Crater-like structures in context with the large flood basalt field
of Al-Haruj al-Aswad (Libya)

Norbert Brügge, Germany
Dipl. - Geol.

 


Al-Haruj al-Aswad

"Basalt and related rocks occur as Tertiary-Quaternary lava flows that cover extensive areas along what appears to be structurally controlled belt that transects Libya from the northwest at the Ghiryan (Gharyan) area to the southeast at the northernmost tip of the Tibesti Basement province. Cursory examination of satellite images of Al-Haruj al-Aswad and Jabal Assoda (as-Sawda), the largest volcanic provinces in northeast Africa and centrally located in Libya, suggests three major phases of volcanic activity.
Lava appears to have emanated to the surface largely through vent and fissure conduits with prevalence of the former over the latter as can be derived from detailed imagery of Jabal Assoda. At this juncture and based on texture-colour variation evident on satellite images, it seems that centres of volcanism within the two said provinces have in general shifted along a westerly direction, but producing significantly smaller volume of basalt and related rocks. On the eastern periphery, it is of interest to note some remnants of gabbroic roots. Isolated volcanic activity had occurred east and south, evidence of which are eroded volcanic conduits and explosive eruption such as that documented for Wow-en-Namous.
Field work in Jabal Assoda and Al-Haruj al-Aswad shows evidence of intermittent, but prolonged lava spewing as attested to by multiple flows on both the macro and mesoscopic scales. A single basaltic boulder or block, for instance, may show varying degrees of vesiculation and flattening. Although vesicle gradation in terms of size and frequency are evident, no generalization can be made as to the rate and amount of flow. However, it can be said that in places, flow and cooling were both rapid as attested to by thin layering of only a few centimeters. Locally, vesicles both circular and flattened can be found filled by calcite.
In contrast to both Al-Haruj al-Aswad and Jabal Assoda, the Ghiryan basaltic field to the northwest has associated with it some phonolite and trachyte, both of which are possible fractionates of an alkaline, silica-depleted basaltic magma. In conjunction with this assumption of a common origin, the structural control of volcanic activity referred to earlier is interpreted as an aborted rifting."
Source: AGCR
  
The northern part the Jebel Al-Haruj is marked as Al-Aswad. The southern part is marked as Al-Abiyad. Here are concentrated 120 single eruptions approximately. To the volcanism of this area was to recently nothing known. Newer research of Hungarian scientists however prove numerous interesting structures in this area and an active volcanism still in the Holocene (the most known volcano is the Wau-an-Namus).
"The Al-Haruj al-Aswad (HA) is part of an alkaline basaltic intracontinental flood basalt field in central Libya (~6 to 0.4 Ma). The study area of 1400 km: contains ~120 vents, comprising shield volcanoes, spatter and scoria cones, formed on a lava plateau. Eighty-four vents fall into an ESE-WNW-trending, ~60-km long, <10-km wide alignment similar to the orientation of the Tibesti Line. Short alignments of 3 lo 5 vents (<3 km) suggest eruptions through fissures. Most of the shield volcanoes had an eruptive phase characterized by gas exsolution, magma disruption and Hawaiian-style lava fountaining indicated by the presence of thin welded agglutinate.
On the flank of the shield volcanoes lava channels are inferred to empty the shallow magma chambers and/or lava lakes, causing subsidence and pit crater formation often with non-uniform collapses. Pyroclastic deposits are massive to poorly bedded, have abundant flattened, deformed spatter, and low proportion of dense coherent lava fragments supported by fine matrix or they are matrix-poor. Spatial relationships between lithofacies, steep dips, and rapid changes in thickness and grain size of the beds allow the interpretation that the deposits are a part of eroded remnants of Hawaiian and Strombolian cones. The lack of accidental lithic rocks and chilling of juvenile fragments indicate that the volcanism of HA occurred in an area of minimum water availability."
Source: GeoLines


Source: AGCR


A photo of the central part of the volcanic chain exhibiting large shield
 volcanoes with wide pit craters on their tops and smaller pit craters
 on their flanks (black arrows)


 


        


         
Flood basalts in the centre of Al-Haruj al-Aswad


You can find beautiful images in a new expedition report from  H.-J. Knoblauch et al.

A selection:

   
This lava ropes in the northern Al-Haruj are thousand years old in maximum                                                             Massive basalt at a hole                                              
 
        
Young lava nearby Al-Fogaha                                                                                                      Typical terrain in the Al-Haruj

        
         Al-Haruj edge-area                                                                                                    Volcanoes in the central Al-Haruj

       
Ruin of a volcano in the southern Al-Haruj area

       
"Black Hole"

Christian Laroubine, Societe Astronomique de France, presents now some Landsat - pictures from centrally Libya, on which a strikingly field of crater-like structures was to be seen. These craters-like structures have a surprising similarity with the craters in the Gilf Kebir (Egypt).
After searching of the craters-field was to be ascertained, that could be a context to the Tertiary flood basalts in the Al-Haruj al-Aswad. The new craters-filed is found near in the southwest of the large basalt shield of the Al-Haruj al-Aswad. There is a large ring structure and many small craters. Some are covered evidently with outflows of basalt. One crater has a similarity to the Oasis crater (nearby Kufra). The large ring-structure is crossed by long dykes. An example for the direct proximity of basalts is the Waw-al-Kebir (see below). Further structures are to be found here.


Basaltic plug Waw-al-Kebir: 16° 43' 25" E and 25° 18' 52" N

The supposed connection of the craters field with the basaltic volcanism in the north would be a further proof, that also the craters-like structures in the Gilf Kebir and in the Kufra region are of subvolcanic origin, as by me was postulated.




Striking section of crater-field: 16 24'  E and 24° 50' N



One crater similar of Oasis crater (Kufra region)
Position: 17° 20' 12" E and 25° 13' 07" N



Details of craters and dykes



Ring-structure with cross-cut of long dykes
 



Stretched craters, locally basalt - outflows probable