TheHajar Mountains (Arabic:جِبَال ٱلْحَجَر,romanized: Jibāl al-Ḥajar,TheRockyMountains[3][4] orThe Stone Mountains) are one of the highest mountain ranges in theArabian Peninsula,[5] shared between northernOman and easternUnited Arab Emirates. Also known as "Oman Mountains",[1][2] they separate the low coastal plain of Oman from the high desert plateau, and lie 50–100 km (31–62 miles) inland from theGulf of Oman.
Al (اَلْ) means "the", andḤajar (حَجَر) means "stone" or "rock". Soal-Ḥajar (اَلْحَجَر) is named as "the stone" or "the rock".
The Hajar Mountains extend for 700 kilometres (430 miles) through theUAE and Oman.[6] They are located on the north-east corner of theArabian Plate, reaching from theMusandam Peninsula through to the east coast ofOman. The range is about 100 km (62 mi) wide, withJabal Shams being the highest peak at 3,009 m (9,872 ft) in the central region of the mountains.[7][8]
The Hajar Mountains are the product of polyphasemountain building. Uplift and deformation of the Arabian passive margin began during the late cretaceous as theAfrican-Arabian Plate began to subduct under theSouth Tethyan Oceanic Plate imitated at anintra oceanic subduction zone. This initiation may have been the result of plate rotation due to the breakup ofGondwana.[14]
Similar to the modern convergence of theAustralian passive margin underEurasian oceanic crust, Arabian passive margin sediments became highly deformed and shortened forming an imbricated thrust belt. This also coincided with the emplacement of theSemail Ophiolite. By the earlyMaastrichtian, deformation ceased, and stablecontinental shelf conditions resumed.
A second episode of deformation began during theEocene Epoch around 45-40 Ma. This episode saw the reactivation of cretaceousthrust faults and the development of long and short wavelength folding of Paleocene marine sediments that infilled previousforedeep accommodation. Low temperature thermodchronometry ofapatite grains has given ages to this deformation and subsequent exhumation of the mountain belt. Exhumation occurred in two states, first between 45 and 40 Ma and again from 20 to 15 Ma. This later unroofing could also be related to tectonic uplift in the nearby ZagrosMountains ofIran.[14]
The high topography is around two major culminations: Jabal Akhdar and Saih Hatat, which are large scaleanticlines.[22] The Saih Hatat culmination containseclogite in the northeast at As Sifah.[23] These rocks weresubducted to about 80 km (50 mi) depth into themantle, and thenexhumed back to the surface.[20] Thisexhumation event created possibly the largestmegasheath fold on Earth, the Wadi Mayh megasheath fold.[24] The common view is that theseeclogites were originallybasic volcanic rocks within the leading edge of the continental crust of theArabian Plate. This leading edge was thensubducted by a NE-dippingsubduction zone.[25][26] However, some geologists have interpreted that theseeclogites were subducted through a SW-dipping subduction zone.[27]
The two culminations are separated by the Semail Gap. This is a prominent linear structure, trending NNE—SSW. However, it is still debated as to what this structure is. Different geologists claim that it is a left-lateral (sinistral)strike-slip fault,[28] anormal fault,[22] a lateral ramp,[29] amonocline due to a blindthrust,[16] or afault with multiple phases of deformation.[30]
There is some debate over whether the topography of the Hajar Mountains is due to thin or thick-skinned tectonics (ifbasement rocks were faulted during collision to create uplift due to thrust faults). RecentBouguer gravity andmagnetotelluric geophysical data suggest deep basement faults that have been activated as thrust faults during the collisional event that obducted the Semail Ophiolite. This has resulted in a much shallower depth to basement in the Hajar Mountains and a deeper depth to basement in the foreland basin to the west.[31] This has implications on economic geology such as the mining for precious minerals and the extraction of oil and gas.
The lateCretaceousobduction event created the proto-Hajar Mountains. However, thistopography subsided and shallow marine sedimentation covered the region, beginning in thePaleocene.[19][32]Paleocene toEocene sedimentary rocks are found at 2,200 m (7,200 ft) above sea level within the Hajar,[29] and arefolded. This indicates that the present daytopography formedafter the lateEocene. The exact timing is debated, and various interpretations indicate the topography formed anywhere between the lateEocene through to theMiocene.[22][32][29][33][34]
The driving forces that formed the Hajar is also debated. Many geologists relate theZagros Collision as the reason for theuplift forming the mountains,[19][32][35][36] as currently theMusandam Peninsula (northwest corner of the mountain range) isuplifting due to this collision. However,Jabal Shams, the highest peak of the central mountains is over 300 km (190 mi) away from this zone. In addition, there is no majorseismicity within the central mountains,[37] indicating that the mountains are not currently deforming, even though theZagros collision is.[29] This indicates that theuplift that created the present daytopography occurred in the past, possibly before the initiation of theZagros collision, by a mechanism that is not fully understood.
Pillow basalts at Wadi Jizz, which is part of the Semail Ophiolite sequence. These were named the Geotimes Pillow Lavas after a photo of them was published on the cover of the Geotimes magazine in 1975.
Oman's geological record is extremely valuable to geologists, and needs to bepreserved.[38] It contains the most completeophiolite on Earth, of which it is most famous for among geologists. Theophiolite sequence has spectacularpillow basalt (Geotimes pillow lava), as well as exposures of the fossil crust-mantle boundary (moho). Generally,ophiolites areobducted prior tocontinental collision, which highly deforms the structure of the originaloceanic crust. However, becausecontinental collision has not occurred in the Hajar, the Semailophiolite is still intact. Oman also has one of the best exposedmega-sheath folds ever discovered, the Wadi Mayh sheath fold.[24] Additionally, the relatively small outcrop ofeclogite is important. Eclogite is rare on the Earths surface, as it is a rock that forms at high pressures deep within thecrust ormantle. Geologists can learn about what is occurring in the Earths interior and tectonic processes from these rocks. There are also various fossil localities in Oman that need to be protected. There is concern in the geological community that with the development of infrastructure these rocks that contain a great deal of information will be excavated and destroyed.[38]
The central section of the Hajar is the highest and wildest terrain in the country.Jabal Shams is the highest of the range,[39] followed byJebel Akhdar. The latter[40] and the smaller Jebel Nakhl range are bounded on the east by the lowSamail Valley (which leads northeast toMuscat).[41]
East of Samail are the Eastern Hajar (Arabic:ٱلْحَجَر ٱلشَّرْقِي,romanized: Al-Ḥajar Ash-Sharqī), which run east (much closer to the coast) to the port city ofSur,[42] almost at the easternmost point of Oman.
The mountains to the west of Sama'il Valley, particularly those in Musandam Peninsula and the UAE,[43] are known as the Western Hajar (Arabic:ٱلْحَجَر ٱلْغَرْبِي,romanized: Al-Ḥajar Al-Gharbī),[44] also known as the "Oman proper". Since Jabal Akhdar and mountains in its vicinity are west of the valley, they may be regarded as Western Hajar.[1][43]
The northernmost mountains of the Hajar range are found on the Musandam Peninsula. For this reason, the phraseRu'us al-Jibal ("Heads of the Mountains") is applied to them, or the peninsula itself. Despite being physically part of the western Hajar, they differ in geology and hydrology to the rest of the range.[1][43] The highest point in the UAE is located atJebel Jais nearRas Al Khaimah, which measures 1,911 m (6,270 ft) from sea level,[54][55] but since the summit is on the Omani side,Jabal ar Rahrah, measuring over 1,691 m (1.051 miles), has the highest peak in the UAE.[56]
Date palms and other trees amongst the Eastern Hajar, near the east coast of Oman
The mountains are rich inplant life compared to most of Arabia, including a number of endemic species. The vegetation changes with altitude, the mountains are covered with shrubland at lower elevations, growing richer and then becoming woodland, including wildolive andfig trees between 3,630 and 8,250 ft (1,110 and 2,510 metres), and then higher still there arejunipers. Fruit trees such aspomegranate andapricot are grown in the cooler valleys and in places there are rocky outcrops with little vegetation. The flora shows similarities with mountain areas of nearbyIran, as well as with areas along theRed Sea in theHorn of Africa. For example, the treeCeratonia oreothauma is found here and also inSomalia.[61]
Like theRu'us al-Jibal,[66] the area of Jebel Hafeet is noted for hosting rare flora and fauna.[67][68] For example, in February 2019, anArabian caracal was sighted here,[69][70][71] and in March, aBlanford's fox,[72][73] which has also been reported in the mountains of Ras Al-Khaimah.[74] In September 2024 it was reported that the rare white-edged rock brown butterflies were spotted in the area.[75]
The Hajar are extensively grazed by domestic goats, camels and donkeys and the landscape has been cleared in parts for urban areas and for mining, which has damaged both vegetation and water supplies and uprooted traditional rural land management behaviours. Poaching of wildlife is another issue. The Oman government has created the Wadi Sareen Reserve and an area of Jebel Qahwan-Jebal Sebtah in the Eastern Hajar, for the protection of Arabian tahr and mountain gazelle. For visitors, there is a road into the mountains from the town of Birkat al-Mawz (on the road toNizwa from Muscat) and a walking route through Wadi al-Muaydin to the Saiq Plateau.[76]
There are 11 marked trails/routes of varying intensity (between Grade 1 to 3) and duration (between 1.5 hours to 18 hours) published by Ministry of Tourism, Oman along the Hajar range.[77] Some areas are inaccessible, and requires special equipment, as shown in aSteve Backshall TV documentary.[78]
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Note: Mountains are sorted in alphabetical order, unless where it concerns ranges. The highest confirmed mountains in each country are indicated with 'HP', and those with the highest peak are indicated with 'HP', bearing in mind that in the UAE, the highest mountain and the mountain with the highest peak are different.Outcrops are indicated with 'OC', andoutliers with 'OL', andanticlines with 'AC'.Volcanoes are indicated with 'V',volcanic craters with 'VC',lava fields with 'LF', andvolcanic fields with 'VF'.
Oman and
United Arab Emirates
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