The oceanic crust is composed essentially of basalt, formed by magnesian silicates (hence formerly called sima). It is 5 to 10 km thick and is denser than the continental crust because it contains more iron. Seismic waves have a speed of 7 km / s.
Ocean dorsals are geologically young relief forms, so none of the existing dorsals are more than 200 million years old, which can be considered very little compared to the 4.6 billion years that Earth has.
