The end of WWII…sound waves.• After WWII, technology advanced and scientists began to

experiment with new tools. • One tool, sound waves, helped to give support to Wegener’s

idea of continental drift.SONAR

SOund Navigation And Ranging

Sound Waves: Echo Sounding• Scientists began to map the ocean floor by using Echo

Sounding.• As sound waves were sent to the ocean floor, they would

bounce back up off the terrain below.• The longer it took for the sound waves to return to the ship,

the deeper the water.

Sound Waves: Echo Sounding• Using the sound waves, scientists discovered systems of

underwater mountains, volcanoes, and valleys. • The world’s largest mountain range is in the Atlantic Ocean…• The Mid-Atlantic Ridge

Pacific Ocean BasinAtlantic Ocean Basin

The Mid-Atlantic Ridge

The Mid-Atlantic Ridge• Harry Hess proposed that

mid-ocean ridges form because areas of the ocean crust is spreading apart (divergent)

• He called this idea, Sea Floor Spreading

Sea-Floor Spreading• The Earth’s lithospheric plates

move along with convection currents in the mantle rifting plates apart along the mid-ocean ridge.

• Molten material erupts through the valley that runs along the center of the mid-ocean ridge.

• As hot magma is forced upward, the sea floor spreads apart.

• This material eventually cools, sinks, and hardens to form the high ridged rock of the ocean floor.

Sea-Floor Spreading• Oceanic crust moves in a conveyor belt fashion.• New oceanic rock is formed along the mid-ocean floor.• Old rock is destroyed farther away in deep ocean trenches where

oceanic plates subduct and sink down into Earth’s interior.