Basalts, basanites and to lesser extent phonolite, nepheline-bearing trachyte, trachyte, and breccia of basaltic composition--all constitute the vast volcanic belt that transects Libya commencing from Gheryan on the northwest to Tibesti to the southwest marking a clear NW-SE trend. The belt is customarily ascribed to Tertiary-Quaternary volcanism. Al Harouj Al Aswad is the largest of volcanic fields and occupies a central position. However, smaller exposures are found in many places throughout the country. Such is exemplified in localities southwest of the city of Al Khums, north of Ben Waleed, south of Wow El Kebir, and in the area of Jabal Eghei in south Libya. Basaltic sills and thin flows as well as pillow lava occur in several, small localities in southern Libya.

The volcanic belt is characterized by basalts that spewed from both fissures and vents. Satellite imagery shows several stages of flows filling plains and wadis, but at the same time forming a a build up reaching tens to hundreds of meters thick. Because of deep level of erosion, gabbroic roots can be seen in Al Harouj Al Aswad and elsewhere within the belt, whereas shallow intrusives made up of phonolite and nepheline-bearing trachyte of alkaline character (e.g. Kaf Tkut, Ras Bu Ghannoosh, and J. Mantroos) are evident in the Gheryan Field. Lherzolite nodules brought up to the surface point out clearly to mantle derivation.

Amongst the pioneers who worked on the geology of Al Harouj Al As wad is Dr. Mabrouk Busrewil, of the University of Tripoli. Recently, Ashraf Ghafeer and Abdussalam Al Sha'affi, both of the University of Benghazi did their Masters degrees on parts of the Gheryan Volcanic Field and Al Harouj Al Aswad, respectively.