Earth:Marrah Mountains

Short description: Range of volcanic peaks in Sudan

Marrah Mountains
Jebel Marra
Inner and outer crater, Deriba Crater
Highest point
Peak	Deriba crater
Elevation	3,042 m (9,980 ft)
Coordinates
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

H. F. C. Hobbs' 1918 notes on the lakes of Jebel Marra indicate that the two lakes at Deriba (the only two in Marra to his knowledge) were situated 1700 feet above sea level. They occupied an amphitheatre-like formation, about 3 to 4 miles in diameter, formed by a circular (or slightly oval) range of steeply sloping elevations, ranging from about 800 to 2000 feet above the surface.

The salt lake, known to the natives as the "female", was the larger of the two and occupied the north-eastern corner of the amphitheatre. It measured about 1950 yards in length, 1350 in width and about 3 miles in circumference. The water in this lake was noted for being very salty, dirty, greenish in colour and having an unpleasant acrid odour. A considerable deposit of salt marked its high water mark and, except at the northern end, the banks gradually sloped into soft, oozing, strongly smelling mud. Hobbs did not take soundings due to time constraints, but deduced that the lake was probably not very deep, except perhaps at its extreme northern end. He carried out a full prismatic compass traverse, walking the distances, with a small closing error, giving relatively accurate results.

The second lake, known as "the male", was about half a mile south of the salt lake and contained more or less fresh water. It was about 1,550 yards long, 900 yards wide, with a circumference of about 1,000 yards, and formed the centre of a large crater, undoubtedly of volcanic origin. The sides of the crater rose almost perpendicularly out of the water, reaching a height of about 400 to 700 feet, except on the north side, where the rim of the crater was considerably lower, allowing a more gradual slope to the water. The water in this lake, like the salt lake, was greenish, clean, clear and had a slight sulphur smell. A sample taken for analysis broke along with the last bottle of whisky.

With no time to take soundings or make a full traverse of the second lake, Hobbs relied on a prismatic compass and a pocket rangefinder for most distance measurements. These figures were regarded as approximate. The lake was regarded with superstition and fear by the inhabitants of Jebel Marra, who believed in its mystical properties. Few natives had actually visited it, but it was considered to be haunted, an oracle to which questions were asked and answers deduced from the colours the water assumed at certain times.

The notes also detail Hobbs' ascent of the south-west peak on 13 March 1918. Despite initial claims by the natives that there was no path, guides were eventually obtained to lead the expedition on a well-trodden path over the most difficult part of the climb. The ascent from lake level took 4.5 hours, with the climb up the ridges surrounding the lakes being almost precipitous. The summit revealed two other peaks, about 1 mile and 4 miles away respectively, 50 and 100 feet higher than the one they were climbing. Another peak, about 20 miles away, with a magnetic reading of 540, was probably the highest. The peak climbed was the most prominent on the south-west end of the mountain and was referred to by the locals as the 'original' Jebel Marra. Due to the limitation of the aneroid barometer, which could only register up to 5000 feet, Hobbs was unable to determine the height of the summit he had reached. However, he estimated by eye that the summit was at least 2000 feet above the lakes, putting it at around 6800 feet above sea level.

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, several hundred samples were collected from a variety of sites including streams, waterfalls, pools, wet rocks, and the two saline crater lakes. Chemical analyses were carried out in the field and further determinations were made on samples transported back to the University of Khartoum. The team also carried out the first bathymetric survey of the two lakes. The bathymetric survey of the Dariba Lakes involved the use of a calibrated echo sounder and an inflatable rubber dinghy for transects, marking the first use of a boat in these waters. In particular, the larger lake was found to be shallow, with a maximum depth of 11.6 metres, while the smaller lake had unique features, 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.

In summary, the expeditions provided important information on the hydrobiological aspects of Jebel Marra, shedding light on the previously unexplored aquatic ecosystems of the region.

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 3042 metres, extends north-south for about 55 miles, widens to 40 miles, and continues north for another 60 miles. 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 1100 metres in the east to 600 metres in the west. Isolated hills and ridges, such as the 1413 metre 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, revealing Archaean rocks beneath sand and clay deposits at distances of 15 to 70 miles 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 1200 metres. 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 100 miles 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 3 miles 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 30 to 50 miles 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". Smithsonian Institution. https://volcano.si.edu/volcano.cfm?vnum=225030.