Sea-Floor Spreading

I. Mapping the Mid-Ocean Ridge

A.     A.  The longest chain of mountains in the world is the mid-ocean ridge.

B.     B.  In the mid-1900s, scientists mapped the mid-ocean ridge using sonar.

1.      Sonar is a device that bounces sound waves off underwater objects and then records the echoes of these sound waves

C.     C.  The mid-ocean ridge curves along the sea floor, extending into all of Earth’s oceans.

D.     D.  Most of the mountains in the mid-ocean ridge lie hidden under hundreds of meters of water.

E.     E. Steep-sided valley splits the top of the mid-ocean ridge for most of its length.

II. Sea-Floor Spreading

  A.  Harry Hess, American Geologist, carefully studied maps of the mid ocean ridge.

  B.  In 1960, Hess suggested that the ocean floors move like conveyor belts, carrying the continents along with them, beginning at the mid-ocean ridge.

1.      The ridge forms along a crack in the oceanic crust.

2.      At the mid-ocean ridge, molten material rises from the mantle and erupts.

3.      The molten material then spreads out, pushing older rock to both sides of the ridge.

4.      As the molten material cools, it forms a strip of solid rock in the center of the ridge.

5.      Then more molten material splits apart the strip of solid rock that formed before, pushing it aside.

6.      This process, called sea-floor spreading, continually adds new material to the ocean floor.

III. Evidence of Sea-Floor Spreading

1. Evidence in Molten Material
   1. In the 1960s, scientists found rocks shaped like pillows in the central valley of the mid-ocean ridge.
   2. Such rocks can form only if molten material hardens quickly after erupting under water.
   3. The presence of these rocks showed that molten material has erupted again and again from cracks along the central valley of the mid-ocean ridge.

1. Evidence from Magnetic Stripes

1.  
   1. Rock that makes up the ocean floor lies in a pattern of magnetized “stripes.”
   2. The pattern is the same on both sides of the ridge
   3. These patterns hold a record of the reversals in Earth’s magnetic field.

III. Evidence of Sea-Floor Spreading

1. Evidence from Drilling Samples

1.      Rock samples were obtained from the ocean floor by drilling into it.

2.      Scientists found that the farther away from the ridge the samples were taken, the older the rocks were.

IV. Subduction at Deep-Ocean Trenches

A.    The ocean floor does not just keep spreading.

B.     It sinks beneath deep underwater canyons called deep-ocean trench.

C.     Subduction takes place at deep-ocean trenches.

1.      Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

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

V. Subduction and Earth’s Oceans

A.    The processes of subduction and sea-floor spreading can change the size and shape of the oceans

B.     Because of these processes, the ocean floor is renewed about every 200 million years.

C.     Subduction in the Pacific Ocean

1.      The Pacific Ocean is shrinking because the many trenches are swallowing more ocean crust than the mid-ocean ridge is producing.

D.    Subduction in the Atlantic Ocean

1.      The Atlantic Ocean is expanding.

2.      In most places, the oceanic crust is attached to continental crust, so as the ocean floor spreads, the continents along its edges also move.