It is believed that when the fountains of the great deep broke open, they did so along the tectonic boundary plate lines shown running around the world. These are the major fault lines (there are other more minor fault lines also):
Along these lines are where a lot of your volcanoes and earthquakes occur
Note: the ring of fire
Basaltic (heavy) rocks make up ocean floor
Granite and sedimentary (lighter) rocks make up continents
Later in Noah’s flood it is believed that the heavier basaltic rocks sank and the lighter granite rocks rose
Continental drift continued
Mountain building, earthquakes, volcanoes, and rifts
Causing the water to run off into the ocean basins
All of which completely changing the world as Noah new it.