Download Power point 9.4

Sea-Floor Spreading

Chapter 9, Section 4
•The Mid-Ocean Ridge
Longest mountain chain in the
world
 It is almost entirely under water,
but some areas do peak out of
the ocean.

•Example – Iceland
Sea-Floor Spreading


As new rock
emerges from
the rift valley,
older rock is
pushed towards
the edge of the
ocean.
http://www.wwnorton.
com/college/geo/egeo
/animations/ch2.htm
Evidence for S.F.S.

Molten Material –
• Pictures of molten material rising
from the mid-ocean ridge have
been taken.

The molten material cools very
quickly due to the contact with cold
ocean water.
http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm
Evidence for S.F.S.
• Magnetic Stripes in the ocean floor


Earth’s magnetic poles (caused by the inner
core spinning) have reversed themselves in
the past.
If they reverse again, our compasses will
point south.
http://www.wwnorton.com/college/geo/egeo/animati
ons/ch2.htm
Evidence for S.F.S.

Drilling Samples
•Scientist drilled the sea-floor to
get rock samples.
The older rocks were found far
from the mid-ocean ridge.
 The younger rocks were found
near the mid-ocean ridge.

Subduction at Deep-Ocean
Trenches



Subduction
means to sink
below.
This allows the
ocean floor to
sink back into
the mantle…a
recycling of
sorts.
http://www.wwnorton.com/college/
geo/egeo/animations/ch2.htm
Subduction and Earth’s
Oceans


The Pacific Ocean is SHRINKING!!!
• More oceanic crust is subducted
than is being produced, because
there are more trenches.
The Atlantic Ocean is
GROWING!!!
• More oceanic crust is being
produced than subducted because
there are fewer trenches.