Deriba (caldera)

Deriba contains two lakes, which are known as the "Deriba lakes". A 11.5-metre (38 ft) deep lake occupies the northeastern area of the caldera,[2] and being located in the lowest part of the caldera floor[14] it is the sink of Deriba.[11] In 1918 it had dimensions of 1,230 by 1,780 metres (1,350 yd × 1,950 yd), but by 1964 the size of the lake had increased.[15] The lake is surrounded by a gradually sloping muddy beach with the exception of the northern shore.[16] This lake is also known as the "female" lake, with green salty water.[8] The salt consists of chloride, potassium and sodium salts.[17]

The volcanic cone has a lake as well, which is 108 metres (354 ft) deep[2] and smaller, with a roughly rectangular shape that extends in north-south direction.[18] In comparison to the other lake 1.21 kilometres (0.75 mi) northwest, this "male" lake had dimensions of 820 by 1,420 metres (900 yd × 1,550 yd) in 1918. Steep slopes surround this lake,[8] which is filled with fresher water.[11] Reportedly, the local Fur people considered the lake haunted,[19] but the lakes were also used as a source for salt.[20]

Water levels in these lakes are fairly stable[21] from season to season[22] although evidence for substantial fluctuations have been found[15][23] that correlate to the regional climate and to fluctuations in the water levels of Lake Chad.[9] Together with several perennial streams they are thus perennial waterbodies.[24] Around the Deriba caldera, drainage occurs either southward or westward,[25] leading into the Bahr El-Arab of the White Nile and the Chari River of Lake Chad respectively.[3] The lakes themselves have no surface outlets.[26]

In the past, larger lakes existed inside the Deriba caldera. The first such lake stage has been dated to 23,000[27] or 19,000 years before present when water levels rose 25 metres (82 ft) above the present-day levels, the second 19,600 - 16,000 years before present when they were 8–5 metres (26–16 ft) higher than currently and the third 14,000 years before present, then 9 metres (30 ft) 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.[28][29] These lake stages have left shorelines[9] and limestone deposits in the caldera,[23] and it is likely that the lakes sometimes overflowed through the eastern caldera rim gap.[30]

Volcanism in the Jebel Marra mountains appears to have begin 15 million years ago and continued in two stages, separated by an erosional period.[43] Volcanism in the southwestern Marra mountains took place between 4.35 million years ago and 60,000 years ago,[36] subdivided into an Old Series and a New Series which is younger than 2 million years ago.[41] After an initial stage, during which olivine basalt and small amounts of pyroclastic material were erupted, trachyte were emplaced.[44] The Deriba caldera probably formed either in the early Pleistocene,[27] 60,000[41] 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.[34]

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 20 metres (66 ft) as far as 5 kilometres (3.1 mi) away from Deriba,[33] while pyroclastic flows have been identified as far as 30 kilometres (19 mi) from the caldera. The eruption has an estimated volcanic explosivity index of 4[1] and may be associated with the volcanic cone inside the caldera.[23]

The volcano was formerly considered to be extinct.[45] Hot springs, whose temperature reaches 65–85 °C (149–185 °F) and which may be supplied by magmatic water,[45] and fumaroles, both within the caldera around the volcanic cone and in the surrounding Marra Mountains,[34][18] indicate that Jebel Marra and the Deriba caldera are a dormant volcano.[18] Reportedly, the exhalations of the fumaroles can kill birds and insects,[45] and some fumarolic vents may have formed between 1964 and 1966,[46] implying that the fumarolic activity may reflect a recent change of volcanic activity.[47]