Unique Landscapes - Coast & Mountains
Coast & Mountains
Abu Dhabi's landscape is most closely associated with seemingly endless sand deserts and dune fields and sabkha formations. Less prominent are the extensive and intricate coastline, remote offshore islands and its singular mountain, Jebel Hafit, which rises high above an otherwise low terrain. These and other formations are integral elements of Abu Dhabi's natural environment. Indeed, it is thanks to the intricacy of the coastal landscape that salt marshes, mangrove forests and other coastal habitats exist and thrive. They are also important for the Emirate's cultural heritage. Thus, the pearling industry, which lasted for thousands of years, is the direct result of the Great Pearl Bank Barrier offshore, which also hosts extensive coral habitats. Inland, the unique geography of Jebel Hafit creates conditions that sustain fragile mountain habitats and a variety of rare species. For thousands of years, the mountain has been a focus of human settlement and cultural development and it still draws visitors keen to explore its unique environment. The prevailing and universal hyper-arid conditions across the Emirate may convey an impression of uniformity, but in fact there are many diverse and unique landscape features.
From Salt to Surface – Island Formation
Several of the islands off the coast of Abu Dhabi are ‘salt domes'. These form as buried salt slowly rises to the Earth's surface, deforming and piercing the overlying layers of rock. Over time, layers of salt were buried by successive layers of sediment. The weight of these upper layers subjected the lower layers to increased pressure. But since salt cannot be compressed, it began an upward migration along faults in the sub-surface geology. As the salt slowly flowed upwards, it deformed the overlying rock layers to produce domed structures. With ongoing upward migration, the salt ‘diapir' eventually broke through these structures, sometimes incorporating large blocks of the host rock into the salt body and carrying them upwards. Eventually the diapir breached the surface to produce the characteristic dome-shape seen on Dalma, Arzanah and Sir Bani Yas, as well as on-shore at Jebel Dhanna. The oldest surface rocks in Abu Dhabi today are shales, dolomites, siltstones and volcanic rocks carried to the surface by these salt diapirs. The flanks of salt domes are often the focus for hydrocarbon exploration, forming ideal traps where oil and gas may accumulate in significant quantities.
Great Pearl Bank Barrier
Abu Dhabi's historic pearl industry was in large measure a result of fortuitous geology. The Great Pearl Bank Barrier, a geologic formation influenced by salt domes, forms a prominent submarine ridge, generally about 5–20 metres below the surface. Its extensive limestone platforms and coral patch reefs, which help to protect the inshore waters, and the carbonate-rich sands deposited here provide an ideal habitat for extensive sea grass beds and associated flora and fauna, including the pearl oyster (Pinctada radiata). Pearls, the only gems produced by living organisms, are formed from nacre, a biomineral consisting primarily of crystalline calcium carbonate.
The dramatic coastline of Abu Dhabi is more than 400 kilometres long, extending from near Ghuweifat in the extreme west to near Ghantoot in the east. With the exception of the 114-metre-high peak of the Jebel Dhanna salt dome, and some low-lying outcrops of Miocene sediments, most of the coastline lies near sea level and is flat and relatively featureless. The feeling of desolation can be powerful and evocative of the desert hinterland. In cross-section, the coastline forms a low angle, gently sloping ramp. Between Jebel Dhanna and Ra's Hanjurah, to the north-east of Abu Dhabi city, the coast is protected from open-marine conditions by a number of peninsulas and near-shore islands. Many of these developed as sediments accumulated around Pleistocene limestone outcrops and Al Dabb'iya, once an island, has now become connected to the mainland. Locally the lateral accretion of sediments has elongated the seaward edge of some islands parallel with the coastline, forming narrow channels and partially isolating coastal lagoons, providing sheltered habitat for mangrove forests. The seaward openings of these channels form deltas and shoals, where a distinctive type of sand grain known as ooids are found.
Ooids are spherical grains formed by the crystallisation of calcium carbonate from seawater. They are found in shallow marine environments where the seawater is agitated and supersaturated in calcium carbonate. Formation typically begins on small grains of sediment, such as a fragment of shell. As these roll around on the sea floor they enter suspension and become coated in concentric layers of calcium carbonate to form an ooid grain. Ooid shoals are highly distinctive when viewed from the air and ooids form a major component of the white sandy beaches. Buried deep underground, they may form rocks that are ideal oil and gas reservoirs.
Mobile Mountains (Jebel Hafit)
Generally, the landscapes of Abu Dhabi display a low-lying relief and gentle topography. However, the isolated mountain ‘jewel' of Jebel Hafit in the north-east of the Emirate, just south of Al Ain, dominates the terrain and is important for water catchment, biodiversity, recreation and cultural heritage. Jebel Hafit is a small outlying fragment of the Hajar Mountains, the product of ancient formative processes. Between 40 and 10million years ago during the Eocene to Miocene epochs, much of Abu Dhabi was covered by the shallow Tethys Sea where diverse marine creatures including bivalves, snails, sea urchins and corals thrived. As these died, their remains accumulated to form thick sequences of limestone. Eventually, the collision between the Arabian and Eurasian plates closed the sea and compressed and buckled these strata into a series of prominent ridges (anticlines) and basins (synclines). Once thrust upwards, the relatively soft rocks of the anticlines were exposed and weathered by wind and rain to form a series of elongate mountains that include the spectacular ~1240-metre-high ridge of Jebel Hafit. The abundant fossils found there are witness to a long and complex history that literally started from the sea and culminated in the summit visible today.