Английская Википедия:Deriba (caldera)

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Шаблон:Good article Шаблон:Short description Шаблон:Infobox mountain Deriba is a Pleistocene or Holocene caldera in Darfur, Sudan. Part of the volcanoes of the Marra 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 basaltic 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 springs and fumaroles 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.

Geography and geomorphology

Файл:Deriba map.jpg

Map of the Deriba caldera

Deriba lies in the Marra Mountains of Sudan,Шаблон:Sfn Africa's geographic centre; Deriba is in the southern sector of these mountains and close to the main summit thereof.Шаблон:Sfn The caldera rim became Sudan's new highest point, after the independence of South Sudan. The town of Nyala lies south of Deriba.Шаблон:Sfn The volcano is poorly studied due to the ongoing Darfur conflict.Шаблон:Sfn

Deriba is an oval, Шаблон:ConvertШаблон:Sfn wide and Шаблон:Convert deep caldera, with a volcanic cone occupying the southwestern part of the caldera floor.Шаблон:Sfn At least five overlapping vents form the volcanic cone.Шаблон:Sfn The rim of the caldera reaches a maximum elevation of Шаблон:Convert in the southwest,Шаблон:Sfn and is steep almost vertical.Шаблон:Sfn The caldera is cut into volcanic ash, lapilli, lavas, obsidian and tuffs, and the floor is strewn with pumice blocks.Шаблон:SfnШаблон:Sfn A gap lies in the eastern caldera wall.Шаблон:Sfn

Lakes

Deriba contains two lakes, which are known as the "Deriba lakes". A Шаблон:Convert deep lake occupies the northeastern area of the caldera,Шаблон:Sfn and being located in the lowest part of the caldera floorШаблон:Sfn it is the sink of Deriba.Шаблон:Sfn In 1918 it had dimensions of Шаблон:Convert, but by 1964 the size of the lake had increased.Шаблон:Sfn The lake is surrounded by a gradually sloping muddy beach with the exception of the northern shore.Шаблон:Sfn This lake is also known as the "female" lake, with green salty water.Шаблон:Sfn The salt consists of chloride, potassium and sodium salts.

The volcanic cone has a lake as well, which is Шаблон:Convert deepШаблон:Sfn and smaller, with a roughly rectangular shape that extends in north-south direction.Шаблон:Sfn In comparison to the other lake Шаблон:Convert northwest, this "male" lake had dimensions of Шаблон:Convert in 1918. Steep slopes surround this lake,Шаблон:Sfn which is filled with fresher water.Шаблон:Sfn Reportedly, the local Fur people considered the lake haunted,Шаблон:Sfn but the lakes were also used as a source for salt.Шаблон:Sfn

Water levels in these lakes are fairly stableШаблон:Sfn from season to seasonШаблон:Sfn although evidence for substantial fluctuations have been foundШаблон:SfnШаблон:Sfn that correlate to the regional climate and to fluctuations in the water levels of Lake Chad.Шаблон:Sfn Together with several perennial streams they are thus perennial waterbodies.Шаблон:Sfn Around the Deriba caldera, drainage occurs either southward or westward,Шаблон:Sfn leading into the Bahr El-Arab of the White Nile and the Chari River of Lake Chad respectively. The lakes themselves have no surface outlets.Шаблон:Sfn

In the past, larger lakes existed inside the Deriba caldera. The first such lake stage has been dated to 23,000Шаблон:Sfn or 19,000 years before present when water levels rose Шаблон:Convert above the present-day levels, the second 19,600 – 16,000 years before present when they were Шаблон:Convert higher than currently and the third 14,000 years before present, then Шаблон:Convert above present-day. Similar lake stages have been documented in Trou au Natron in Tibesti, 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 shorelinesШаблон:Sfn and limestone deposits in the caldera,Шаблон:Sfn and it is likely that the lakes sometimes overflowed through the eastern caldera rim gap.Шаблон:Sfn

Geology

Deriba is part of the Jebel Marra volcano, which together with the Tagabo Hills and the Meidob Hills is one of three volcanic fields in Darfur;Шаблон:Sfn these form the little known Darfur Volcanic Province.Шаблон:Sfn The Jebel Marra volcano is maximally Шаблон:Convert wide and Шаблон:Convert long in north-south direction, where the northern segment is centered around Jebel Gurgei.Шаблон:Sfn

Jebel Marra consists of a pile of mostly basaltic lava which has been covered by pumice and volcanic ash as well as pyroclastic rocks and ignimbrites. The Deriba caldera which forms the top of the entire complex,Шаблон:Sfn which around Deriba has the appearance of a large ash coneШаблон:Sfn or shield volcano.Шаблон:Sfn Other, less spectacular vents are found elsewhere in the Marra Mountains. Erosion has cut canyons into the volcanic complex.Шаблон:Sfn The occurrence of volcanism has been explained with a mantle plume centered between Meidob and Jebel Marra.Шаблон:Sfn

The basement is formed by crystalline rocks, mainly metamorphosed gneisses and schists,Шаблон:Sfn and is a mobile belt of Panafrican age.Шаблон:Sfn They are in part covered by the Nubian Sandstones of Cretaceous age and aeolian sands,Шаблон:Sfn and by the Jebel Marra massif which occupies a surface of Шаблон:Convert.Шаблон:Sfn Tectonic uplift of the Darfur dome commenced in the Cretaceous and resulted in a noticeable upwarp of the basement beneath Jebel Marra. Two major tectonic lineaments intersect at Jebel Marra,Шаблон:Sfn one trending south-southeastward and the other east-northeastward.Шаблон:Sfn

Eruptive history

Volcanism in the Jebel Marra mountains appears to have begin 15 million years ago and continued in two stages, separated by an erosional period.Шаблон:Sfn Volcanism in the southwestern Marra mountains took place between 4.35 million years ago and 60,000 years ago,Шаблон:Sfn subdivided into an Old Series and a New Series which is younger than 2 million years ago. After an initial stage, during which olivine basalt and small amounts of pyroclastic material were erupted, trachyte were emplaced. The Deriba caldera probably formed either in the early Pleistocene,Шаблон:Sfn 60,000 or 3,520 ± 100 years before present (BP), but activity continued with the volcanic cone inside of the caldera and peripheral vents, possibly into historical time.Шаблон:Sfn

The 3,520 ± 100 BP eruption was a Plinian eruption that deposited pyroclastic material containing blocks of basement material. The ash fall from the eruption reaches thicknesses of Шаблон:Convert as far as Шаблон:Convert away from Deriba, while pyroclastic flows have been identified as far as Шаблон:Convert from the caldera. The eruption has an estimated volcanic explosivity index of 4 and may be associated with the volcanic cone inside the caldera.Шаблон:Sfn

The volcano was formerly considered to be extinct. Hot springs, whose temperature reaches Шаблон:Convert and which may be supplied by magmatic water, and fumaroles, both within the caldera around the volcanic cone and in the surrounding Marra Mountains,Шаблон:SfnШаблон:Sfn indicate that Jebel Marra and the Deriba caldera are a dormant volcano.Шаблон:Sfn 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.Шаблон:Sfn

Climate

Jebel Marra has a humid subtropical climate (Cwa) bordering on a subtropical highland climate (Cwb). Precipitation on Jebel Marra is about Шаблон:Convert,Шаблон:Sfn the mountain lies at the eastern margin of the Sahel and receives more precipitation than the surrounding region.Шаблон:Sfn Between 12,000 and 8,000 years before present, the climate was wetter as a consequence of a northward shift of climate zones.Шаблон:Sfn

Biology

Vegetation around Deriba consists of grassland with scarce trees such as the wild olive.Шаблон:Sfn The plant life is classified as Afromontane, it also includes species from temperate climates.Шаблон:Sfn It is likely that past humid periods permitted the expansion of Mediterranean species into the Jebel Marra mountains.Шаблон:Sfn Presently, Jebel Marra is a suitable site for agriculture and used as such by the Fur people.Шаблон:SfnШаблон:Sfn Semi-desert and desert landscapes dominate the region around the Marra Mountains.Шаблон:Sfn

The lakes are salty, oxygen poor and remote and thus contain little plant or animal life.Шаблон:Sfn Some vegetation grows around the smaller lake, as well as in other parts of the Deriba caldera where water is available.Шаблон:Sfn

Copepods live in the smaller lake, while the larger one is populated by blue-green algae and rotifers.Шаблон:Sfn Spirulina occurs in the large lake,Шаблон:Sfn and Melosira and Nitzscia diatoms have been identified in the small lake.Шаблон:Sfn The copepod Eucyclops gibsoni has been encountered in the small lake. Among the rotifers are Brachionus dimidiatus, Brachionus plicatilis, Hexarthra jenkinaeШаблон:Sfn and Lecane bulla, some of which also occur in the smaller lake.Шаблон:Sfn Ephedra flies are widespread around the large lake, and other insects were collected on the small lake.Шаблон:Sfn Birds such as greenshanks, little grebes, sacred ibis and stilts are uncommon.Шаблон:Sfn

Gallery

• Sketch of the Deriba lake looking west

  Sketch of the Deriba lake looking west

• Northeast lake in Deriba, looking west

  Northeast lake in Deriba, looking west

• The southwestern lake

  The southwestern lake

• The caldera

  The caldera

References

Sources

• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite web
• Шаблон:Cite book

External links

• Satellite image (large scale) : [1]
• Oblique aerial photo on page 27 (large scale) : [2]
• Deriba Caldera at NASA Earth Observatory