How do convection currents form underwater mountains?

The convection currents are the ones that create the force that breaks up and moves the crustal plates on the surface of the planet. By moving the crustal plates, the convection currents contribute to the formation of different types of plate boundaries, including the divergent plate boundaries.

The divergent plate boundaries are the boundaries where two plates are moving away from one another because the convection currents are dictating that kind of movement from bellow them. As they move away from each other, they leave a gap between them, and the convection currents push up a lot of magma towards the surface. The magma cools of very quickly in the water, thus forming rocks constantly, rocks that little by little start to make a big mas, that eventually turns into an underwater mountain, or rather a mid-ocean ridge.