The oceanic lithosphere is composed of two parts: the oceanic crust (in blue on the animation) and the upper mantle (dark green). These two regions are made up of rocks of different nature:
- Low density basalt rocks (≈ 2,900 kg/m³ at the ridge level) for the oceanic crust.
- High density peridotite rocks (≈ 3,300 kg/m³) for the upper mantle.
The discontinuity of Mohorovičić (or Moho), is the boundary between these two zones. Given that it separates two rocks of different nature, it is considered a geochemical separation.
The asthenosphere is located under the lithosphere. Like the upper mantle, it is composed of peridotite, but its peculiarity is to be ductile whereas the peridotite of the upper mantle is rigid. Depending on the pressure, it is around 1300 °C that the physical properties of the rock change from solid to ductile.
The 1,300 °C isotherm is a physical boundary that separates the lithosphere (rigid rocks) from the asthenosphere (ductile rocks). This isotherm is generally between 50 and 100 km deep but rises to the surface when it is above a hot spot or ridge, where the asthenospheric mantle undergoes partial melting.
Part of the peridotite will melt to give magma. This magma, which is less dense than the surrounding rock, will rise and accumulate in a magma chamber within the oceanic crust. The magma present in the magma chamber is animated by convection movements. Under pressure, part of the magma can fracture the rock to create veins (dikes), some of which will reach the surface. The magma that emerges from these dikes will adopt the characteristic shape of a cushion, hence the name "pillow lava". Basalts are magmatic rocks derived from rapidly cooled magma made up of small crystals.
Another part of the magma follows the convection loops and rubs against the colder walls of the magma chamber. This slow cooling of the magma is at the origin of another type of rock according to the principle of fractional crystallization: gabbro (consisting of larger crystals than than basalt).
Basalt and gabbro are the two main components of the oceanic crust.
It is at the ridge that the formation of a new sea floor is made by magmatism, but it is not this magma surge that causes the plates to move apart. It is the subduction phenomenon at both the right and left ends of the plates that is responsible for the divergence of the plates.