This age is determined by the process of sea floor spreading. New oceanic crust is continuously formed at mid-ocean ridges, pushing older crust away from the ridge. As a result, the oldest oceanic crust is found furthest away from the mid-ocean ridges.

For the Atlantic Ocean, the oldest crust is found near the eastern coast of North America and western coast of Africa, where the Atlantic Ocean began to open up during the Jurassic period.