Divergent plate boundaries cause similar effects in oceans as they do on land. New seafloor erupts through rift valleys when plates diverge at mid-ocean ridges. The flow of magma at mid-ocean ridges form convection cells, and this movement is very likely to cause earthquakes.
A transform fault is when one plate slides past another, causing earthquakes on land and divide mid-ocean ridges into segments.
Divergent plate boundaries on land rip apart continents and create new ocean basins; this is called continental rifting. Magma rises and splits, and new ocean crust erupts through the space left behind. The Atlantic Ocean is a final stage example of continental rifting where a whole ocean now separates the Americas from Europe.