How are new seas formed?

Seas arise at places where the Earth's crust cracks open due to expansion. Around 200 million years ago the Atlantic was formed in this way. Today in East Africa, a crack from Malawi to Ethiopia has formed as a result of the eruption of a volcano. If the soil sinks further, water from the oceans will enter the region to give rise to a water body. 

This has already happened in the case
of Red Sea, the continuation of the East African rift system. If the plate continues to
expand, then at some point of time it breaks completely giving rise to a long mid-
ocean volcanic ridge.