Divergent plate boundaries occur when two plate are moving away from each other. If this type of boundary occurs on a continent it will eventually form an ocean basin. Fisrt though it will begin as a rift like the one shown below.
This rift which is part of the Great Rift Valley in Africa will eventually over millions of years seperate the two land masses and form an ocean basin.
Not all divergent plate boundaries occur on continents, in fact the most active occur on the sea floor. When the two plates diverge on the sea floor magam rises up from the earth producing new oceanic crust.
The Mid-Atlantic Ridge is an example of a divergent boundary as two plate move away from one another new earth is being created. This process is actually making the Atlantic Ocean larger everyday!
Below is a diagram of a mid-ocean ridge as it spreads. The blue and red arrows represents the magnetic pull of the earth when the rock was created. Scientists use these marks to determine how old the ocean is.
