|"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
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
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."
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
You can find beautiful images in a new
expedition report from
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 Al-Haruj
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.