Marrah Mountains

Marrah Mountains
Jebel Marra
Inner and outer crater, Deriba Crater
Highest point
Peak	Deriba crater
Elevation	3,042 m (9,980 ft)
Coordinates	12°57′00″N 24°16′12″E
Geography
Location in Sudan
Country	Sudan
Region	Darfur
Geology
Type of rock	Volcanic field
Last eruption	2000 BC

The Marrah Mountains or Marra Mountains (Fur, Fugo Marra; Arabic: جبل مرة, Jebel Marra) are a range of volcanic peaks in a massif that rises up to 3,042 metres (9,980 ft). They are the highest mountains in Sudan.

Geography

Waterfall.

The mountains are located in the center of the Darfur region of Sudan on the border of the states of South Darfur and Central Darfur, with a smaller part of the range in the state of North Darfur. The highest point is Deriba Caldera. The upper reaches of the massif is a small area of temperate climate with high rainfall and permanent springs of water amidst the dry savanna and scrub of the Sahel below.

Topographic map of the Marrah Mountains

Detail of the Marrah Caldera

Apart from the Aïr Mountains in Niger which are on the border of the Sahara proper, the Marrah Mountains are the only major mountain range in the otherwise flat Sahel, rising up to 2,600 metres (8,500 ft) above the plain, but are relatively unknown owing to lack of development and political conflict in the region.

The last eruption occurred around 1500 BC. The centre of activity was Deriba Caldera, and involved caldera collapse following the eruption of pumice and pyroclastic flows which travelled over 30 kilometres (19 mi) from the volcano.

The vegetation was described by Gerald Wickens.

Description of the lakes

The two lakes at Deriba, as described by Hobbs' 1918 notes, were presumably the only two in Jebel Marra. Hobbs situated them at an altitude of approximately 518 m (1700 feet, as indicated in his note) above the plain and 1,463 m (4,804 feet) above sea level (modern measurements, however, indicate 2100 m). They were described as being located within a vast amphitheatre, approximately 4.8 to 6.4 km (3 to 4 miles) in diameter, formed by a continuous circular (or slightly oval) range of steeply sloping heights, varying from about 244 to 610 m (800 to 2,000 feet) above the surrounding area.

The larger lake, known as the "female", lay in the northeast corner of the amphitheatre. It measured approximately 1,780 metres (5,840 ft) in length, 1,230 metres (4,040 ft) in width, and had a circumference of about 4.8 kilometres (3.0 mi). Its water, with a high salinity, appeared murky green and emitted an unpleasant odour. Although time constraints prevented soundings, it was inferred that the lake was shallow except in its northern part. The smaller lake, dubbed as "male", situated around 800 metres (2,600 ft) south, had fresher water. Its dimensions were approximately 1,420 metres (4,660 ft) in length, 820 metres (2,690 ft) in width, with a circumference of about 910 metres (2,990 ft). Rising nearly vertically from the water's edge, the crater walls reached heights of around 120–210 metres (390–690 ft), except for the northern rim, which sloped gently. Like its counterpart, this lake exhibited a greenish hue and a faint sulphuric aroma.

Since the first exploration by Hobbs and Gillan in 1918 (Gillan 1918, Hobbs 1918.), Jebel Marra has been visited by many geologists, botanists and zoologists. In 1964 an expedition made the first biological survey of the streams and two Deriba lakes. The hydrobiological importance of Jebel Marra stems from the isolation of these waters near the geographical centre of the African continent, coupled with the known chemical differences between the various streams and lakes (Hunting Technical Services, 1958). During the five-week expedition, numerous samples were collected from various sites, including the two saline crater lakes. Chemical analyses were conducted both in the field and at the University of Khartoum. The team also conducted the first bathymetric survey of the lakes, using a calibrated echo sounder and an inflatable rubber dinghy. The larger lake was found to be shallow, with a maximum depth of 11.6 metres (38 ft), while the smaller lake featured unique characteristics, including a conical funnel.

Further investigation, including depth checks using a variety of methods, revealed evidence of significant water level rises in both lakes in relatively recent times. Clear evidence of former beach levels suggested abrupt rises, probably caused by large landslides on the unstable inner walls of the crater. Evidence from dead trees and comparison with historical maps and photographs revealed changes in lake dimensions and levels over time. The study concluded by suggesting a steady rise in the level of the great lake due to the influx of sediment, which could lead to its eventual disappearance.

History

During the War in Darfur, the Marrah Mountains came under the control of the rebel Sudan Liberation Movement/Army faction loyal to Abdul Wahid al Nur. The mountains remained one of the group's most important strongholds, housing several of its bases, as of 2021.

Geology

Jebel Marra, an extinct late Tertiary volcanic massif with a peak elevation of 3,042 metres (9,980 ft), extends north-south for about 89 kilometres (55 mi), widens to 64 kilometres (40 mi), and continues north for another 97 kilometres (60 mi). The Tagabo Hills and the Meidob Plateau, to the north-east of the range, are thought to be of volcanic origin. The plateau rests on Archean rocks on an uplift between the Chad and Middle Nile basins and extends westwards to the Sudanese border, forming an undulating peneplain called the Basement Complex, with elevations ranging from 1,100 metres (3,600 ft) in the east to 600 metres (2,000 ft) in the west. Isolated hills and ridges, such as the 1,413 metres (4,636 ft) Tebella Massif, possibly remnants of an older erosional surface, dot the peneplain.

To the southeast and south of Jebel Marra, the plateau maintains an elevation of 600–700 metres (2,000–2,300 ft), revealing Archaean rocks beneath sand and clay deposits at distances of 24–113 kilometres (15–70 mi) from the base of the mountain. Similar features are observed in the west, including small plateaus and inselbergs such as the Dagu Hills and the Gennung at 1,200 metres (3,900 ft). The eastern and northern regions of Zalingei are dominated by acid crystalline schists and gneisses, while the western counterpart consists mainly of paraschists, calc-silicate gneisses and other rock formations.

South of the Tebella massif, the Wadi Debarei basin appears to be a clinal trough intruded and occupied by foliated unfoliated granite. Geologists from the Sudan Geological Survey have recently carried out work near El Fasher and Nyala, but their results are not yet published. Jebel Nyala, the Dagu Jebels and the Wana Hills show granitic and gneissic compositions, while the areas between El Fasher and Nyala show predominantly quartzose rocks.

About 160 kilometres (100 mi) east of Jebel Marra, the Archaean rocks are covered by Nubian sandstones, with evidence of a westward extension near El Geneina. Recent geological surveys suggest the extensive presence of Nubian sandstones southwest of El Fasher under wind-borne sands, challenging previous beliefs. Limestone near Zalingei, derived from calcareous waters from springs, is the only recorded sedimentary deposit within the Archaean outcrop.

Andrew (1948) suggests that volcanic activity in Jebel Marra began in the Upper Tertiary (Miocene). The Deriba crater, thought to be a recent culmination, has a diameter of over 4.8 kilometres (3 mi) and contains two lakes with distinct saline characteristics. Lava peaks surround the crater, one of which may be the highest point in the range. The volcanic history shows periods of continuous lava eruption, erosion and explosive events that shaped the existing large crater. Outlying volcanic areas and an intrusive phase represented by dykes near Kutum remain topics for further study.

The region is characterised by widespread surficial deposits of fluviatile and aeolian origin, with the Wadi 'Azum and its tributaries showing broad channels and terraces. The eastward and southeastward flowing rivers have sandy beds that change to silts within 48–80 kilometres (30–50 mi) of their sources. A large silt or clay plain north-east of Kutum is thought to be a playa, while a former erg, the qoz, retains dune relief, immobilised by the prevailing savannah. The W. Ibra conveys river runoff south of Jebel Marra, and the Qoz Dango continues the extension of the erg, with no western counterpart on the volcanic highlands.

In summary, the geological and topographical features of Jebel Marra include its volcanic origin, diverse rock compositions, sedimentary deposits and history of volcanic activity. Ongoing geological surveys and the need for further investigation underline the complex nature of this region.

References

• "Jebel Marra". Global Volcanism Program. Smithsonian Institution.