Arabia [Geography and environment]

The Arabian Peninsula’s unique geography and environment encompass a variety of landscapes and geological formations. Bordered by the Red Sea, the Arabian Gulf, and the Indian Ocean, the region’s arid climate exhibits significant variations in temperature, with sporadic rainfall. Intermittent watercourses and aquifers are vital water sources. Tropical cyclones can bring heavy rains, and flash floods pose water-management challenges.

The Arabian Peninsula consists morphologically of a tilted block of pre-Cambrian platform, shaped like an immense desertic quadrilateral of 3.3 million km². Crossed by the Tropic of Cancer, with latitudes spanning 13°N to 31°N, and a longitudinal range from 33°38’E to 60°04’E, the Arabian Peninsula is the largest peninsula in the world. It is bordered by three seas: to the west the Red Sea, passing through the Suez Canal and Europe; to the east, the Arabian Gulf, extending from the Shatt al-Arab delta to the Strait of Ormuz; and to the south, the Indian Ocean, which connects Africa and Asia. The margins of Jordan, Syria and Iraq form the continental northern border. The peninsula is currently composed of Kuwait, Oman, Qatar, Saudi Arabia, the United Arab Emirates and Yemen, and southern portions of Iraq and Jordan (Fig. 1).

Due to its position within the sub-tropical high-pressure belt, the Arabian Peninsula is one of the most arid subcontinents on Earth, but despite this general aridity, varied climatic conditions prevail within the peninsula. Combined with the rich geological and geomorphological history of the peninsula, the variety of climatic conditions generate a wide range of reliefs and colourful landscapes.

The Arabian Peninsula comprises several physiographic units, from west to east: the Tihama plain (a narrow coastal plain extending from the Gulf of Aqaba to Yemen); the western Arabian Shield area, from the western highlands (chains of Hijaz, Madyan, ʿAsīr, and Yemen) to the central plateau (Ḥismā and Najd); the northern interior (the northern cuesta-like area, the steppe lands and the Black Desert); the Arabian sand sea (the Nafūd and Rubʿ al-Khālī desert); the coastal area of the Arabian Gulf (al-Ḥasā plain, United Arab Emirates low-lying coast); the Hajar mountains and its piedmonts (Northern Oman); the southern coastline (from Ṣawqira Bay to the Ḥaḍramawt plateau).

Geology

The Arabian Peninsula lies on the Arabian plate, a minor tectonic plate located between a convergent boundary with the Eurasian plate (Makran trench and Zagros fold and thrust belt) and with the Indian plate (Owen fracture zone), and a divergent plate boundary (the Red Sea rift).

The collision of the Eurasian and Arabian plates started around 100–90 Ma, with an intra-oceanic thrusting that formed the Samail arc (Gray & Gregory 2004). This compressive system led to a famous geological process, the formation of the world’s largest complex of ophiolites in Oman, composed of a thrust slice of oceanic crust and upper mantle formed around 96 to 95 Ma, and deep-sea sediments, obducted onto the Arabian continental margin during the Late Cretaceous Period (95 to 70 Ma).

The Arabian plate was separated from the African plate with the opening of the Red Sea rift during the Eocene (56 to 33.9 Ma), and was part of the African (Nubian) plate until the Oligocene (33.9 to 23.03 Ma). Since then, the Arabian plate has been gradually moving towards the Eurasian and the Turkish plates. As the Red Sea grew, the northern portion of the plate was pushed slowly northward, producing crustal deformation in the form of multiple ripples of syncline and anticline in the Zagros Mountains of Iran, and closing the intervening Neotethyan Ocean. Spreading in the Red Sea began along its entire length 13 Ma ago and this extensional motion still continues today as the spreading rate on both sides of the Red Sea is currently about 1 cm/yr (Augustin et al. 2021). Seismic hazards are common in the western portion of the peninsula, major earthquakes occur with a 20–30-year recurrence interval in Yemen (Mohindra et al. 2012). Tsunamis can form along the Makran fault zone, the Owen fracture zone and the Red Sea Rift (Salamon et al. 2021).

This highly disturbed geological history provided Arabia with a rich lithology (Fig. 2). Western Arabia is composed of the oldest exposed rocks, and the western Arabian shield must be regarded as a continuation of the adjacent African shield. It is made up of Precambrian gneiss (dated between 2.6 billion and approximately 541 Ma), overlain by sedimentary materials or basaltic, volcanic rocks related to rifting activity along the Red Sea (Brown et al. 1990). Flood basalts can reach, in some places, thicknesses of up to 3,000 metres, and craters are numerous. A huge thrust sheet of ophiolites occupied the eastern top of the Arabian continental margin (Oman mountains). Thick layers of younger sedimentary materials (Late Cenozoic) cover the majority of the peninsula, demonstrating that the Arabian Peninsula was covered by shallow marine conditions up until 20 Ma, before the uplift and the large-scale thrusting and folding of the peninsula. These rocks host immense reservoirs of gas, oil and water, for instance the onshore Dunkhan oil field in the Qatar peninsula.

Geomorphology

The most represented Quaternary units are sands and gravels covering the broad area of the central lowlands. The huge Arabian sand sea (Holm 1960) accounts for nearly a third of its land area (around 800,000 km²). The Nafūd and the Rubʿ al-Khālī (Empty Quarter) are linked over 800 km by a thin arc of dunes called the “sand river”, al Dahna. The Rubʿ al-Khālī is the largest continuous expanse of active sand sea in the world, with an area of between 500,000 and 600,000 km², and linear ridges up to 300 km long. Several erosive (reg) to accumulation (erg) landforms can be observed, including large crescentic forms called mega-barchans around Liwa. Along the piedmont area, vast coalescing alluvial fans called bajada, formed during past pluvial periods of the Pleistocene, cover extended parts of the continental plain, for instance, in Oman in the southern piedmont of the al-Hajar Mountains. Quaternary lava fields are found in Central Arabia, for example at Ḥarrat Rahaṭ, but mainly in Western Arabia, like Ḥarrat Khaybar (latest historic eruption from early Islamic times at AD 650). Eruptions are mainly effusive. The general organisation of the eastern low-lying coast is extended prograding Holocene flats, with supratidal mud to sand flats called sabkha, low dunes, shallow lagoons called khor, sandpits, mangroves and coral barrier systems. This pattern is favoured by gentle and irregular subsidence which can be interrupted by salt diapirs, such as the Jabal Dhanna (Abu Dhabi).

Climate

In most parts of the country, moisture availability is low and evaporation rates are high. The whole Arabian Peninsula has a hot desert climate (BWh), as defined by the Köppen climate classification (mean temperatures around 25°C and rainfall around 100 mm/yr). The aridity index subdivides scales of aridity depending on annual rainfall, ranging from hyper-arid conditions between 50–100 mm; arid conditions between 100–200 mm; semi-arid conditions between 200–600 mm and sub-humid conditions up to 600 mm (Le Houerou 1982) (Fig. 3). Seasonality is often described as low, but the temperature discrepancy between winter and summer is significant and two distinct thermal seasons can be differentiated. In winter, mean temperatures range between 10°C to 22°C and are influenced by the latitudinal position. In summer, all mean temperatures are approximately 30–33°C and maxima easily rise to 48°C. The all-time highest measured “feels like” temperature is 81°C at Dhahran (Saudi Arabia) on the 8th of July 2003 (Tétart 2020).

Annual rainfall is concentrated in space and time and interannual variations can be very high. For instance, at Medina, mean annual rainfall is at 39.2 mm/yr but for a single year rainfall can range from 2 mm to 89 mm. In most parts of the peninsula, rainfall occurs between December and April in the form of violent downpours, amounting to around 100 mm/yr. In the Nafūd and the Rubʿ al-Khālī, mean annual rainfall is less than 50 mm/yr and in the al-Hajar Mountains (Oman) rainfall can reach 300 mm/yr (Sayq plateau). In the southern part of the peninsula, summer precipitations are higher because of the Indian monsoon influence. Combined with the topography, summer rainfall can reach 500 mm from the ʿAsīr Mountains (Saudi Arabia) to the Jabal Ḥarāz (Yemen). Ibb (Yemen) cumulates around 900 mm/yr. Distance from the coast can also affect annual humidity.

Tropical cyclones developing in the north Indian Ocean during the pre-monsoon (May) and the post-monsoon (October-November) periods can affect the eastern coast of the peninsula and generate heavy rains, storm surge and flood.

Hydrology

Due to aridity, there are no permanent wadi (local term for stream or valley), water-courses are intermittent, dry most of the time, and drainage basins are endorheic, except in small coastal watersheds. Inland depressions act as an outlet for continental streams, like the vast inland sabkha of Umm al-Samin (Oman). Stream flows are mainly torrential and watercourses can fill rapidly after rainfall although flooding is generally short-lived. Flash floods commonly occur after thunderstorms, and catastrophic floods combined with prolonged periods of drought are the main water challenges facing Arabia.

Regarding the general hydrography, in the northern part of the peninsula, flows run towards the Arabian Gulf basin, as is the case for the Wādī al-Rima—al-Bāṭin which sources in Medina and ends in the northern part of Kuwait (Fig. 4). In the western part, waterways crossing the Tihama plain end in the Red Sea. But most of them are short and greatly divided, except the Wādī al-Dawāsir, Bīsha and Tathlīth (Sanlaville 2000). The al-Hajar Mountains (Oman) are the starting point for numerous wadis in Oman. Wādī Ḥalfayn can be considered as the longest Omani wadi, connecting the Hajar Mountains with the Gulf of Filim.

In Central and Eastern Arabia, permeable bedrock led to the development of extensive and copious aquifers, like the Umm al-Radhuma—Dammam Aquifer System. Karst springs and artesian wells are common and greatly exceed the total rainfall of the area. They are famous in the area of al-Ḥasā or Hofuf, along the tectonic depression of the Wādī Sirḥān or at Taymāʾ (Saudi Arabia). Most of the groundwater resources are fossil, linked to ancient recharge related to a more humid climate at the end of the Pleistocene and the Early Holocene. Renewable water resources are higher around the al-Hajar Mountains and the Yemeni highlands than elsewhere as a result of the combination of permeable bedrock and elevation.

Tara Beuzen-Waller

References and suggested reading

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