Geological Survey Open-File Report SGS-OF-2002-3
are known to occur in the Phanerozoic limestones of northern and eastern Saudi
Arabia. They formed in past intervals of moist climate rather than the
present hyperarid one. Few caves are known in the west because the
predominant rocks are metamorphosed, tectonized and crystalline, comprising the
Precambrian shield. There are however two geological units younger than
the crystalline basement in western Saudi Arabia, where caves might be found.
These are: (1) raised Quaternary coral limestones along the Red Sea coastline;
(2) the extensive Cenozoic basalt lava fields.
present report describes the first and successful search for lava-tube caves in
the Cenozoic basaltic lava fields of Harrat Kishb (area 5,892 km2).
Cenozoic lava fields, or harrats, occupy 80,000 km2 of western
Saudi Arabia, where they mainly occur as 12 large lava fields in a chain
extending from Syria in the north, across easternmost Jordan, southward for the
entire length of western Saudi Arabia; and into Yemen in the south.
investigation reported here was largely confined to an area of central Harrat
Kishb. Here, six lava-tube caves were located in three closely spaced
areas in central Harrat Kishb in basalt lavas of four different stratigraphic
units. All occur in lavas of the Cenozoic Hil basalt, which is less than
one million years old. The Hil basalt is subdivided into six subunits.
By far the largest and longest lava tube occurs on the western side of the
scoria cone of Jabal Hil, which erupted a few thousands of years ago and belongs
to the youngest subunit Qh6 of the Hil basalt. A detailed geologic map was
made of the surface features of a major arterial lava tube extending for 3 km to
the west of the parent cone. Basalt lava flows were extruded from five
points along the arterial lava tube to build a ridge comprising a chain of low
rootless shield volcanoes. Later, when the eruption ceased, the arterial
lava tube drained and subsidence occurred into it. The interior of the
lava tube is visible in several of these collapses. It has a height of
over 20 m and the floor has a maximum below-surface depth of 42.5 km.
Nearby, in stratigraphic subunit Qh4, a single lava tube cave (First Cave) is
visible at the bottom of a 26.5 m deep shaft. The sides of this shaft
are dangerously loose and the cave was not explored. Farther east, in
subunit Qh3, three closely spaced lava-tube caves were found and are here named
Ghostly Cave, Bushy Cave and Kahf al Mut’eb
caves. The largest of these, Ghostly Cave has two passages extending 143 m
and 140 m from the entrance. Nearby, in the same stratigraphic
subunit, a small erosional cave (Window cave) only one meter deep is present in
weakly lithified agglomerate. This is the only example of an erosion cave
known on Harrat Kishb. A sixth lava-tube cave (Dahl Faisal), some 22 m
long, was located on northern Harrat Kishb in subunit Qh1 of the Hil basalt.
The lava-tube caves are elliptical in cross-section and have short lava
stalactites on the roof and lava levées on the walls. They contain
evidence of ancient man in the form of defensive walls, and two throwing sticks
of possible Neolithic age were found. These tubes are half-filled with
fluviatile sands and wind-blown dust, which may preserve a stratigraphic record
in pollen and spores.
tubes form in basalt lavas after eruption ceases to extrude molten lava.
At this stage, basalt lava may continue to flow downslope away from the volcano
by draining the main feeder tube inside the flow at its highest point.
Theoretically, every lava flow can have a lava tube. There are over 2,000
basaltic volcanoes standing in western Saudi Arabia, many of which have produced
multiple lava flows. This fact and the success of the first search for
lava-tube caves suggests that numerous such caves may exist in the Cenozoic lava
fields of western Saudi Arabia, and further searches are recommended.