Arusha National Park (Mount Meru)
The Arusha National Park, northern Tanzania is dominated by Mount Meru, which at 4,565 m is Africa’s fourth highest summit. Meru is a giant stratovolcano, part of the Younger Volcanism located on older, faulted volcanic terranes in the northern Tanzania divergence. The main cone has a diameter of 25 km. It was built up from numerous, explosive, Plinian-style eruptions that occurred between 0.20 Ma and 80,000 BP. The most spectacular feature of the volcano is a horseshoe-shaped caldera with an estimated age of 7,800–7,000 BP. The western side of the caldera reveals sheer inner walls and is capped by a rocky summit ridge. The caldera contains the 1,067-m-high Ash Cone, an unconsolidated pyramid of ash and cinders, which is one of the youngest volcanic features of Meru. The collapse of the eastern sector of the cone produced a large debris avalanche deposit (DAD), the Momella event, which extends over 35 km onto the lower slopes of Kilimanjaro. The caldera collapse and Momella event can be compared with the catastrophic eruption of Mount St. Helens in 1980. The avalanche at Meru was far larger and carved out a distinctive undulating terrane that contains the Momella Lakes, important habitats for migrating birds. The montane forests that girdle the lower and central slopes of the mountain are particularly extensive and are refuges for large mammals and numerous species of birds. The slightly older Ngurdoto Volcano, which is situated in the southeastern arm of the park, includes a well-preserved summit crater protected from visitors. Meru is gazetted as an active volcano (the last activity was in 1910) and should be monitored as potentially hazardous, particularly in light of the explosive style of volcanism and proximity to the regional town of Arusha.
KeywordsAsh cone Caldera DADs Meru Momella Lakes Sector collapse Stratovolcano
The name Meru is derived from ancient Hindu and Buddhist religious scripts, in which a mythological mountain is described as a sacred place in the centre of our universe, in both a physical and spiritual sense. The concept of a mountain with seven rings, separated by water, is central to the mythology of several ancient cultures. The active Mediterranean volcano of Santorini with its sea-filled caldera enclosed by a ring of islands may in part fit these descriptions. Both Meru and Santorini experienced highly explosive, Plinian-style eruptions that included caldera events. Multiple caldera events can occur and seven such events have been recognised at Santorini (Druitt et al. 1999). Volcanism is typically rejuvenated in the centre of calderas and can form new cones, such as the active Nea Kameni Volcano, a newly formed island in Santorini, and the Ash Cone at Meru.
13.3 Older Volcanic Terranes
13.4 Main Volcanism
Volcanism associated with the main phase of activity at Meru peaked at 0.20 Ma–80,000 BP, after Little Meru had become extinct (Wilkinson et al. 1986). This coincided with the waning of activity in the Kibo Volcano (Kilimanjaro), an indication these two giant volcanoes may share a common plumbing system. The Main Cone Group is the most extensive of the volcanic subdivisions recognised at Meru (Fig. 13.3). This group is dominated by phonolite breccia and tephra that formed from repeated Plinian-style eruptions, a style of volcanism that characterises continental rifts (Sect. 5.10). Formation of the main cone (which may originally have attained a height of over 5,000 m) was followed by a period of quiescence with cycles of intense erosion occurring during the Main Ice Age. Numerous deep gullies and ridges developed during this period.
The parasitic activity was disrupted by Plinian-style events with the eruption of pyroclastic flows high on the flanks of the main cone and with the formation of extensive deposits of pumice and ash on the outer slopes. These deposits mapped as the Mantling Ash blanket large areas of the northern, western and southern slopes (Fig. 13.3). They are well exposed in some river beds, particularly on the western slopes, where they may be as much as 20 m in thickness (Vye-Brown et al. 2014). The ash columns associated with these relatively young Plinian eruptions could have attained heights of 23 km, similar to those described by Pliny the younger in conjunction with the 79AD eruption of Vesuvius (Box 5.1).
13.5 Sector Collapse and the Momella DAD
After the persistent activity of the Late Pleistocene, the Holocene saw a resurgence of catastrophic volcanism at Meru. The eastern sector of the main cone disintegrated at approximately 7,800–7,000 BP to create the horseshoe-shaped caldera (Wilkinson et al. 1986). The caldera has a length of 8 km, width of 5 km and is entirely open to the east (Fig. 13.2). The 1,300-m-high, near-vertical internal walls on the western face include numerous layers of lavas and ash, as well as evidence of block faulting, and feeder dykes (Plate 13.3b). Formation of the caldera and partial collapse of the cone at Meru can be compared with the 1980 eruption of Mount St. Helens (Box 13.1), as originally suggested by Roberts (2002). This would have involved a far greater explosive force than at Mount St. Helens as the caldera and cone are far larger. Major seismic shocks would have preceded this event. The collapse of entire sectors of volcanic cones results in debris avalanche deposits (DADs), as discussed in connection with Meru by Delcamp et al. (2015). The lahars of the earlier mapping (Fig. 13.3) can be reinterpreted as DADs.
The Momella DAD formed during the hot and humid climatic regime of the Early Holocene, and some parts of the deposit have been redistributed by fluvial activity into fan and fluvio-volcanic sequences. The lateral blast or surge associated with the sector collapse would have affected a far larger area than the caldera or avalanche and most of the forest over many tens of km2 would have been destroyed. The blast would have had catastrophic effects on the early inhabitants of the region, such as the Hadzabe tribe. The sector collapse that produced the Momella DAD may also have created a thin layer of ash that mantles large parts of the surviving cone (Delcamp et al. 2015). These events may be correlated with formation of a small lava dome and nephelinite flows within the caldera. One of the nephelinite flows contains xenoliths that may have been transported from the mantle (Roberts 2002).
Box 13.1: Meru and Mount St. Helens
13.6 Momella Lakes
The three large lakes and smaller lakes, ponds and marshes in the northeastern segment of the park, occur in areas of hummocky ground associated with the Momella DAD. The lakes fill hollows in the debris deposit and can be assumed to have a similar maximum age, i.e. approximately 7,800–7,000 BP. Big Momella Lake is the deepest of the lakes (10–30 m) and is moderately alkaline. The more scenic Small Momella Lake is shallower (4–10 m) and although alkaline and salty in the central parts, includes freshwater sections in which Hippopotamus and aquatic birds thrive. The smaller Rishateni Lake is unusually rich in dissolved fluorine, possibly the highest ever recorded in natural lakes. The fluorine is derived from the erosion of the alkaline-rich volcanic rocks. The water used for domestic purposes in the local villages, and also in Arusha may similarly have anomalously high contents of fluorine. Both the Big Momella and Rishateni Lakes contain sufficient cyanobacteria for migrating flamingo (Lihepanyama 2016) (Plate 13.4b). Flamingo may also occur on the small alkaline lakes of Elkekhotoito, Jembamba and Tululusia.
At approximately 1,800 BP, minor seismic activity caused the course of the Ngare Nanyuki River that drains the eastern slopes of Meru to change in a northerly direction. As a result, most of the Momella Lakes are now fed by groundwater within the porous debris deposits and by limited surface run-off and precipitation. Only the Small Momella Lake remains part of an underground river system; the Large Momella and Rishateni Lakes are mostly stagnant. The Momella Lakes are important stopovers for a large variety of birds that migrate between Europe/East Africa and southern Africa.
13.7 Ash Cone
The giant pyramid-shaped body of ash and cinder, known as the Ash Cone rises 1,067 m above the floor of the caldera in the northwestern corner (cover). This feature is correlated with the most recent activity at Meru, i.e. after the sector collapse. In 1910, small amounts of ash were ejected from the Ash Cone for a few days. Up until 1954, fumaroles were recorded in the ash cone, but in 1974 a survey revealed no activity. The unvegetated lava flows on the high, northeastern flanks of the Ash Cone may have erupted as recently as 1877, although there is no consensus or accurate dating of these events. The central parts of the caldera include small seasonal lakes (pans), recorded incorrectly as craters on tourist maps, that dry to reveal deposits of alluvium and salts.
The Arusha National Park has been described as one of the ‘hidden gems’ of Africa, as it has much to offer and yet receives far less visitors than Kilimanjaro and the more famous parks farther west. The biodiversity conservation and equitable management of natural resources in this area are hampered due to the relatively large population within the rural community (Istituto Oikos 2011). Mount Meru has a important role in ensuring climate stability and water supply for a large area, including a rapidly growing urban population based on the regional centre of Arusha. A fundamental principle is the protection of the montane forests and fertile foothills of Meru, large parts of which occur outside of the park.
- Cattermole, P. (1982). Meru—A Rift Valley giant. Volcano News, 11, 1–3.Google Scholar
- Dawson, J. B. (2008). The Gregory Rift Valley and Neogene-recent volcanoes of northern Tanzania. Geological Society London Memoir, 33, 102 p.Google Scholar
- Delcamp, A., Delvaux, D., Kwelwa, S., Macheyeki, A., & Kervyn, M. (2015). Sector collapse events at volcanoes in the North Tanzanian divergence zone and their implications for regional tectonics. Geological Society of America Bulletin, 128, 169–186.Google Scholar
- Druitt, T. H., Edward, L., Mellors, R. M., Pyle, D. M., Sparks, R. S. J., Lanphere, M., Davies, M., & Barreiro, B. (1999). Santorini Volcano. Geological Society London Memoir, 19, 169 p. Google Scholar
- Glicken, H. (1996). Rockslide-Debris avalanche of May 18, 1980, Mount St. Helens Volcano, Washington. US Geological Survey Open-File Report 96–677, 90 p.Google Scholar
- Guest, N. J., & Leedal, G. P. (1956). The volcanic activity of Mount Meru (Records 3, 40-46). Geological Survey of Tanganyika.Google Scholar
- Guest, N.J., & Pickering, R. (1966). Notes accompanying quarter degree sheet 40: Gelai and Ketumbeine. Mineral Resources Division of Tanzania.Google Scholar
- Istituto Oikos. (2011). The Mount Meru challenge: Integrating conservation and development in northern Tanzania. Milano: Ancora Libri (Italy), 69p.Google Scholar
- Lihepanyama, D. G. (2016). Ecology of Lesser Flamingos in them Momella lakes, Arusha National Park, Tanzania. Unpublished B.Sc. thesis, University of Dar-es-Salaam, 87 p.Google Scholar
- Roberts, M. A. (2002). The geochemical and volcanological evolution of the Mt Meru region, northern Tanzania. Unpublished Ph.D. thesis, University of Cambridge.Google Scholar
- Vye-Brown, C., Crummy, J., Smith, K., Mruma, A., & Kabelwa, H. (2014). Volcanic hazards in Tanzania. In: British Geological Survey Open File Report OR/14/005, 29 p.Google Scholar
- Wilkinson, P., Downie, C., Cattermole, P. J. & Mitchell, J.G. (1983). Notes accompanying quarter degree sheet 55: Meru. Geological Survey of Tanzania.Google Scholar