the rocks
reverse back and forth over time in strips
parallel to the mid-ocean ridges.
• A strong magnetic reading is recorded
when the polarity of a rock is the same as
the polarity of Earth’s magnetic field today.
• The normal polarity show up as large
peaks, thus providing strong support for
the seafloor spreading theory.
• The magnetic reversal showed that new
rock was being formed at the mid-ocean
ridges, helping to explain how the crust
moves and supporting the continental drift
theory.