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Yemen’s Physical Geography: Volcanism, Jol, Wadis, and Rub al-Khali
The Republic of Yemen (henceforth, Yemen) is located on the trailing (southern) aspect of Arabia and the Arabian plate in the Middle East (1). Yemen borders the Gulf of Aden to the south, the Red Sea to the west, Saudi Arabia to the north, and Oman to the east. Yemen owns half of the Bab el Mandeb, the strait linking the Red Sea and the Gulf of Aden, possibly the point of the earliest migrations of modern humans out of Africa around 60,000 years ago when oceans were much lower and currently site of the world’s most active shipping lanes. Yemen is slightly more than twice the size of the state of Wyoming in the USA (around 204,000 square miles).
The Middle East, as defined here (1), originally formed part of the African Plate throughout the greater part of the Phanerozoic, i.e., roughly the past 545 million years; the Phanerozoic includes the Paleozoic, Mesozoic and Cenozoic eras. (2) The African plate began to break up during the early part of the Mesozoic era (beginning around 252 million years ago), with the opening of the so-called Neotethys Ocean, which resulted in separation of the Iranian segment from the African plate. (2)
During the late Tertiary (around 2.5 million years ago), a spreading ridge or “rift” developed and propagated from the Indian Ocean to form
Together, the Gulf of Aden and the Red Sea rifts form the southeastern and southwestern boundaries of the Middle East area and of Yemen. (3) These two rifts are very young, compared with the countless other rifts in Earth’s geologic history. As a result, the development of a coastal plain in both rift regions is minimal. This geologic fact explains why Yemen lacks a well-developed coastal plain on its southern aspect (facing the Gulf of Aden rift), and on its western aspect (facing the Red Sea rift).
Geologists have observed good agreement between the geologic structures on either side of the Red Sea rift, i.e., those found in western Yemen (the eastern shore of the Red Sea) and the lands (e.g., Ethiopia, Djibouti) on the western side of the Red Sea. That said, the Yemen side of the Red Sea has much greater relief (differences in elevation and slope between the higher and lower parts of the land surface of a given area) than the opposite African shore. On the Yemen side, uplifted Precambrian rocks and their sedimentary and volcanic cover create a 932-mile-long mountain chain parallel to the Red Sea, which the Saudis call the Asir Mountains and the Yemeni call the Highland Mountains of Yemen. This mountain chain continues in an “L” shape by turning the corner at the southwestern-most aspect of Yemen. The mountains continue for a couple hundred miles eastward along the southern coast.
The highest mountain in this chain, Jabal an Nabi Shu’ayb, located in Sana’a Governorate in northeastern Yemen, is almost 12,000 feet above sea level. (4-5) The ancient city Sana’a is both the center of Sana’a Governorate and the capital of Yemen. Sana’a lies at an elevation of 7,218 feet above sea level, making it one of the highest capitals in the world. (6)
The Highlands of northwestern and western Yemen gradually decline to the north (toward Saudi Arabia) and to the east (toward Oman). For example, as one travels from Sana’a eastward to ancient Marib (Yemen), a distance of about 75 miles (as the crow flies), elevation rapidly declines from around 7,200 feet (Sana’a) to 3,200 feet (Marib airport). When one continues traveling eastward from Marib to Tarim (Wadi Hadhramaut, Yemen), a distance of about 250 miles, elevation gradually falls from 3,200 feet to around 2,400 feet. In sum, one loses about a mile in elevation from Sana’a in the west to Tarim in the east across the midriff of Yemen.
Yemen’s vast Highlands as described above are composed of “flood basalts,” which are “the repeated effusion of huge batches of basaltic magma, around 38 to 380 square miles per eruption, over relatively short periods of time,” as described elsewhere. (8) Basalt is dark grey to black in color and fine-grained due to its rapid cooling after extrusion from the deep hot Earth. The basalts are called “flood” because they have low viscosity, meaning they are “runny” and spread out relatively quickly across a surface.
The basalt plateaus of Ethiopia and Djibouti on the western side of the Red Sea and Yemen on the eastern side of the Red Sea “form a single petrologic province whose origin is directly related to rifting in this area that started during the Tertiary Period [late Tertiary, around 2.5 million years ago, see above] and continues to the present.” (9) The Tertiary basalt plateaus of Ethiopia and Yemen formed after a large asthenospheric plume (called Afar plume) reached the base of the lithospheric mantle in the Afar region of Ethiopia (northeast Ethiopia) about 30 million years ago. (9) The Afar region of Ethiopia is extremely geologically active today. The asthenosphere is the “mechanically weak ductily-deforming region of the upper mantle of the Earth. It lies directly below the lithosphere at depths of between 62 and 124 miles below the Earth’s surface,” perhaps even deeper in some places. (10) The lithosphere is the Earth’s crust and uppermost mantle, which constitute the hard and rigid outer layer of the Earth. A plume is a column of hot, solid material that originates deep in the mantle, probably at the core-mantle boundary. (11) A plume includes innumerable fissures through which ascending magma reaches the surface of the Earth. Flood basalts are the first eruptive products of a new mantle plume. The volumes of basalt produced during these events are enormous, say around 1,000 to 1,500 miles in diameter per eruption. (11)
The number of lava flows and their thickness in Yemen, Saudi Arabia, Ethiopia and Djibouti decrease with increasing distance from the major rifts. The Ethiopian basalt plateau originally covered an area of about 290,000 square miles, whereas the basalts in Yemen and Saudi Arabia had an area of about 50,000 square miles. (9) “The main episode of volcanic activity started at 30 million years ago and lasted for about ten million years. However, volcanic activity in the area of the Main Ethiopian Rift started about 15 million years earlier about 45 million years ago. About 20 million years ago, volcanic eruptions ceased in Yemen and became localized in the main rift valley of Ethiopia. A second phase of volcanism started at 4.5 million years ago throughout the region and coincided with renewed widening of the Red Sea.” (9) Six volcanoes, which date to Upper Miocene to Pliocene age (around 5 million to 2.5 million years ago), along the southern coast of Yemen extend from the city of Aden westward to the straits of Bab el Mandeb at the entrance to the Red Sea. The lavas extruded by these volcanoes rest on top of the earlier flood basalts that extruded in the Tertiary, as described above. Thus, new magma rose from the deep hot Earth, piercing through basement rock, Upper Jurassic limestones, and the Triassic flood basalts to create new volcanoes on top of flood basalts. The city and port of Aden actually lie in the crater of the now extinct Aden volcano, which is on top of flood basalts.
Aden harbor was the site of the Al Qaeda-inspired bombing of the U.S. Navy destroyer U.S.S. Cole on October 12, 2000. Seventeen American soldiers were killed and many more injured. Smaller volcanic cones and lava fields, interspersed with sand, cover the southern coast of Yemen eastward to Bir Ali.
The Jol is the stony uplifted plateau wasteland in the Hadhramaut region of Yemen. The Jol is located eastward of the flood basalts that characterize the western side of Yemen. The Jol is also the southern fringe of the vast Arabian tableland that tilts downwards from Petra (Jordan) to Oman. (12) To reach the Jol, one may start from Mukalla at sea level on the southern coast of Yemen and head northward 40 miles (as the crow flies) up the Jol’s southern flank to reach its highest point located on its east-west backbone, at around 5,000 feet, almost a mile high. (There are occasional mountains on the Jol, such as Kor Saiban, that are up to 7,000 feet above sea level.) The Jol backbone is a watershed, meaning monsoon rainfall flows to the north or south, depending on where it strikes in relation to the watershed. The north side of the Jol backbone slowly loses elevation all the way to Petra. At the boundary between Yemen and Saudi Arabia, the Jol is between 1,000 feet above sea level (east, on the Oman side) to around 4,000 feet as one approaches the flood basalts in western Yemen. One ascends the Jol’s south and north flanks via valleys of various widths, lengths and conformations, called wadis (more below).
Explorer Freya Stark (1893-1993) described the Jol in this way: The Jol has usually been dismissed by travelers as a piece of dull dreariness, a plateau where heat and cold are alike unbearable, where food is quite, and water almost, non-existent, a hard, inhospitable, flat expanse. To me this was not so. The Jol has the fascination and the terror of vastness not only in space, but in time. As one rises to its sunbathed level, the human world is lost; Nature alone is at work, carving geography [e.g., wadis] in her millennial periods, her temporal abysses made visible in stone. On that upland we tread the ancient floors of seas. It has been lifted, sunken and re-lifted perhaps, how often? Its shells are those which, before the beginning of man, lay in unnavigated oceans. They have been raised 7,000 feet and more into the sunlight. The sea-bottom has hardened to limestone; it rolls away now to south and north in shining stony spaces. On its uplifted substance the Sculptor is at work. (12) Wadis begin on the Jol. “Here the shallow catchment area gathers torrential rains, whose waters, swirling round to find the weakest loophole, eat out with increasing volume the ruinous gorges below; here [up on the Jol] they are but small steep whirlpool hollows, funnel tops down which the water rushes [e.g., in wadis to the north or to the south]. Like a shallow amphitheater, the ground converges onto these valley heads in tiers so strangely regular that they look like the foundations of walls set with loose stones. But they are not loose; though scored and wind-bitten to a semblance of separate boulders, you find them immovably set there with edges rectangular in spite of time, a part of the mountain core.” (13) Another observer notes: “The Jol isn’t flat at all, thousands of local depressions—baby wadis—are wedged out of the rock, leaving table mountains reaching up to the “original’ sediment level.” (14) Many Beduin tribes of Yemen live on the Jol in carefully demarcated territories. Peasants live in the wadis. Wadis (Arabic for valleys) are drainage courses formed by water that is intermittent or ephemeral. The word wadi is similar in meaning to the Spanish word “arroyo,” meaning a wash or dry bed that fills with water temporarily after a rain and then becomes dry once again. Wadi floors are usually dry (with some exceptions) all year round, except after rains. When there is a rain, the wadis rapidly fill up with sediment-heavy water (“seil”) that has poured off the Jol at the heads of wadis and down the sides of wadis onto the floor of the wadis. Wadi walls are usually made of limestone riddled with caves in which humans have lived for millennia. The walls may be vertical and more than a 1,000 feet high, or they may have a slope configuration. The air in the wadis, as compared with the air of the Jol, is heavier, according to Stark. (15)
Two of the most famous wadis in Yemen are the Hadhramaut and the Doan (Dowan), a tributary of the Hadhramaut. Both are in the Hadhramaut Governorate. Descending off the Jol steeply into a wadi can be difficult. Stark wrote about descending into the Doan wadi, north of the backbone of the Jol: The difficulty of the Hadhramaut lies in the steep descents into these wadis, the “’aqabas” [“aqabats”] whose causeways are built into the precipice. They are probably very old, paved with cobbly boulders…A Queen of Yemen in the fourteenth century left endowments for “drinking fountains in the valley roads” and repairs to the steep hill-side paths and “staired ways.” Down our particular ‘aqaba, scooped in the cliff and sometimes overhanging, we made our way and took fifty minutes to reach the rubble slopes where the limestone precipices are bedded on sandstone. (15)
The wadis of Yemen have dozens of towns and ruins with distinctive Yemeni mud and straw skyscraper architecture. The towns usually are located away from the wadi streambed to avoid damage and loss of life from the intermittent flooding. However, Tropical Storm 03B hit Yemen on October 24, 2008, causing severe flooding over the eastern Governorates—Hadhramaut and Al-Mahara—for about 30 hours. The heavy rainfall caused flood surge heights in the wads exceeding 30 feet in some areas, as described elsewhere, with loss of life and heavy property damage. (16)
Yemen catches the southern tip of the Empty Quarter (Rub’ al Khali) of the Arabian Peninsula. The Empty Quarter spills across four Arab nations and is the world’s largest sand desert, or sand sea. Its 250,000 square miles make up about one-fifth of the Arabian Peninsula. It holds roughly half as much sand as the Sahara, which is 15 times the Empty Quarter’s size but composed mostly of graveled plains and rock outcrops. (17) The Empty Quarter is 620 miles long and 310 miles wide and is either uninhabited or inhabited with Beduins (Bedouins). Summer temperatures reach 131 degree Fahrenheit. Vast oil reserves and a vast water aquifer exist beneath the surface of Rub’ al Khali. (18) Today, a paved road connects Marib to Shabwah, on the other side of the Rub al Khali. One can view the Empty Quarter in Yemen by car.
Yemen is a land of varied topography, including flood basalts and volcano basalts atop a plateau in the west; the narrow coastal areas dotted with volcanic lava, volcanoes and cones; the Jol; the wadis; and the Rub’ al Khali. Notes:
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