The oldest rocks found in these basins are approximately 200 million years old. This is a lot younger than the oldest continental rocks which have ages greater than 4 billion years. The reason for this discrepancy is simple. Oceanic rock is returned to the Earth's mantle when oceanic crust is subducted . Many of these subduction zones occur at the continental margins where oceanic crust meets continental crust .
Subduction also creates the ocean's deep trenches .
Most of the general information concerning the depth of the ocean basins were made after World War I when the echo sounder was developed for military purposes. This instrument accurately determines the time between the emission of a strong acoustic pulse and the detection of its echo. Using this principle scientists can determine the distance from the sounder to the ocean bottom. The following image displays the topography of the Earth's terrestrial land surface and ocean basins. In the ocean basin, the gradation from red to yellow to green to blue indicates increasing depth.