!Oman - The Earth for beginners
Maybe instead of a few drinks in hotel (unlikely in Oman, anyway) you want to have some fun trying off-road driving to see some of the Earth's guts accidentally thrown out by merciless nature? Definitely better than sitting in the hotel...
Yeah, so get in the car and drive from Muscat to Wadi Abyad (see photo below). It is only an hour or two drive from the city centre. A nice, short brake for geologizing.
Let us start the real "Journey to the Center of the Earth". (If you want to know a bit more about Earth’s interior go and read our previous post)
How to get there?
From Muscat take a Motorway (Muscat expressway) heading east (in the Barka Direction. Take the road 13 towards Nakhl. Then continue your ride (towards Al-Awabi) and Approximately 18 km (11.2 miles) from the roundabout in Nakhl you will see a small side paved road towards Subaykhad (Wadi-Far). Turn there and continue for approx. 2 km, until you pass a concrete bridge at the wadi level. Then you will see a few buildings on your left. The concrete road finishes there. You can park and walk from there but that would be at least 2 hrs walk so it is much better to have a 4WD car (please note 4x4 car is a must here!). You can drive just following tracks in the gravel-filled valley until you will not be able to drive any further. Make sure you are on 4WD mode as the tracks are really not good, periodically changing and is quite easy to slip away. You will probably be bounced by the gravels quite often so some minor damages may happen. Be aware of water and flash floods. Before you go check the latest weather conditions.
It is approx. 20 mins drive from the last concrete parking site until the place where you will probably not be able to drive any further, however this also depends on the time of the year and the water level in the valley. Of course if you have a good car with high suspension you probably would be able to drive even further and eventually you could get to another village - Al Abyad. Otherwise there will be about 20-30 minutes walk to the MOHO exposure.
Please make sure you have enough petrol and water before you go. Be aware that there very little shade in the valley and it can get really hot so do not forget a hat or a scarf. Wild animals may be wild so avoid bushes and uncovered boulders as some snakes may be taking a sunbath. Mobile signal is rather good however you may miss it sometimes.
So let's go and see this unique boundary and part of the awesome oceanic crust section. Welcome to Nakhl Massife!
Look at the rocks outcropping on your left and right. They seem to be brownish-greenish in colour with some patches of grey-green minerals. These rocks seem to be cut by some younger "veins" and these are known as a dikes (More specifically, these are known as rhodingite dikes or serpentinites. Peridotites contain some pyroxene cummulates). These rocks you can see now are known as peridotites. By definition, rock is an aggregate of minerals, and this particular rock is composed mainly of beautiful minerals called olivines and pyroxenes. The brown parts are mostly olivines whereas green patches are mainly composed of pyroxenes (very important rock-forming mineral with quite complicated chemical formula). Sometimes you can also see some tiny black dots - these are spinels; chromites ((Fe, Mg)Cr2O4).
Based on its geochemistry, peridotite by definition is an ultramafic rock i.e. it contains less than 45% of silica. Two olivine minerals, known as forsterite and fayalite, are in fact a solid solution of (Mg, Fe)2SiO4). Forsterite is more magnesium rich whereas fayalite is ferrous, i.e. contains more iron. The dominant chemical composition of the olivines in the perodite rock sample is very important as they can tell you about conditions prevailing when the rocks turned solid from the melt. Peridotite, which is composed mainly of olivines, is called dunite. There are also three other kinds of peridotite called lherzholite (mostly olivine, clinopiroxenes and orthopiroxenes - most common perodite type); harzburgite (olivine and orthopyroxens) and wehrlite (olivine and clinopiroxens). The best way to determine of the rock type is by looking at the microscopic images of the thin sections).
Peridotite is a dominant rock in the upper mantle. Geologists think that this rock represents the old record of the Earth's evolution (accretion). Peridotite is like a mother for other dense rocks. The physical-chemical process of melting and "precipitation" of peridotite will eventually give birth to rock more common at the Earth’s surface, known as basalts. Peridotite are also known from elsewhere in the Solar System. You may be really surprised but some meteorites are in fact peridotites. (In fact, geologists very often analyse meteorites to understand the geochemistry of the mantle).
Olivines are very unstable minerals and they really do not like low temperatures (well, Oman is hot, but for them is still cold) and pressures, therefore they are very often altered (weathered). In the past, some fluids (hydrothermal fluids) were circulating through fracture network developed in peridotites altering them and creating another type of minerals with the generic name of serpentinites (asbestos for instance). Now you can see them on both sides of the valley just below the quite modern conglomerates.
If you look at individual pebbles in this conglomerate (rounded clasts) you will see that they consist of similar rocks to those outcropping around. If you wonder what is it that glues the clasts together - it is calcium carbonate mostly. The same calcium carbonate is responsible for the beautiful white and blue colours in the small ponds you will see on your way. This glue is called cement.
Remember the gravel you were driving on? With time (and if conditions allow) these loose pebbles might also become cemented together creating another conglomerate.
(You can also see red coloured streams and ponds -this is a result of iron which is leached from olivines. Some of the cements may also be made of magnesite which is magnesium carbonate. The carbonates are probably delivered from Hajar mountains located southward).
Where the river valley turns eastwards you will see two hills with two slightly different colours. Between the two colours is what we are looking for - Mohorovic (MOHO) boundary. The mantle-crust boundary. It may not be as exciting as you expected, but there are only a few places in the world where you can see that. It is best to see it from a short distance. It is not a sharp boundary, rather a gradual change in rock colour and composition
Just before the boundary, the peridotite composition is changing. The mantle segment (peridotite) seems to be laminated suggesting that magma flowed in the past. Also the mineral composition seems to different. Closer to the MOHO boundary we have more and more olivines. This relatively thin sequence is sometimes referred to as MOHO transition zone.
Just above MOHO we can see a totally different type of rock. You can also see some darker lenses of rocks within the peridotite interval. Theses rocks are known as gabbros. A very characteristic feature of gabbros over there is their layering. It is the lowermost part of the oceanic crust.
Gabbros are greenish or dark-coloured. The mineral composition is mostly pyroxene (mostly clinopyroxene), plagioclase and minor amounts of amphibole and olivine.
Geochemically gabbros contain more sillica than peridotites but still a considerable amount of iron-magnesium bearing minerals - gabbros are classified as mafic rocks.
If you look at the hill (jebel) to the left you may see some lighter coloured rocks which seem to be embedded in the gabbro background. These are known as a wehrlite. It is yet another peridotite typy and is a mixture of olivine and clinopyroxene.
So you have crossed the boundary. You are now in the oceanic crust. Gabbros are another layer in the ophiolite sequence. In Oman you can see also other rock types which form other layers of the oceanic crust.The Batinah coast sections in the west are perhaps the most complete in terms of the ophiolite succession. If you want to know more I suggest you read an article by Searle (2014; see references).
Due to the fact that entire Omani Mountains are so important for geology, the famous British geologist Michel Searle proposed a special protection status for them (world heritage sites).
U-Pb dating of zircons which were extracted from the trondhjemites (rock composed of quartz, plagioclase feldspar and some amount of biotite) in the gabbros suggest the ophiolite was created approx.. 95Ma (Cretaceous, Cenomanian). The detailed study of the entire Omani ophiolite structure helped to understand how the magma is created, transported towards the surface and then expelled in the mid-ocean ridges. Based on recent data, the oceanic crust outcropping in Sumail Ophiolite, was formed in a fast spreading ridge. There is a still ongoing discussion about tectonic position of the ridge (mid ocean ridge or back arc ridge). In some places in Oman it was possible to map out magma chambers along the ridge axis and to follow magma ascent pathways from gabbros through sheeted dykes to pillow lavas (Searle, 2014).
What else to do?
18 km eastward you can stop in the wonderful fort in Nakhal. The fort was build on the pre-islamic foundation but its current look dates back to 19th century. The entire fort complex is located at the entrance of beautiful Wilayt Nakhal in Wadi Ar Raqeem valley (look at the wonderful Jabal Akhdar mountains in the distance). The museum inside is open Saturday-Thursday 9am-4pm, and admission fee was 500 baisa. Parking is free. There are some information posters but unfortunately most of the information is in Arabic. A few old canons and guns are hanging on the walls.
(Fort Nakhal, phone: +968 9982 1975; the tour takes approximately 45 mins, 500 baisa ($1.3), free parking).
You can go even further and visit local Ain A'Thawwarah hot springs After about 10 minute drive through date plantation (there are signs to the hot springs on the way) you will see a small parking (free parking, not entry fee). Note that during holidays and on the weekends it may be quite busy as it is a popular site with local people and people are coming here even from Muscat just to take a bath.
I can't go to Oman to see ophiolite, what now?!
It is sad but there are only a few other places in the world where you can see similar outcrops although probably not as spectacular as in Oman. In the UK you can visit Lizard Peninsula in Cornwall. The genesis is similar (although the age is differ rent - Carboniferous). There will be another post about Lizard ophiolite complex.
Searle, M.P. 2014. Preserving Oman's geological heritage: proposal for establishment of World Heritage Sites, National GeoParks and Sites of Special Scientific Interest (SSSI) [in:] Rollinson , H. R., Searle, M. P., Abbasi, I. A., Al-Lzki ,A.&Al-Kindi , M. H. (eds) 2014. Tectonic Evolution of the Oman Mountains. Geological Society, London, Special Publications, 392, 9–44.
Rajendran, S, Nasir S, 2015. Mapping of Moho and Moho Transition Zone (MTZ) in Samail ophiolites of Sultanate of Oman using remote sensing technique. Tectonophysics.