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Vocabulary

 * Mid-ocean ridge**:the longest chain of mountains in the world.
 * Sonar**:a device that bounces sound waves off underwater objects and then records the echoes of the waves.
 * Sea-floor** **spreading:**the process that continually adds new material to the ocean floor.
 * Deep-ocean trenches**:deep underwater canyons.
 * Subduction**:the process by which the ocean floor sinks beneath a deep-ocean trench.

**Mapping the Mid-Ocean Ridge**

 * Sonar maps sea-floor ridge
 * most mountains in the sea-floor are usually hidden under hundreds of meters of water.
 * The island of Iceland is an example of a mid-ocean-ridge

**Evidence for Sea Floor spreading**

 * Evidence from molten material

Scientist found strange rocks that looked like toothpaste squeezed from a tube,on the sea floor


 * Such rocks can only be found when molten material hardens quickly after erupting under water.

Evidence from magnetic stripes

 * Scientists discovered that rock that makes up the ocean floor lies in a pattern of magnetic stripes
 * Evidence from drilling samples

Samples from the sea floor were brought through pipes of a drilling ship. They found that further away from the ridge the samples were taken, the older the rocks were.The youngest rocks were always in the center of the ridges.

**Deep ocean trenches**
Subduction takes place in deep-ocean trenches.
 * A deep-ocean trench forms where oceanic crust bends downward.


 * At deep-ocean trenches, subduction allows part of the ocean floor to sink back to the mantle, over tens of millions of years

**Subduction and Earth's oceans**

 * The process of subduction and sea-floor spreading can change the size and shape of the oceans.
 * Subduction in the pacific ocean

The pacific ocean is shrinking, because the deep-ocean trench has swallowed more oceanic crust than the mid-ocean ridge. > and the ocean gets wider. back to homepage
 * Subduction in the Atlantic Ocean
 * The Atlantic Ocean is expanding because it only has a few short trenches. The spreading sea floor moves the continents,