Transcript Sea-Floor Spreading The mechanism for continental drift Theory of Continental Drift • Remember Wegner proposed that over time, continents drift apart? – Well how.

Sea-Floor Spreading
The mechanism for continental drift
Theory of Continental Drift
• Remember Wegner proposed that over time,
continents drift apart?
– Well how did this happen?
Mid-ocean Ridges
• Scientist (Harry Hess) noticed that there are
mountain ranges that exist under water in the
Oceans
– Ex: Mid-Atlantic Ridge
• These mid-ocean ridges exist due to
SEA-FLOOR SPREADING
Sea-Floor Spreading
• The process by which new oceanic lithosphere
is created as older materials are pulled away.
• As tectonic plates move away from each other
the sea floor spreads apart and magma rises
to fill in the gap.
Sea Floor Spreading Video
Sea-Floor Spreading Explained
Sea-Floor Spreading
• Sea-floor spreading creates new oceanic crust
– Newer crust is found at the center or near the
ridge
– Older crust is found further away from the ridge
(because it is being pushed away from the center
of the ridge by new rising magma)
Magnetic Reversals
• Strongest evidence for sea-floor spreading:
– Magnetic Reversals recorded in ocean floor
• Magnetic reversals:
– The switching of Earth’s north and south pole over
time
Magnetic Reversals and the Sea Floor
• Magnetic minerals found in the magma that bubbles up at the
ridge align with the Earth’s magnetic field.
– Once the molten rock cools, it’s like tiny compasses set in stone
• This record is carried slowly away from the center of the ridge,
forming bands of alternating magnetic reversals
– Every time there was a magnetic reversal, a new band was formed
Magnetic Reversals
• Why would Magnetic Reversal bands be
evidence of sea-floor spreading?
– These bands show that different parts (bands) of
the crust were formed over different time periods
– Because with each band, the rock patterns that
form that part of the crust are different from the
other bands nearby
Magnetic Reversals
Magnetic Reversals