Divergent plate boundaries on continents: rift valleys
Schematic diagram of continental rifts (modified from USGS material).
Spreading-plate boundaries do not only (although mostly) exist in the oceans, but on continents as well. After all, this process is believed to have separated the continents in the remote past of the earth. At continental spreading boundaries, a continental plate is in the progress of being ripped apart along a so-called rift, a set of parallel fractures that form a graben. If the process continues long enough and the continental crust is effectively separated, the continental rift graben turns into a normal mid-oceanic ridge.
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.
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).
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.