Scientists used sound waves (echo sounding) to map the ocean floor and discovered underwater mountain ranges, volcanoes, and valleys. This helped support the idea of continental drift. The Mid-Atlantic Ridge was found to be the largest undersea mountain range. Harry Hess proposed the theory of sea floor spreading to explain how new crust forms as the lithospheric plates move apart at mid-ocean ridges due to convection currents in the mantle.

1. 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.
SOund Navigation And Ranging

2. 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.

4. 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

5. The Mid-Atlantic Ridge

6. 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

7. 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.

8. 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.