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Sea-Floor Spreading
- The mid-ocean ridge is the longest
chain of mountains in the world.
- Scientists mapped the mid-ocean ridge
using sonar.
- At the mid-ocean ridge, molten material
rises from the mantle and erupts. The
molten material spreads out, pushing older
rock to both sides of the ridge.
- Youngest rocks in the ocean are at the
mid-ocean ridge; and the oldest are
at the trench in the subduction zone.
- Sea-Floor Spreading is the process
where the new crust is formed at the
mid-ocean ridge and destroyed at deep
ocean trenches melting back into the mantle.
Evidence for Sea Floor Spreading
New material is erupting along the midocean ridge. The presence of pillow
shaped rocks has showed that molten
material has erupted again and again.
Oceanic rocks shows that Earth’s magnetic
poles have reversed; iron bits lined up in
the opposite direction.
Drilling into the sea floor shows
the youngest rock closer
to the mid-ocean ridge;
the older rock are farther
Ridge push occurs when new crust forms at the
mid-ocean ridge and pushes older crust away
Slab pull occurs when a slab of old crust gets
pulled down by gravity at the trench
The processes of subduction and sea-floor spreading
can change the size and shape of the oceans.
 Because of this process, the ocean floor is renewed
about every 200 million years.
The Pacific Ocean is shrinking at an average of 9 cm/year
(3½ inches)! This is due to the fact that a deep-ocean
trench is swallowing more crust than the mid-ocean ridge
can produce.
The Atlantic Ocean is expanding an average of 2.3 cm/year
(1 inch)