Our transcription: But marine geologists need more than physical samples to understand the sea floor.
It's like trying to understand an entire city by only visiting certain buildings and street corners.
By examining the shape and global distribution of submarine land forms and determining their rock composition as well, geologists have been able to link the processes which create these land forms with the formation of the ocean crust itself.
Most topographic mapping techniques used in seafloor exploration are based on the principal of sound waves bounced off the sea bottom, a technique called "Echo Sounding."
First developed for the military detection of enemy submarines in the 1920s, echo sounding devices emit a pulse of sound toward the ocean bottom and record the amount of time it takes for that sound to bounce off the bottom and back to the device.
By converting echo time into water depth, an echo sounding device draws a two dimensional profile of the ocean floor beneath the research vessel.
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