The ocean basins are the result of tectonic forces and processes. All of the ocean basins were formed from volcanic rock that was released from fissures located at the mid-oceanic ridges. 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.