Wadi Abyad

May 2010


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Walking through the Moho.
Al Abyiad, Abyad, Abyadh, ophiolites, gabbro, peridotites

Wadi Abyad is well know with geologists for its walk through the Moho (more of that below). It is also a very nice walk for non-geologists through a wadi that always carries water and features 'blue & milky white' pools with white cabonate clouds precipitating from the water. The whitish pools, according to popular here-say, give wadi Abyad its name, the 'white wadi', contrasting with the surrounding dark ophiolite rocks.

Somehow this wadi escaped being written up for a long time, but that is not because we had not been there before. This visit was triggered by an e-mail in February 2010 from a lady called Nadège: 

"........I am a French journalist and I work for the society Adamis who prepares a film collection on "global warming and its consequences on men". The goal of those films is to inform on the global warming, to make people sensitive to it and to show that, today all over the world men have already to face the consequences of global warming and try to find solutions. Those films are 52 mn,  and will be first broadcasted on "France 5", french channel, then in internationals channels. You can watch a presentation video of the series in English on J'ai Vu Changer La Terre    ......"

She wanted me to explain about the Oman ophiolites and the significance of these rocks for CO2 sequestration as part of the global CO2 cycle. That's where the link with global heating comes in. She was the second French journalist who contacted me for the same topic in a few months time. Indeed a hot topic.

The reason ophiolites got into international attention is recent research indicating the potential of these rocks to absorb huge amounts of CO2 from the atmosphere (CO2 sequestration). To understand this we have to go back to plate tectonics. A large part of Oman's ophiolites consist of what originally were igneous rocks from deep below a now vanished slab of an oceanic plate. Some 90 million years ago, a large ocean, known as Tethys, closed and part of it was pushed on top of the eastern edge of the Arabian plate. The Oman ophiolite, as the huge slab of oceanic crust and underlying earth mantle is called, is the largest and best exposed succession of oceanic rocks in the world (check-out a small write-up for the PDO Al Mahal magazine). For more on ophiolites see also Schreurs (2003, 2010). The mantle rocks are the ones that are important for the CO2 story. They originate at depths of more that 6 to 7 kilometers below the once Tethys Ocean, once partly molten and consisting of weird rocks with weird names (peridiotites, harzburgites). They are rich in dark minerals that are low in silica compared to the overlying oceanic crust. The main minerals are olivines and pyroxenes (Magnesium, Iron Silicates) that crystallised at high pressures and temperatures in the Earth Mantle. Now, at the earth surface these minerals rapidly weather and convert into minerals that are more stable at low pressure and temperature. Weathering olivines can grab a lot of CO2 from the atmosphere and through a series of chemical reactions form magnesite, a stable magnesium carbonate; the abundant white veins that one can see in the ophiolites. Scientists have come-up with ideas of how this natural process can be enhanced and may help to reduce atmospheric CO2 levels. See for example the story in Reuters and a good paper on this topic in the July 2009 issue of the Geological Society of Oman as well as  Kelemen & Matters (2008).

Wadi Abyad provides an easy drive and walk through one of the larger chunks of ophiolite, the Nakhl Rustaq ophiolite massif , just north of the Al Hajar Mountains. A slice from oceanic crust down into the earth mantle.

The transition between the earth mantle and the overlying earth crust is known as the 'Moho' after Mohorovičić, a Croatian seismologist who observed seismic velocity changes associated with this transition. 

A good geological description can be found on the website of the "Laboratoire de Tectonophysique de Montpellier" a Virtual fieldtrip through the Oman ophiolites, specifically a fieldtrip to Wadi Abyad that describes this geological walk both from the south as well as from the north side.  See also Hanna (1995).

Google Earth map Wadi Abyad, which can be reached from the Nakhl to Rustaq road. For Google Earth track, see WAbyad.kmz

Detail Google Earth map wadi Abyad. See Google Earth track file WAbyad.kmz
How to get there:
From Barka Roundabout drive to Nakhl and continue in direction of Rustaq.From here the road follows the gap between the southern edge of the Nakhl - Rustaq ophiolite massif in the north and the rising dipslopes of the Al Hajar Mountains to the south. Approximately 2.7 km from junction to wadi Mistal is a relatively new tarmac road signposted 'Subaykah' ( 23°21'50.47"N -  57°40'14.94" E), which will lead you into Wadi Abyad. Follow the track through the wadi north. At the end (some 7.7 km from junction) are some higher terraces where you can safely park your car, or even camp (in 2010 some tourists had to be recued from this wadi when they were surprised by a sudden flooding of the wadi, so better be safe than sorry).
Driving any further is not possible. From here you can walk to the village of Abyad some 8km further. Take enough water and your swimsuit. You will meet the Moho where the wadi sharply turns west.
The TV crew was keen to take also good shots on route through the wadi, with video cameras fixed on the roof, on the front and mirror, giving different perspectives of the drive. Henk Droste (on holiday in Oman) joined to provide more interesting imagery of two rather than one geologist exploring the wadi.

The drive starts in the dark brown rocks of the upper mantle, consisting of serpentinized oilivine (brown), orthopyroxene (light green) and chrome spine (black), compositionally a harzburgite with some lenses of dunite (only olivine and spinel). The banding further in the wadi represent different upper mantle magma's.  It ends in oceanic crust at first with layered gabbros (very coarse grained and massive) and followed by basalt dykes (sheeted dykes). A careful search will reveal also the fibrous asbestos in many places.  

Fixing a camera on the roof.

The TV crew including cameraman, journalist and soundman

With camera and sound-gear walking into the wadi

One of the blue pools in the wadi with gas bubbling (methane?) in the water and carbonate precipitating as as fine crust on the algal mats. The white cloud of fine carbonate material in suspension gives the wadi its name:  'Wadi Abyad', the white wadi.

Wadi Abyad Satellite map (above) and draped with Geological map (below). The contact between mantle rocks (south, green) and crustal oceanic rocks (north, reddish) is clearly recognisable on the satellite map where the wadi turns sharply west. This is also the place where the falaj to the gardens of the village of Abyad starts. 

Henk looking at the find carbonate mud. The pool in front of him is lined with green algal slimy mats that become encrusted with carbonate precipitating from the water.

Wrinkled algal mats encrusted with carbonate.

Wadi Abyad downstream, descending to the Moho

Preparing the camera.

The wadi narrows, cutting through its own gravels that are cemented by carbonate; as hard as concrete
Children playing with donkeys in the wadi near the village of Subaykah. Fun in the water, and providing a nice backdrop of the ophiolite story.

One of the messages I tried to give to the filmmakers is not only CO2, ophiolites and global heating, but also the rest of the geological equation. If we would not have the huge masses of igneous rocks in for example India (Deccan traps) and in many other places in the world, the CO2 level in the atmosphere would be much higher today. Emphasis on Oman's ophiolites may lead to forgetting that the main Al Hajar Mountains consist of limestone like so many other mountain ranges in the world. Their weathering and erosion leads to the massive release of CO2 in the atmosphere. Simply stated: the impact of global tectonics on the earth atmosphere is huge. We should be more surprised about having had several thousand years of relatively benign tectonics rather than the huge natural catastrophes that are normal on a geological timescale. That is not to say that we should continue with unrestricted pollution of our world. Certainly not, but in addition to more environmental friendly living we should also be prepared for big natural problems. A relatively small volcano in Iceland totally disrupting global air traffic just when we were visiting Wadi Abyad makes the point.  


  • Hanna, S., 1995. Field Guide to the Geology of Oman. Published by the Historical Association of Oman
  • Kelemen, P., J. Matter and S. Al-Busaidi, 2009. Mineral Carbonation in Peridotite for CO2 Capture and Storage (CCS) in the Context of other CCS Techniques. Al Hajar 14, pp 10-12.
  • Kelemen, P. and J. Matter, 2008. In situ carbonation of peridotite for CO2 storage.  PNAS, vol. 105, no. 45, pp 17295–17300
  • Millson, J. and J.Schreurs, 2006. Ophiolites, a natural wonder. Al Mahal, no. 3, pp 3-8.
  • Schreurs, J. 2003 2010, Wadi Jizzi Geological Excursion.

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@ J. Schreurs May 2010