Transcript Sea Floor Spreading

Seafloor Spreading
Seafloor Spreading Video
Wegener could not explain how the
continents moved but . . .
• Harry Hess explained HOW continents
moved and he called his idea Seafloor
Spreading
During World War II:
• Scientists began to experiment with new tools
and equipment to learn about the oceans
• SONAR (SOund Navigation And Ranging) was
one new technology used to map the sea floor
• Sound waves reflect off of any hard
surfaces that they hit, so…
1. Sound waves were sent to the
bottom of
the ocean floor from a
“transmitting” device held off a boat
2. Sound waves reflected back to
another “receiving” device on the
boat
3. The longer it took the sound waves
to return to the boat, the deeper the
water.
• Sonar technology also is what helps fisherman
catch fish and our military to look for enemies
So what have we
discovered about the
ocean floor?
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloorspreading.html
The seafloor contains
mountains, volcanoes
and valleys
Including the MidAtlantic Ridge
• The World’s Largest
Mountain Range
• And it is
UNDERWATER!
Process of Seafloor Spreading:
1. Magma is forced upward at the mid-ocean
ridge (divergent boundary)
2. Magma cools and sinks as it flows away
forming a high ridge
3. The magma then cools to solid crust forming
new ocean floor
This occurs ~2.5
cm/yr in Atlantic
Ocean
This is happening all over the Ridge
Areas of Seafloor Spreading
Features of Sea Floor Spreading
• Trenches form away from the divergent
boundary near subduction zone.
– Subduction – more dense crust (oceanic) gets
pushed under less dense crust (continental)
back into the mantle.
Proof of Sea Floor Spreading
1. Rock samples
– Youngest rock near the
ridge (where magma
cooled and hardened
most recently)
– Oldest rock near shore
(where magma cooled
and hardened ~180
million years ago
2. Magnetic Clues
• Some minerals are magnetic
• Magnetic minerals are attracted to Earth’s
magnetic field.
• When magma containing these minerals cools,
it freezes the magnetic mineral grains so they
point in the direction of the Earth’s magnetic
field.
Over time, Earth’s
magnetic field
changes (reverses
direction)
-As a result, the
direction the minerals
point changes with it.
Earth’s magnetic
field caused by
movement in
outer core
• These changes result in Paleomagnetism magnetic striping along the ocean floor.
Each beige and red stripe indicates a change in Earth’s magnetic field.