C.K.K.K.+1.4

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=**Section 4 (1-4) Sea- Floor Spreading**=

**Vocabulary**
mid- ocean ridge: the longest chain of mountains in the world. sonar: a device that bounces sound waves of underwater objects and then records the echoes of these sound waves. sea- floor spreading: the process that continually as new material to the ocean floor. deep- ocean trenches: a deep under- water canyon. subduction: the process by which the ocean floor sinks back into the mantle.

**Mapping the Mid- Ocean Ridge**

 * in the mid 1900's scientists mapped the mid- ocean ridge using sonar.
 * it extends through all the world oceans.
 * most mountains that are in the mid- ocean ridge lie under water, but in some places the ridge pokes above the surface.

**Evidence for Sea- Floor Spreading**

 * a man named Henry Hess, an American geologist, studied the mid- ocean ridge and carefully studied maps of the mid- ocean ridge.
 * evidence from molten material
 * rock shaped like pillows and toothpaste
 * evidence from magnetic strips
 * iron lined magnetic pole
 * evidence from drilling samples
 * youngest rock near center of ridge, the older the more far away

**Subduction at Deep- Ocean Trenches**

 * deep- ocean trenches subducts the ocean floor
 * ocean floor sinks back into the mantle

**Subduction and Earth's Oceans**

 * ocean floor renewed every 200 million years subduction in the Atlantic and Pacific ocean
 * Pacific ocean covers one third of the planet but is shrinking
 * Atlantic ocean is expanding
 * less short trenches, more ridges