Two tectonic plates under water move apart, and magma fills the space between them. this is an example of

Sea floor spreading

Explanation:

In case of a divergent plate motion, two plates move away from another. Due to this motion, there occurs separation of the seafloor, resulting in the addition of new materials to the crust along the mid oceanic ridge. This mid oceanic ridge marks the boundary of separation of the oceanic plates. It often generates deep focus earthquakes and forms volcanoes or chains of volcanic arcs.

Thus, the separation of the two plates under water causes seafloor spreading,  and the magma fills the space along the mid oceanic ridge.