LAVA FIELDS OF SAUDI ARABIA

DR. ROOBOL: We’ve been working in the lava fields of Saudi Arabia for the past 20 years. First we went to map them and date and analyze the lavas. Next we returned for geohazard studies locating areas of microseismic activity and geothermal phenomena. We went again to look for gemstones. After that, we returned to site quarries for industrial materials such as for lightweight aggregates and pozzolan and basalt for fusing to make rock wool and fused basalt castings. Now with the Saudicaves project we have returned to locate lava tubes and we are expecting to cooperate with the Department of Antiquities to excavate caves with indications of occupation to levels of ancient man whose stone tools abound on the desert surface. There are many publications on the lava fields both in the international literature and in internal reports. There are also a set of 1:250,000 colored geological maps available through the Saudi Geological Survey. [See references at the end of this interview – Ed]

SAUDICAVES: Saudi volcanoes are related to the opening of the Red Sea, aren’t they?

DR. ROOBOL: That is what most people think but it’s not that simple. The Red Sea is oriented NNW and there are two types of basaltic lava-field in western Saudi Arabia. It is the older type which is deeply eroded and yields radiometric ages of 15 to 30 million years, that are associated with the opening of the Red Sea. These older lava-fields are so deeply eroded that there are no morphological volcanoes left.

The volcanoes that we can see today, e.g. by the side of the pilgrim expressway when we drive from Jeddah/Makkah to Madinah, belong to a much younger set of basaltic lava-fields with age dates from 10 million years right up to the historic eruptions. These young volcanoes and lava-fields form a distinct N-S chain in western Arabia and the vents are aligned N-S, quite different from the Red Sea direction. This chain is 600km long and we have called it the Makkah-Madinah-Nafud volcanic line. It forms the axis of uplift of western Saudi Arabia. Between Makkah and Madinah is the 20,000km2 Harrat Rahat lava-field with 644 scoria cones, 36 shield volcanoes and 24 domes. Between Madinah and the Great Nafud are the coalesced harrats Khaybar, Ithnayn and Kura with an area of 20,560km2 and 327 scoria cones, 46 basaltic shield volcanoes, 20 domes, 5 tuff cones, one basaltic stratovolcano (Jebel Qidr which looks like Mt. Fuji in Japan) and 39 massive and very long lava flows that we have named “whaleback flows” from their unusual appearance in the field. These the biggest and longest basalt flows in western Saudi Arabia and of course contain the biggest and most extensive lava-tubes.

All these volcanoes of the MMN volcanic line are younger than 10 million years and they are NOT related to the NNW opening of the Red Sea. They do in fact represent a completely new N-S crustal rift that began forming only 10 million years ago in western Saudi Arabia. So in a few tens of millions of years there might be a new ocean along this line similar to the Red Sea.

DR. ROOBOL: All basalts look the same – black dense rock usually with a few shiny crystals in long flows often about 10m thick with the famous pahoehoe and aa surfaces (Hawaiian names for ropey or blocky surfaces). However for the past century petrologists have been looking carefully at the chemistry and mineralogy of black basalt lavas and today the basalts of the different tectonic settings such as ocean floors, continental rifts, hot spots and lunar can be distinguished on their contents of such elements as Na, K, Y, Nb and Zr. The lava-fields of western Saudi Arabia are regarded as forming in a continental rift setting and their chemistry is described as alkaline in composition. However the lavas and volcanoes of the MMN line are only mildly alkaline with modest Na and K contents. The rock names for the MMN line lavas are: alkali olivine basalt (AOB); hawaiite; mugearite; benmoreite; trachyte and comendite. In contrast the young lava fields on either side of this line (such as Harrat Kishb) are strongly alkaline with the rock names: alkali olivine basalt, basanite, phonitephrite, tephriphonolite and phonolite.

Now, there’s a reason for this. The lavas along the high axis represent a rift zone that is forming today and within that rift, the magma is trapped inside the crust and undergoes a process of differentiation (removal of olivine, pyroxene and feldspar crystals) to produce extreme felsic compositions. In contrast, the lavas on either side, such as Harrat Kishb, are very primitive. They are mainly basanites coming direct from the mantle, without stopping on the way up and transporting with them pieces of the mantle in the form of peridotite.

SAUDICAVES: Would these differences in chemistry and mineralogy affect the creation of lava tubes?

DR. ROOBOL: No, this chemical difference is extremely subtle and we don’t expect it to have any influence whatsoever on the formation of lava tubes and the physical features of the flows and volcanoes themselves. It is the presence of water and volatile components within the lava that controls viscosity and physical forms. Water in particular can be picked up by lava near the surface.

DR. ROOBOL: The distribution of the lava tubes is influenced by the physical features of the lava flows. We have found in all the harrats along the MMN Line, that there are three stratographic units. The oldest is characterized by extremely large volume lava flows that cover great distances. They are of enormous volume and it is in these large lava flows (which we have named “whaleback”) that you will find the longest and the most frequent lava tubes. As the harrat evolves, the lavas become smaller in volume with time and so the flows become shorter. The volume erupted gets smaller and smaller. So maps of the harrats show three stratigraphic units superimposed one on top of another. The youngest one has a small area, the middle one has a medium area and the oldest parts of the harrat extend furthest on either side.

So, it’s very much the older parts of the harrats with these very large-volume and very long flows which have the longest lava tubes within them. But there are many smaller lava tubes in the youngest lava flows.

DR. ROOBOL: It depends which harrat you’re dealing with. In the case of Harrat Rahat, they would go back to about ten million years. In the older parts of Khaybar to about five million years and the older parts of Kishb to only two million years, based on the existing potassium-argon dating.

SAUDICAVES: What is the age of the lava tubes we found near Jebel Hil in Harrat Kishb?

DR. ROOBOL: The Kishb lava tubes belong to the youngest stratographic unit which contains the shortest lava flows. Jebel Hil and its lava tube is as young as 2000 years but the adjacent eroded flows with caves may be as old as one million years. These younger lava tubes are only partially filled with sand and debris. Where the lava flows are five, eight and ten million years old, the lava tubes are either eroded off the surface or filled with erosional debris.

SAUDICAVES: Isn’t it likely that tubes in the older lava would have collapsed due to the lava breaking up?

DR. ROOBOL: I haven’t seen that much erosion. No, I think the collapses on them are long chains of collapse craters that probably date from the time of the eruption, when the lava was removed from the tube and the roof collapsed from lack of support, rather than late erosion. This is indicated by lava draping down into some of the collapses. The late erosion is still very slight.

SAUDICAVES: Can you tell us something about Harrat Kishb where the first reconnaissance for lava tubes took place?

DR. ROOBOL: Harrat Kishb is one of the smaller lava fields in Saudi Arabia. It’s about 5,890 km2 in area with 162 scoria cones 1 shield volcano, 9 domes, 6 tuft rings and 1 whaleback lava flow. It has been divided into three stratigraphic units. It’s quite difficult to date the oldest lavas because they’re covered in caliche and salts caused by water evaporation. So we are not entirely delighted with our numbers of samples dated, but the oldest rocks appear to be only two million years old.

There’s a wide variety of land forms on Kishb. They are mainly built up from monogenic – that’s one eruption – volcanoes, producing scoria cones and basalt lava flows. But in the youngest parts of the harrat, you get differentiated products of phonolite domes with small areas of pyroclastic flows and fall fields around them. It’s very distinctive, green weathering phonolite.

Harrat Kishb is very well known in Arabia because of its abundance of peridiotite nodules carried in the dominant rock type, which is basanite, a very primitive basalt. The attraction, of course is that some of these peridiotite nodules carry large crystals of gem-quality olivine and Saudi Arabia will shortly have a gem industry, as we have drilled one prospect which has been licensed, which contains 7,000 kgs of gem-quality peridot in a very small area, in gravels less than seven feet thick.

DR. ROOBOL: There are three lava-tube sites that have been identified on Kishb in our preliminary reconnaissance. This is interesting, because they show that the lava tubes formed in three different parts of a typical lava flow. The big ones on Jebel Hil formed right next to the vent, and this is the simplest way they form. Lava continues to flow after leaving the vent and the flow front is fed by draining the near-vent area of the flow. So you’ve got a long, empty tube forming next to the volcano.

The ones to the east of this area formed in the middle of a flow, in a very interesting place, because the lava flow divided around an old scoria cone, building up as a series of arcuate ridges that wrapped around the older cone. These ridges are topographically higher than the surrounding flow. Only the high parts of the ridges drained to form local lava tubes, where lava has piled up behind an obstruction.

The third cave, Dahl Faisal, is formed somewhere in the middle of a very long flow. In this case, it was a pahoehoe flow with a thin roof and as the flow was continuing to propagate, the roof was inhaled as a conical funnel which popped at the bottom to form a nice, smooth, round, funnel-shaped entrance and air was sucked into the interior to form a lava tube only from that point downslope.

So, there are three different mechanisms for the three localities that have been found on the very first lava-tube reconnaissance of the lava fields.

SAUDICAVES: Thank you very much for spending this time with us.

The scientific results of the petrology of harrats were published in the international scientific literature available in geology and earth science libraries around the world. The colored maps are on sale at the Saudi Geological Survey. Data on industrial uses, gemstones, seismic hazard, geothermal phenomena and lava tubes remain in internal reports of the Deputy Ministry of Mineral Resources, Jeddah, some of which are confidential. A full set of these internal reports are available in the library of the Saudi Geological Survey, Jeddah.

Camp. V.E., Hooper, P.R., Roobol, M.J., White, D.L., 1987, The Madinah eruption, Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types: Bulletin of Volcanology, volume 49, pages 489-450.

Camp, V.E. and Roobol, M.J., 1989, The Arabian continental alkali basalt province: Part 1. Evolution of Harrat Rahat, Kingdom of Saudi Arabia: Bulletin of the Geological Society of America, volume 101, pages 71-95.

*Camp, V.E. and Roobol, M.J. 1991, Geologic map of the Cenozoic lava field of Harrat Rahat, Kingdom of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources Geosciences Map GM-123, scale 1:250,000, with text 37 p.

Camp, V.E., Roobol, M.J. and Hooper, P.R., 1991, The Arabian continental alkali basalt province: Part II. Evolution of Harrats Khaybar, Ithnayn and Kura, Kingdom of Saudi Arabia: Bulletin of the Geological Society of America, volume 103, pages 363-391.

Camp, V.E., Roobol, M.J. and Hooper, P.R., 1992, The Arabian continental alkali basalt province: Part III, Evolution of Harrat Kishb, Kingdom of Saudi Arabia: Bulletin of the Geological Society of America, volume 104, pages 379-396.

Camp. V.E., and Roobol, M.J., 1992, Upwelling asthenosphere beneath western Arabia and its regional implications: Journal of Geophysical Research, volume 97, number B11, pages 15,255-15,271.

*Roobol, M.J. and Camp, V.E., 1991a, Geologic map of the Cenozoic lava fields of Khaybar, Ithnayn and Kura, Kingdom of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources Geoscience Map GM-131, scale 1:250,000, with text 40p.

*Roobol, M.J. and Camp, V.E., 1991b, Geology of the Cenozoic lava field of Harrat Kishb, Kingdom of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources Geoscience Map GM-132, scale 1:250,000, with text 34p.

*Roobol, M.J., Pint, J.J., Al-Shanti, M.A., Al-Juaid, A.J., Al-Amoudi, S.A. and Pint S., with the collaboration of Al-Eisa, A.M., Allam, F., Al-Sulaimani, G.S. and Banakhar A.S., 2002. Preliminary survey for lava-tube caves on Harrat Kishb, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2002-3, 35 p., 41 figs., 1 table, 4 apps., 2 plates

* For sale at: Publications Dept., Saudi Geological Survey, P.O. Box 54141, Jeddah 21514, Saudi Arabia. Fax (966-2) 619 7704