An illustration of the likely formation history of the Southern Oklahoma Aulacogen. In A, volcanoes form along a continental plate boundary. B shows the triple junction's basic appearance following its formation. C shows that one of the three "arms" has not split while the other two do, continuing into D, in which the Iapetus Ocean forms.
An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Rift zones are places where new crust is formed. An aulacogen is a rift zone that is no longer active.
