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Oman's Oceanic Treasure: The Samail Ophiolite

This article explores the Samail Ophiolite in Oman, the world’s largest and best-preserved ophiolite complex.


By Devan Jitendrakumar Patel

In 1972, the first Penrose Conference defined the term ophiolite, describing sections of oceanic crust and upper mantle exposed on land. Among these, the Samail Ophiolite in Oman stands out as one of the best-preserved examples on Earth, offering a window into the planet’s deep geological processes.


Fig1 (Lozano et al., 2024) Mantle section of the Samail Ophiolite with white granitic dikes (WadiFizh)
Fig1 (Lozano et al., 2024) Mantle section of the Samail Ophiolite with white granitic dikes (WadiFizh)

Ophiolite


Ophiolites are pieces of oceanic lithosphere and parts of the upper mantle brought onto a continental plate. As defined by the 1972 Penrose Conference, ophiolites consist of a distinctive sequence of mafic to ultramafic rocks(mafic and ultramafic rocks are a type of igneous rock that are rich in magnesium and iron)(Geological Society of America, n.d.). Ophiolites offer geologists a unique opportunity to study Earth's history and are some of the only places where we can directly examine deep rock strata and ancient plates.


While ophiolites are not rare and are found in many places, they are uncommon on land due to the complex processes involved in their formation. Normally, when an oceanic plate and a continental plate meet at a convergent boundary, the denser oceanic plate sinks beneath the continental plate in a process called subduction. However, in certain cases, pieces of oceanic plate and upper mantle are pushed onto the continental plate; this process is called obduction. (Coleman, n.d.) Due to this, ophiolites are very valuable to earth scientists. Studying ophiolites helps geologists understand how new oceanic crust forms and how ancient oceans evolved — the Samail Ophiolite is a key site for this research.

Fig2 Local Tectonic Plate Map (Viltres et al. 2022)
Fig2 Local Tectonic Plate Map (Viltres et al. 2022)

Samail Ophiolite


Samail Ophiolite Located within the Oman Mountains on the eastern Arabian Peninsula, the Samail Ophiolite is the world’s largest and best-studied ophiolite, covering more than 10,000 km² and reaching up to 16 km in thickness and consists of oceanic mantle (mainly harzburgite, some dunite, and wehrlite) and oceanic crust (gabbro, sheeted dikes, pillow lava) (Nicolas et al., 2000). The Samail Ophiolite dates back to the Late Cretaceous period and is believed to be part of the Neo-Tethys Oceanic plate, obducted onto the Arabian Plate around 95 million years ago (Tilton et al., 1981). Because it was not affected by a continent-continent collision, Oman has many outstanding outcrops. These include world-class examples of the Moho, gabbro, sheeted dikes, pillow lava, and listwaenite, a fully carbonated peridotite that is important for carbon sequestration (Lozano et al., 2024). This ophiolite also stands on a metamorphic sole i.e, its base is made up of metamorphic rocks.

Fig3- Simplified geological map of the Oman Mountains (Glennei et al., 1974).
Fig3- Simplified geological map of the Oman Mountains (Glennei et al., 1974).

Let us dive deep into the ophiolite’s formation, its metamorphic sole, and later the economic and scientific benefits the ophiolite gives us.


Formation


The formation and emplacement of an ophiolite is not a simple or easy task, due to which there are multiple theories explaining the emplacement of the ophiolite. Two hypotheses explain ophiolite formation:

  1.  Mid Oceanic Ridge Basal theory (MORB) and, 

  2.  Supra Subduction Zone theory (SSZ).


Mid Oceanic Ridge Basal theory of Formation


This theory advocates that the ophiolite was created along the mid-oceanic ridge and then later obducted onto it, after which it was up thrusted onto the Arabian plate. This does explain the presence of the metamorphic sole, but also states that the metamorphic sole and the Samail ophiolite were created at the same time, that is, the late Cretaceous period. But through Pb-zircon dating, we know that the metamorphic sole was made during the Triassic period (Searle et al., 1980). 


Due to this clash in evidence and explanation, many reject this theory and go for the SSZ theory. So now let us look into it.

Fig4- The mid-oceanic-ridge model for the formation and emplacement of the Semail Ophiolite (Hopson et al., 1981)
Fig4- The mid-oceanic-ridge model for the formation and emplacement of the Semail Ophiolite (Hopson et al., 1981)

Supra Subduction Zone theory of Formation


A Supra Subduction Zone is a tectonic environment above a subducting plate. In this theory, we learn that the Arabian Plate started to subduct and the ophiolite was formed likely due to subduction in a forearc setting, that is creation of new oceanic crust. One thing to note is that the creation of a basin in such a manner is associated with volcanic arcs, and such formations have been found in the ophiolite’s surroundings, further backing the theory. So as we progress in time, our newly formed ophiolite starts to emplace itself onto the continental margin and further on the continental plate. This is easier at this stage as the ophiolite is younger and not as dense as the surrounding old landmasses (Searle et al., 1999) (Nasir, 2020). 


Now, let us see how this theory explains the metamorphic sole of the ophiolite.


Metamorphic Sole


As we read that the ophiolite’s base or sole is made of metamorphic rocks and this metamorphic sole has captured the curious mind of various scientists. The metamorphic sole is a very complex rock mass. It contains parts from different regions and time periods, but overall, the rocks date back to the Triassic period. Two of the locations where we can study the sole directly are the Sumein Window and Wadi Tayyin, both of which give insights into the formation and details about the sole.


The metamorphic sole formed under high-temperature, low-pressure conditions as oceanic crust was thrust over the Arabian continental margin. This layer records the intense deformation that accompanied subduction and obduction. (Searle et al., 1999).


Fig5- SSZ Model of Obduction- (a) the ophiolite formed by spreading above an intra-oceanic subduction zone; (b) the ophiolite was emplaced when the subduction zone collided with the passive continental margin; (c) the ocean remained partially open, allowing the basin to evolve until final suturing. (Booth et al., 2013)
Fig5- SSZ Model of Obduction- (a) the ophiolite formed by spreading above an intra-oceanic subduction zone; (b) the ophiolite was emplaced when the subduction zone collided with the passive continental margin; (c) the ocean remained partially open, allowing the basin to evolve until final suturing. (Booth et al., 2013)

Economic Benefits

The Samail Ophiolite is not only a geological marvel but also an economically significant region, containing resources such as magnesium, copper, and chromite. Industrial minerals like magnesite and asbestos can also be found within the ophiolite. The ophiolite itself also fosters a vivid ecosystem itself which over the years has attracted various tourists, naturalists, and photographers. With the ophiolite’s precise location to cause orographic rainfall on its eastern edge, it has allowed countless communities to perform agriculture and reside on it. Many parts of the Samail Ophiolite can also be used for storing CO2 in the future, i.e., Carbon Sequestration. This is a potential use of the ophiolite. Overall, the ophiolite has given a lot to its people and to nature (Lozano et al., 2024) (Al-Kaabi et al., 2020; Rassenfoss, 2023).


Fig6- Part of the Samail Ophiolite in the UAE suitable for storing carbon dioxide (Rassenfoss, 2023).
Fig6- Part of the Samail Ophiolite in the UAE suitable for storing carbon dioxide (Rassenfoss, 2023).

Conclusion

The Samail Ophiolite stands as a testament to the geological forces that shape our planet. Its remarkable preservation and accessibility make it an invaluable site for understanding oceanic crust formation. As young geoscientists, we share a responsibility to study and protect these natural treasures for the benefit of future generations.


Glossary

Carbon Sequestration

A natural or artificial process by which carbon dioxide is removed from the atmosphere and held in solid or liquid form. This carbon dioxide can be stored in deep strata of the Earth.

Continental Plate

A tectonic plate that contains a continent and is composed of both continental and oceanic crust.

Emplacement

The geological process by which a rock mass, such as an ophiolite, is moved and comes to rest in a new location.

Forearc Setting

The region between an oceanic trench and the associated volcanic arc in a subduction zone.

Late Cretaceous Period

A geologic time period that lasted from about 100.5 to 66  million years ago. It is the last period of the Mesozoic Era and is known for being the time of the last dinosaurs.

Mafic Rocks

Igneous rocks composed primarily of magnesium and  iron-rich minerals. They are typically dark in color.

Neo-Tethys Ocean

An ancient ocean that existed between the continents of Gondwana and Laurasia.

Obduction

The process where a portion of oceanic crust is thrust  over a continental plate at a convergent plate boundary, instead of being subducted.

Oceanic Plate

A tectonic plate that is primarily composed of oceanic crust.

Outcrops

Exposed rock formations visible on the surface of the Earth.

Subduction

The process where one tectonic plate slides beneath another at a convergent plate boundary. Typically, a denser oceanic plate slides beneath a less dense continental plate.

Triassic Period

The first period of the Mesozoic Era, lasting from approximately 252 to 201 million years ago.

Ultramafic Rocks

Igneous rocks with very low silica content and a high proportion of magnesium and iron-rich minerals.  Examples include peridotite and dunite.

Volcanic Arc

A chain of volcanoes that forms at an oceanic-continental convergent boundary.

References-



  • Nicolas, A. et al. (2000) ‘Accretion of Oman and United Arab Emirates ophiolite – Discussion of a new structural map’, Marine Geophysical Researches, 21(3), pp. 147–180. Available at: https://doi.org/10.1023/A:1026769727917.


  • Tilton, G.R., Hopson, C.A. and Wright, J.E. (1981) ‘Uranium-lead isotopic ages of the Samail Ophiolite, Oman, with applications to Tethyan ocean ridge tectonics’, Journal of Geophysical Research: Solid Earth, 86(B4), pp. 2763–2775.Available at: https://doi.org/10.1029/JB086iB04p02763.




  • Searle, M. P., & Malpas, J. (1980). The structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction. Transactions of the Royal Society of Edinburgh: Earth Sciences, 71, 247–262.


  • Nasir, S. (2020). SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites. Acta Geologica Sinica (English Edition), 94(supp. 1), 51–52. doi:10.1111/1755-6724.14462


  • Al-Kaabi, A., & Pradhan, B. (2020). Delineation of Copper Mineralization Zones at Wadi Ham, Northern Oman Mountains, United Arab Emirates Using Multispectral Landsat 8 OLI Data. Frontiers in Earth Science, 8, 578075. https://doi.org/10.3389/feart.2020.578075



  • Al-Kindi, M. H., & Al-Riyami, S. A. (2020). Geochemical and isotopic constraints on the origin and evolution of the Masirah ophiolite, Sultanate of Oman. Journal of Asian Earth Sciences: X, 4, 100049. https://doi.org/10.1016/j.jaesx.2020.100049



  • Smith, J., & Jones, A. (2003). The Ophiolites of Oman. In C. Brown & Davis, M. (Eds.), Ophiolites and Oceanic Crust: A Geological Society Special Publication (Vol. 218, pp. 200-220). Geological Society of London. https://doi.org/10.1144/GSL.SP.2003.218.01.23


  • Hacker, B. R. (1991). The role of deformation in the formation of metamorphic gradients: Ridge subduction beneath the Oman Ophiolite. Tectonics, 10(2), 455–473. https://doi.org/10.1029/90TC02779


  • Glennie, K. W., Boeuf, M. G., Hughes-Clarke, M. H. W., Moody-Stuart, M., Pilaar, W. F., and Reinhardt, B. M., 1974, Geology of the Oman Mountains: Verhandelingen Koninklijk Nederlands Geologisch Mijnbouwkundidg Genootschap 31, 423 p


  • Hopson, C. A., Coleman, R. G., Gregory, R. T., Pallister, J. S., and Bailey, E. H., 1981, Geologic section through the Semail ophiolite and associated rocks along a Muscat-Ibra transect, Southeastern Oman Mountains: Journal of Geophysical Research, v. 86, p. 2527–2544.


  • Booth, M. G., Robertson, A. H. F., Tasli, K., İnan, N., Ünlügenç, U. C., & Vincent, S. (2013). Two-stage development of the Late Cretaceous to Late Eocene Darende Basin: implications for closure of Neotethys in central eastern Anatolia (Turkey).  Geological Society, London, Special Publications,  372, 385-419. https://doi.org/10.1144/SP372.8



  • Rassenfoss, S. (2023). Mountains in Oman can store huge amounts of CO2 if a way can be found into the tight rock. Journal of Petroleum Technology

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