Divergent boundaries are plates that move away from one another causing underwater volcanoes and the spreading of the seafloor.
The Spreading Seafloor
When divergent plates move they move away from each other. When two oceanic plates move away from each other the seafloor starts to spread apart making more and more and more ground at the bottom of the sea.
Why do they move apart?
Deep in the mantle are huge convection currents. When they move up the plates start to move apart because of the rising convection currents pushing on the bottom of the asthenosphere.