"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."
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
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."
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
You can find beautiful images in
a new expedition report from H.-J. Knoblauch et al.
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
Volcanoes in the central Al-Haruj
Ruin of a volcano in the southern
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.