1. Sea-Floor Spreading

2. - The mid-ocean ridge is the longest chain of mountains in the world
- 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.
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3. Evidence for Sea Floor Spreading
New material is erupting along the mid-ocean 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
away.

4. 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

5. 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.

6. The Pacific Ocean is shrinking at an average of 9 cm/year (3½ inches)
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)