oceanic lithosphere

The oceanic lithosphere is the rigid, rocky outer layer of the Earth that forms the ocean floor. It is part of the larger lithosphere, which is about 60 miles (100 km) thick and comprises the Earth's crust and the solid outermost layer of the upper mantle.12

This oceanic lithosphere is composed of basaltic rock, which is denser than continental crust. It is formed at divergent plate boundaries through seafloor spreading, where magma wells up from the mantle, cools, and creates new crust. As oceanic crust ages, it cools and becomes denser, eventually leading to its subduction beneath other plates at convergent boundaries. This continuous process of formation at mid-ocean ridges and destruction at subduction zones means that oceanic crust is relatively young, with a mean age of about 60 million years.3456