When three of these combine, one of them usually forms an Aulacogen.  
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}]