If you look up 'aulacogen' in a dictionary of geological terms, the entry will read something like: "A failed rift arm." To the professional geologist with years of schooling in plate tectonics, seismics and fluid dynamics, that definition should suffice. But to the layperson, that explanation means nothing.

Formation
Below the Earth's relatively thin outer crust lies a thick layer of superheated, plastically-behaving rock called the mantle. As very hot liquid is wont to do, it moves around in massive convection cells, assisted by the planet's heat flow patterns, the rotation of the Earth and the movement of the tectonic plates on the surface.

All of this is rather chaotic, so sometimes large masses of magma push upwards via a convection cell and rise towards the surface through a phenomenon called a mantle plume. When a mantle plume occurs underneath an oceanic plate, this creates a "hot spot" and usually results in the creation of island chains like Hawaii (over a span of tens of millions of years). If a plume is generated underneath a land mass, however, this causes rifting -- the splitting apart of a continent.

Most rifting takes place on ocean floors, along what are called mid-ocean ridges. These ridges, now massive complexes stretching across the length of the oceans, once were subtle dome features on dry land. Because the Earth's surface is curved, it looks concave from within (the perspective of a mantle plume). And because of this curve, pressure from within doesn't snap the Earth's crust in two like it would a planar surface. Instead, it causes a "triple junction" of three planes of fracture, each roughly 120 degrees from one another. (Think of a chick emerging from an egg. When it first cracks the eggshell, generally three cracks in the same triple junction pattern are generated.)

When dealing with the Earth's crust, we'd have three active rifts forming new rock at the surface and pushing the existing rock outwards. I've rendered a plan view of an idealized rift system below, in ASCII-mation:


                         A                                                                  B 
                                                                                  
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                        @  @                                 \                               @      @                                                           
                        @  @                       -----------\                              @      @                                                           
                        @  @                       -----------/                              @      @                                                           
                        @  @                                 /                                                                                      
                        @  @                                                               @@                                                                    
                        @  @                                                            @@@                                                                    
                        @  @                                                         @@@             **                                                        
                     @@@    @@@                                                    @@@         @@      **                                                        
                   @@@        @@@                                               @@@         @@@          **                                                    
                @@@   @@@  @@@   @@@                                          @@@        @@@               **                                             
              @@   @@@        @@@   @@@                                    @@@         @@@                   **                                         
           @@@   @@              @@@   @@@                               @@@        @@@                        **                                          
        @@@   @@@                  @@@   @@@                          @@@          @@                            **                                       
     @@@   @@@                        @@@   @@                                  @@@                                **                                    
        @@@                              @@                                  @@@                                     **                                   
     @@@                                   @@@                              @@                                         **                                     
                                                                                
@ - Rift        * - Aulacogen                                

In Diagram A, the triple junction has formed, with the characteristic three arms at 120 degree separation. New crust is formed along the axis of the arms, and is pushed by newer rock outward at a direction perpendicular to the axis. Thus, new crust generated along the north/top arm would be pushed either east or west and rock generated at the southwest/left axis would be pushed either to the northwest or the southeast. Got it? Good.

In a perfect world, a mantle plume would rise straight up-and-down, and all three rift arms would propogate, right? In reality, convection implies circular motion, meaning that there is some lateral component to the direction of the plume when it reaches the surface. This, combined with inconsistencies in the thickness and relative strength of the crust means that the rifts will propogate at different rates. This imbalance usually results in one of the three arms ceasing to become active. Once the arm "fails" to spread, the rift arms is known as an aulacogen.

In Diagram B above, the triple junction from Diag. A has been aged by oh, let's say five million years. The north and southwest arms are still active and spreading is still taking place, but the southeastern arm has ceased to become active and is now an aulacogen. Once dormant, an aulacogen may reactivate, depending on subsurface activity.

Geology
As aulacogens essentially are rift valleys, they become sedimentary troughs. This means that sediment from other parts of the continental plates often finds its way to an aulacogen, where it is deposited. As the valleys are topographical lows, usually river systems run through them. Also, as the two active rift arms from the original triple junction grow into mature rift systems (or a solitary rift system, as may be the case), the rift becomes submerged under a nascent ocean, further increasing the chance for an aulacogen to play host to a river system.

Aulacogens are typified by graben structures when viewed in cross-section. Rocks will show signs of brittle stress, including compaction structures, transform faulting and intense fracturing. These fractures often play host to secondary ore deposits yielding copper, zinc and lead.

Modern Examples
The most commonly cited example of an aulacogen is the East African Rift, which is a reactivated aulacogen. Look at a map of eastern Africa and the Arabian peninsula, and note how the Red Sea and the Arabian Sea appear to be orientated at an angle of nearly 120 degrees from each other. Use those two rift arms to plot the third, through the length of Ethiopia and into Kenya. There's your aulacogen.

Other hypothesized -- remember, geology is an interpretive science -- aulacogens include the Benue Trough and at least fifteen suspected structures crisscrossing North America.

Sources
Tectonics (1995), Moores & Twiss, W.H. Freeman & Co.
Univ. of Sydney, Division of Geology and Geophysics - http://www.es.usyd.edu.au/geology/people/staff/prey/Teaching/Geol-3101/EReport03/GroupB/Report1/styles.html
Schlumberger Oilfield Glossary - http://www.glossary.oilfield.slb.com/Display.cfm?Term=aulacogen

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