An overview of Origin, Morphology and Distribution of Desert
Forms, Sabkhas and Playas of the Rub’ al Khali Desert of the Southern Arabian Peninsula
Arun Kumar and Mahmoud M. Abdullah
Center for Petroleum and Minerals, Research Institute
King Fahd University of Petroleum and Minerals
Dhahran 31261, Saudi Arabia
Email: arunkumarlko@hotmail.com
Open access e-Journal -Earth Science India, eISSN: 0974 – 8350 - Vol. 4(III), July, 2011, pp. 105-135
Abstract
Rub’ al Khali or the “Empty Quarter” is the largest erg or continuous sand desert in the world and occupies southern part of the Arabian Peninsula. It is a broad depression that covers approximately 560,000 km2 extending from United Arab Emirates in the east to ~1500 km west to the hills of southwestern Saudi Arabia and Yemen. There are several types of sand dunes, vast sand sheets and rare ephemeral water bodies like lakes and ponds also occur in this region. This paper describes these morphological features and discusses their origin and evolution in the light of Pleistocene-Holocene climate change that alternated between humid and arid phases. Primarily two parameters have influenced the evolution of landscape in Rub’ al Khali, firstly, very low rainfall (< 50 mm/year) resulting in low natural vegetation cover, restricted soil formation, increased erosion, and expanded aeolian sand bodies and secondly, the dune morphology, which in this region is influenced by the changes of wind direction during Pleistocene-Holocene epochs. Strong wind in Rub’ al Khali plays an important role in sand drift and dune migration. Unidirectional winds form barchan dunes that range in size from small (4 m high) to large megabarchan dunes (250 m high). There are two dune-forming winds in this region; the Shamal that blows mainly from northeast to southwest direction and the Southwest monsoon that is active from June to August. During Quaternary repeated advances and retreats of polar ice caps resulted in shifting of desert belt towards the Equator that resulted in change in direction and intensity of dune-forming winds.
Key Words: Arabian Peninsula, Deserts, Rub’ al Khali, Sabkhas, Sand Dunes, Desert Lakes
Introduction
About one third of the Arabian Peninsula is covered by sandy deserts and the Rub’ al Khali or the “Empty Quarter” is the largest erg (sand sea or desert) or continuous sand desert in the world (Fig.1). It is a broad depression that covers approximately 560,000 km2 (522,340 km 2 : Edgell, 2006) and spreads across Saudi Arabia, United Arab Emirates (UAE), Oman and Yemen (Wilson, 1973; Glennie, 2005). It extends from UAE in the east to ~1500 km west to the hills of Yemen (Breed et al., 1979). Rub’ al Khali is a Neogene basin that is characterized by large sand dunes measuring 50 to over 250m high separated by up to 2 km wide flat valleys. The valley floors are either inland sabkhas, sandy or gravely or bed rocks. There are several types of sand dunes, vast sand sheets and few ephemeral water bodies (playas) reminiscent of lakes and ponds also occur in this region. Linear dunes, commonly 200 km long, barchans, barchanoid ridges and megabarchans cover large areas of this desert.
The sand dunes in Rub’ al Khali are basically of two types: transverse and linear. Transverse dunes are oriented perpendicular and linear dunes are parallel to the dominant wind direction. Unidirectional winds form simple crescent shaped dunes, known as barchan dunes ranging in height from 4 to 250 m as in the case of megabarchans. When these dunes are linked on their slip face, a row of dunes called a barchanoid ridge. When the barchan dunes have three or more arms with slip faces, they are called star dunes. When winds blow in two opposing directions, linear dunes are formed and they are of three main types (a) simple linear dunes; (b) compound linear dunes; and (c) complex linear dunes.
Sand dune morphology (see Fig. 2) primarily depends upon the amount of sand supply, shape, size and composition of the sand grains, surface topography, wind speed, direction and duration, humidity, and nature of the surface vegetation (Edgell, 1990). The deposition of sand and subsequent formation and accumulation of dunes generally occurs as a result of an interruption of sand-laden wind masses due to the presence of obstacles or variations in the ground relief (Bagnold, 1951). A complete understanding of the mechanisms of dune formation is still poorly understood (Warren and Knott, 1983). A review of sand dune studies and distribution of types of sand dunes using Landsat Imagery in Rub’ al Khali is described by McKee (1979). In deserts, the dry sand on an avalanche slope has a maximum angle of repose of 340 and the minimum height for an avalanche slope to form is 30 cm. Transverse dunes migrate downwind as sand is deflated from its windward slope and is redeposited on the leeward avalanche slope (Glennie, 2005).
Fig.1: Location of Rub’ al Khali desert in the Arabian Peninsula showing the prevailing wind direction.
Fig.2: Morphology of sand dune (modified after http://en.wikipedia.org/wiki/Dune).
Edgell (2006) gave the most comprehensive account of different topics on study of deserts covering not just the Rub’al Khali desert but also all deserts of the Arabian Peninsula and modifies some of the existing geomorphological details. According to Edgell (op. cit.) the Rub’ al Khali covers an area of 522,340 km2 (not ~ 560,000 km2 as stated earlier) with huge linear dunes and its types, transverse dunes including barchan dunes, megabarchan dunes and barchnoid ridges, and large inter-dunal gravel deposits and sand sheets. The inter dune areas are greater than the areas covered by dunes. This is a sedimentary basin whose axis trends NE to SW and is bordered to the north and northwest by the Arabian Shield, and to the south and southeast by Hadramwat – Dofar arch. The basin opens in the north in the Arabian Gulf and northeast by the arc of mountains in Oman. Topographically, this basin gradually rises towards the southwest from < 50 m in the United Arab Emirates (UAE) territory to > 900 m near the eastern ridge of the Asir Plateau of southwestern Saudi Arabia.
Fig. 25-A: Landsat ETM image showing star dunes.
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