by Marijcke Jongbloed 

The geology of the UAE is very interesting, because traces of the ancient events that shaped the land can still be seen now, due to the scarcity of vegetation and the ways in which erosion took place.

The rigid outer shell of the earth (the “lithosphere”) is fragmented into plates that can move, propelled by “currents” in the underlying layer (the “asthenosphere”). Plates can drift apart or closer together, a process called tectonic movement.

Geologically, the UAE is part of a body of continental rock known as the Arabian platform which includes the Arabian Gulf and the Zagros mountains of Iran as well as the area we know today as the Arabian peninsula. For most of its history the Arabian platform has been part of the larger Afro-Arabian continent.
Around 500 million years ago, in the Cambrian era, much of the Arabian landmass was covered by shallow seas that evaporated in the hot climate, leading to the formation of thick salt deposits. Then movements of the Afro-Arabian tectonic plate caused it to pass near the South Pole where it went through an ice age, some 400 million years ago.
Later more folding and faulting occurred in Arabia. The seas flooded the area once again, in the early Cretaceous (circa 130 million years ago). In the major tropical ocean (the sea of Tethys) that separated the Afro-Arabian continent from the Eurasian landmass thick layers of limestone and dolomite rocks were deposited. These rocks of the late Permian to late Cretaceous era (250-65 million years ago) are the rocks where the UAE’s oil is found. These hydrocarbons were created from abundant organic material (algae and other micro-organisms) that were deposited in the warm tropical seas of the time. The deposited organic material was buried deeper and deeper and was broken down by heat to form oil and gas.

Fossil of the now extinct gastropod Acteonnellid

During the Palaeocene and Oligocene (65-23 million years ago) sea levels rose and fell several times. The coastal region was periodically covered with warm tropical waters. After that the area became tectonically more stable but it began to separate from Africa along the Red Sea Rift about 25 million years ago and is currently moving northwards at an average rate of 5 cm per year. As the Afro-Arabian plate moved north and was pushed under Eurasia, the Zagros mountains in Iran were formed. The western part of the sea of Tethys disappeared and the straits of Hormuz closed. In the rapidly subsiding basin thick layers of salt and gypsum were deposited. The collision created large scale folding in the Emirates, evidence of which can be seen in Jebel Fayyah and Jebel Hafeet.
The UAE coast rose above sea level in the late Miocene/early Pliocene (5 to 2 million years ago) and the Arabian Gulf filled with water again about 4 million years ago. Fossil evidence shows that in the late Miocene there were lush riverine valleys in the west of Abu Dhabi with savannahs populated by elephant, giraffe, rhinoceros and many other animals.
About 8000 years ago the lowered sea level allowed sand to be blown into Abu Dhabi from Saudi Arabia and in the last 6000 years the Arabian climate has become steadily drier.


The layered and folded rocks of the Hajar mountains can be studied at the surface

Most oceans have a ridge of volcanic mountains running along the centre. Due to volcanic activity, such a ridge in the middle of the ocean can split, forming a new part of the lithosphere, the so-called oceanic crust. The two sides drift apart, a process called seafloor spreading.

In the late Cretaceous, there was movement originating from volcanic action in the part of the Tethys sea that still remained to the east of the Arabian landmass. This caused dark, dense crustal magma extruded by the submarine volcanoes to be pushed up over the edge of the Arabian landmass, together with rocks from the layer below, giving rise to a tremendously thick sequence of so-called ophiolite rocks, which are the main type of rock in the Hajar mountains. Ophiolites are typical of such spreading centres and provide the geologist with relatively rare access to rocks that are amongst the most representative of the deepest parts of the Earth’s crust. In fact, the Hajar Mountain range holds the most extensive area of ophiolites in the world and is one of the few places on earth where these oceanic crust rocks can be studied on land.

The beaches that bordered the ancient ocean folded into mountains also. These are the fossil bearing limestone outcrops that separate the gravel plains from the Hajar range.
Erosion by wind and water has worn down many of the mountaintops, while copious rainfall in the past thundered down the mountains to form deep wadis down their sides and wide gravel plains at their feet.


The sands that cover the UAE from the Arabian Gulf coast south to the uninhabited sands of the Empty Quarter, the Rub’ al-Khali, are a more recent geological feature than the oil-producing rocks of the ancient Arabian landmass. It is the result long periods of erosion and re-deposition in a dry environment. The sand overlies the oil- and gas-producing rocks that are not exposed to the surface and are only known from drilling.

An aerial view of Fossil rock partly covered with red sand

Throughout the UAE the sand differs in colour and composition, depending on its source.
Near the coast the sand is white, made up of calcium carbonate, derived from the carbonate sediments, seashells and coral reefs of the coast.
Further inland the sand consists of quartz crystals, a stable end-product of the chemical weathering of most types of rock. It can acquire various colours through impurities or coatings. For instance, red sand is quartz with amounts of oxidised iron.
Black sand is derived from the igneous rocks of the Hajar mountains.
A special type of white sand with perfect spheres called ooliths is formed in tidal channels between islands.
Fossilised sea urchin Goniopygus superbus


In principle sand dunes are formed by the force and direction of the wind acting on the supply of sand. In detail not much is known about the formation of dune patterns.

The scale of the sand patterns ranges from ridges of up to kilometers in length, dunes that are measured in meters or multiples of meters. And ripples that can be from a few to a hundred centimeters in length.

There are three distinct types of sand dunes – their shape depends on the direction and the strength of the wind as well as on the type of sand in the area.

Barchan dunes are sickle shaped, with a steep concave slope on the down-wind side and gentler convex slope on the windward side. They tend to form where sand is relatively scarce and can often be found on gravel plains or salt flats.

Transverse dunes are elongated sand ridges perpendicular to the prevailing direction of the wind. They usually lie parallel with flat sandy plains in between. Again, the downwind side is steep, while the windward side has a gentle slope. Most of the dunes in the UAE are of this type, with very high (up to 150 meters) transverse ridges in and to the south of the Liwa oasis.

Longitudinal dunes are formed parallel to the prevailing wind direction.
The processes that formed these long sand dunes (called seif-dunes) are not clearly understood but it is thought that they were formed during the last ice-age, when winds were much stronger than today. In the UAE they only occur in the extreme southwest of the country, continuing deep into the Empty Quarter.

The sand deserts of central and western parts of the UAE have relict longitudinal dunes aligned in more or less west-east orientation with a more recent pattern of linear east-west ridges superimposed.

Of course, combinations of sand dune shapes can occur, so the patterns are often difficult to recognize.

The major dune ridges in the UAE were formed in the most recent ice age, some 18.000 years ago. Glaciation helped the sand dunes to develop because there were strong winds in the narrow area between the ice front and the equator. In addition the global sea level fell and this caused the Arabian Gulf to fall dry, exposing the masses of loose sediment that could be blown into dunes.

The prevailing wind direction in the UAE today is from the north-west. Therefore active dune crests are usually aligned from the north-east to the south-west, with the steep face to the south-east.

In the west of Abu Dhabi emirate, there is an area where in the Miocene period (6-8 million years ago) a major river system existed in a subtropical savannah. Many fossils of animals and plants that no longer occur in the UAE were found deposited in what is known as the Baynunah Formation. Palaeontologists and geologists have carried out extensive research there, most recently under the auspices of the Abu Dhabi Islands Archaeological Survey (ADIAS) and the Environment Agency-Abu Dhabi (EAD). Some of the 51 species of fossils found there are the remains of monkeys, hippopotamus, crocodiles and elephants. The existence of these animals of African origin is proof of the fact that for a long time, until about 5 million years ago, a land connection existed between Africa and Arabia.

A very different array of fossils can be found in the east of the country, in the row of Tertiary limestone hills and mountains aligned along the western flanks of the Hajar range. These sediments used to be beaches and were folded up through the spreading of the seafloor due to volcanic action of submarine volcanoes.
Fossils of various species of gastropods

Many types of fossilized sea urchins, gastropods and bivalves are present in places like Jebel Huwayyah, Jebel Buhays and Jebel Fayyah, exposures of what is called the Simsima formation. Of the 45 species of fossil sea urchins found in the Simsima formation, 14 were new to science and some of these have been named either after the place where they were found or after the person who was instrumental in finding them (for instance Heterodiadema buhaysensis and Codiopsis lehmannae – the latter named after a friend of the author)
One gastropod, the Acteonellid, became extinct more than 50 million years ago but can still be found as a fossil, sometimes in large numbers. Another marine creature – an odd claw-shaped bivalve called a rudist - became extinct at the end of the Cretaceous period, 70 million years ago.

Jebel Hafeet is the largest of these fossil-bearing mountains – a major topographic high rising nearly 1300 m above sea level, located immediately south of Al Ain and straddling the border between the UAE and Oman. It is easily visible from space, and from below it looks like “beached whale”, as Wilfred Thesiger once described it. It is asymmetrical – on the west it dips at about 25 to 30 º while on the east it is much steeper. To geology students it is of special interest, because the mountain is eroded in such a way that the composing layers can be easily observed at the surface.
In geology jargon Hafeet is the type locality for two major formations: the Hafeet formation and the Seniya formation. Both are rich in deep sea planktonic fossils called foraminifera. At the foot of the mountain, close to where the road from the cement works passes through a man-made gap, fossils of branching corals, oysters, gastropods and more rarely sea urchins and pieces of barnacles and crab claws can be found. One type of fossil that is special to this mountain is called Nummulites fichteli. They are thin round pieces of rock, often a bit curved, the size of a coin (hence the name). Where they are broken, a delicate structure of circular and transverse septi can be seen. These nummulites are the remains of a now extinct one-celled organism.

The UAE are not only interesting as a finding place for fossils, but also for rocks and minerals. On Sir Bani Yas, a salt plug from the inner core of the earth broke through to the surface carrying with it ancient core rocks. One of these very old (620 million years!) rocks can be seen (and touched) in the Sharjah Natural History Museum.
In the Hajar mountains beautifully coloured interesting rocks are common – cherts, micah, quartz, garnet, and calcites. Copper was present in large enough quantities to be mined, making the area famous in the days the Portuguese. The copper used to be put in earthenware pots that had a small round space in the bottom. The pot was heated to such a high temperature that the copper melted and collected in the bottom of the pot, which was broken after cooling so that the coppers ingot could be retrieved. The remains of mine shafts and heaps of copper slag and pot shards are found throughout the Hajar mountains.

Whereas the Hajar (rocky) mountains were aptly named, Jebel Hafeet is a misnomer for a mountain that is all but empty! For those who develop an eye for nature, every corner of the country holds surprises and the possibility to discover sometimes still unknown objects.


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