Three of these often form a "triple junction" which, when passed over a hotspot, can lead to sea-floor spreading.  
When three of these combine, one of them usually forms an Aulacogen.  These features display on the earth's surface as a series of alternating uplifted areas called horsts, and sunken areas called grabens.  Three of these often form a "triple junction" which, when passed over a hotspot, can lead to sea-floor spreading.  Distinct from divergent boundaries and mid-ocean ridges, for 10 points, name these regions where the crust and lithosphere are being pulled apart, exemplified by a Great Valley of this type in Africa that contains Lake Tanganyika.
When three of these combine, one of them usually forms an Aulacogen.  
{rifts} [accept {rift valleys}]