Divergent plate boundaries on continents: rift valleys
Volcanism at rift-zones is very much analogous to mid-ocean ridges, but more complicated because the thickness of the continental crust is much larger and rising magmas can undergo more differentiation and other chemical reactions with the crust material. Thus, volcanoes on continental rifts display a more complex variety of features and are much less well-understood.
The most prominent example of a continental rift is the East-African Rift Valley and its continuations into the Red Sea and the Gulf of Aden. If the rifting between East Africa and the rest of the continent continues, a new ocean will eventually be formed here. In the Red Sea, where sea water already has flooded the rift, this can already be observed - the youth stadium of a new ocean.
Rifting has been pulling Arabia away from the African continent and a new ocean is developing in the Red Sea and the Gulf of Aden. Another rift, the East African Rift Valley, is still in the phase where it forms a graben that is trying to rip East-Africa apart from the rest of Africa. (modified from USGS).