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Deriba is a Pleistocene or Holocene caldera in Darfur, Sudan. Part of the volcanoes of the Marrah Mountains, it lies on the Darfur Dome and like the Tagabo Hills and Meidob Hills volcanism may be the product of a mantle plume. After the separation of South Sudan, the highest point of Sudan is on the margin of the caldera.
The caldera lies atop a shield volcano or ash cone in the southern Marra Mountains, which developed first as a pile of Basalt lava flows and later as layers of volcanic ash and tuff, including the eruptions that formed the caldera. A large eruption occurred about 3,520 ± 100 years before present, and Hot spring and Fumarole are active to the present day.
Deriba contains two lakes, one in the northeastern side of the main caldera and the other in a cone in the southwestern sector of the Deriba caldera. The fresher southwestern lake is smaller but considerably deeper than the saltier northeastern lake; in the late Pleistocene the caldera was filled with a larger lake.
Deriba is an oval caldera, wide and deep, with a volcanic cone occupying the southwestern part of the caldera floor. At least five overlapping vents form the volcanic cone. The rim of the caldera reaches a maximum elevation of in the southwest, and is steep almost vertical. The caldera is cut into volcanic ash, lapilli, , obsidian and , and the floor is strewn with pumice blocks. A gap lies in the eastern caldera wall.
The volcanic cone has a lake as well, which is deep and smaller, with a roughly rectangular shape that extends in a north-south direction. In comparison to the other lake northwest, this "male" lake had dimensions of in 1918. Steep slopes surround this lake, which is filled with freshwater water. Reportedly, the local Fur people people considered the lake haunted house, but the lakes were also used as a source for salt.
Water levels in these lakes are fairly stable from season to season although evidence for substantial fluctuations have been found that correlate to the regional climate and to fluctuations in the water levels of Lake Chad. Together with several perennial they are thus perennial waterbodies. Around the Deriba caldera, drainage occurs either southward or westward, leading into the Bahr al-Arab of the White Nile and the Chari River of Lake Chad respectively. The lakes themselves have no surface outlets.
In the past, larger lakes existed inside the Deriba caldera. The first such lake stage has been dated to 23,000 or 19,000 years before present when water levels rose above the present-day levels, the second 19,600–16,000 years before present when they were higher than currently, and the third 14,000 years before present, then above present-day. Similar lake stages have been documented in Trou au Natron in Tibesti Region, where shifts of the position of the subtropical jet stream and the tropical depressions associated with the jet stream have been invoked as an explanation. These lake stages have left and limestone deposits in the caldera, and it is likely that the lakes sometimes overflowed through the eastern caldera rim gap.
Jebel Marra consists of a pile of mostly lava which has been covered by pumice and volcanic ash as well as Pyroclastic rock rocks and . The Deriba caldera which forms the top of the entire complex, which around Deriba has the appearance of a large ash cone or shield volcano. Other, less spectacular Volcano are found elsewhere in the Marra Mountains. Erosion has cut into the volcanic complex. The occurrence of volcanism has been explained with a mantle plume centered between Meidob and Jebel Marra.
The basement is formed by crystalline rocks, mainly metamorphosed and , and is a mobile belt of Panafrican age. They are in part covered by the of Cretaceous age and aeolian sands, and by the Jebel Marra massif which occupies a surface of . Tectonic Geologic uplift of the Darfur Dome commenced in the Cretaceous and resulted in a noticeable upwarp of the basement beneath Jebel Marra. Two major tectonic intersect at Jebel Marra, one trending south-southeastward and the other east-northeastward.
The 3,520 ± 100 BP eruption was a Plinian eruption that deposited Pyroclastic rock material containing blocks of basement material. The ash fall from the eruption reaches thicknesses of as far as away from Deriba, while have been identified as far as from the caldera. The eruption has an estimated volcanic explosivity index of 4 and may be associated with the volcanic cone inside the caldera. There are no historical records or oral tradition linked to this or other events at Jebel Marra, however.
The volcano was formerly considered to be extinct. , whose temperature reaches and which may be supplied by magmatic water, and , both within the caldera around the volcanic cone and in the surrounding Marra Mountains, indicate that Jebel Marra and the Deriba caldera are a dormant volcano. Reportedly, the exhalations of the fumaroles can kill birds and insects, and some fumarolic vents may have formed between 1964 and 1966, implying that the fumarolic activity may reflect a recent change of volcanic activity.
The lakes are salty, oxygen poor and remote and thus contain little plant or animal life. Some vegetation grows around the smaller lake, as well as in other parts of the Deriba caldera where water is available.
live in the smaller lake, while the larger one is populated by and . Spirulina occurs in the large lake, and Melosira and Nitzscia have been identified in the small lake. The copepod Eucyclops gibsoni has been encountered in the small lake. Among the rotifers are Brachionus dimidiatus, Brachionus plicatilis, Hexarthra jenkinae and Lecane bulla, some of which also occur in the smaller lake. Ephydridae flies are widespread around the large lake, and other insects were collected on the small lake. Birds such as , , sacred ibis and are uncommon.
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