What happens when a continental plate splits apart

The two parts of the continent start to move away from each other

causing rifting of the continental plate, the center or the rift becomes thinner, weaker and starts to sag. The basin area in the middle which has sagged is usually filled up with water, forming a sea. (e. g. the red sea)

The thinner plate in the center then becomes oceanic crust from this, and in the center a new constructive plate boundary is formed. The plates continue to move apart from one another and magma erupts from the constructive/divergent plate boundary, producing new oceanic crust formed mainly of basalt.