Arusha National Park (Mount Meru)

  • Roger N. Scoon


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.


Ash cone Caldera DADs Meru Momella Lakes Sector collapse Stratovolcano 

13.1 Introduction

Despite its relatively small size (137 km2), the Arusha National Park is one of the most spectacular in East Africa. The dominant feature is Mount Meru, a giant stratovolcano (Cattermole 1982) that includes a horseshoe-shaped caldera with a largely unconsolidated cone of ash and cinder (Plate 13.1). The volcano has a diameter of 25 km and rises more than 3,500 m above the regional plateau to the north of the regional town of Arusha (Fig. 13.1). The montane forests that girdle the central and lower slopes are extensive and include stands of giant Podocarpus trees. The forests are an important habitat for large mammals and host numerous bird species. Most landforms in the park have been created and shaped by geological processes associated with explosive phases of volcanism and erosion during the Late Pleistocene–Holocene. Meru is capped by a summit ridge that leads to the rocky protuberance of Socialist Peak, which at 4,565 m is the fourth highest in Africa (Plate 13.2a). Many of the most significant features, including the caldera, ash cone and summit ridge are visible on a three-dimensional satellite image (Fig. 13.2). Meru is gazetted as an active volcano as there was minor activity in the past century (Guest and Leedal 1956; Guest and Pickering 1966).
Plate 13.1

The giant, horseshoe-shaped caldera at Mount Meru contains the Ash Cone which last erupted in 1910. The unvegetated flows in the foreground are also evidence of relatively recent activity although they have not been radiometrically dated

Fig. 13.1

The Arusha National Park is dominated by the giant cone of Mount Meru.

Source Google-Earth Image 2017, DigitalGlobe, CNES/Airbus

Plate 13.2

a View of the summit ridge of Meru looking south includes gently dipping layers of well-bedded lavas and ashes with the rocky, dome-shaped outcrop of Socialist Peak visible in the background; b The resistant rock dome of Socialist Peak is comprised of lavas of the Summit Group, one of the youngest components of the volcano

Fig. 13.2

Three-dimensional satellite image looking northwest over Mount Meru towards the older Monduli and Tarosera volcanoes. Meru includes an annular ring of forest on the lower slopes and a massive, partially collapsed caldera, open to the east, with a central cone of ash and cinder. Little Meru is visible as a small triangular-shaped cone on the northeastern slopes. The Ngurdoto Crater occurs in the foreground. The hummocky ground and lakes associated with the Momella DAD occur east of the Meru Caldera and north of the Ngurdoto Crater.

Source NASA Landsat 7 ETM+ image mosaic for the year 2000 from the University of Maryland Global Land Cover Facility, overlaid on elevation data from the Shuttle Radar Topography Mission, courtesy of Philip Eales, Planetary Visions

13.2 Mythology

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

The Meru Volcano is built upon an older, faulted, volcanic terrane within the structurally-complex northern Tanzania divergence (Fig.  5.4). The region includes two principal fabrics, the northwest–southeast trending Lembolos Graben and the north–south trending Oljoro Graben. The volcanic rocks in the vicinity of Meru may have arisen from magmas fed through weakness associated with these grabens (Dawson 2008). Some of the Pliocene and Early Pleistocene volcanism that pre-dates Meru, i.e. the Older Volcanism, is shown on the simplified geological map (Fig. 13.3). The latter includes the phonolite lavas of the Meru West Group (3.1–2.4 Ma), the alkali basalt-phonolite lavas of the Oljoro Graben (2.5–2.3 Ma), and the flows of alkali basalt-phonolite on the lower, western slopes of Meru (1.5 Ma). Several volcanic centres which are part of Younger Volcanism (Late Pleistocene) and yet pre-date Meru are also identified. Little Meru (0.40–0.30 Ma) is a subsidiary peak on the northeastern slopes of the main cone comprised of explosive nephelinite breccias with clasts of older material (including phonolite) which may have been derived from Meru West (Wilkinson et al. 1986). The Ngurdoto Volcano is protected by an arm of the Arusha National Park that projects southeast from Meru. This severely eroded cone includes a well-preserved summit crater with a diameter of 4 km by 3 km and a depth of 360 m. Ngurdoto is associated with alkaline silicate (nephelinite) and natrocarbonatite lavas (Roberts 2002) and although it has not been dated is older than Meru, as is the adjacent Matuffa Crater.
Fig. 13.3

Geological map of the Arusha National Park and surrounding area simplified from the Geological Survey of Tanzania 1:125,000 quarter degree sheet 55. The original terminology, relative ages and mapping units of Wilkinson et al. (1983) are retained, e.g. use of lahar rather than DAD. Recent studies suggest the Summit Group may be younger than the Momella Lahar (or DAD)

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.

At approximately 60,000 BP, a new burst of activity at Meru resulted in formation of the Summit Group (Fig. 13.3). This unit of phonolite and nephelinite lava flows forms the summit ridge with its prominent 150-m-high rock dome (Plate 13.2b). The phonolite lavas may contain phenocrysts of alkali feldspar. Deposits of fine- and coarse-grained ash are intercalated with the lava flows on the summit ridge (Plate 13.3a). Formation of the Summit Group was followed by a renewed period of quiescence, albeit with extensive parasitic activity. Parasitic features are particularly well developed on the lower, southern slopes. Some parasitic craters are partially infilled by lakes, many of which are very scenic, e.g. the Duluti Crater at an elevation of 1,300 m, located 13 km to the east of Arusha.
Plate 13.3

a The flanks of the Meru Caldera include inter-layered deposits of fine- and coarse-grained ash; b The internal western wall of the Meru Caldera is built up of multiple beds of lava and ash. The prominent contact in the upper part of the face may be related to the Momella event

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 is associated with the sector collapse that created the caldera. This feature was originally mapped as extending over 35 km to the lower slopes of Mount Kilimanjaro, and with a surface area of approximately 400 km2 (Fig.  12.3). Recent studies have suggested this deposit is far larger and can be conjoined with lahars that were originally mapped as separate, older features. The areal extent of the Momella DAD is now estimated as approximately 1,249 km2 and the volume is calculated as 18 km3, constituting one of the largest DADs ever recorded (Delcamp et al. 2015). DADs create characteristic undulating terranes, as the fast-travelling avalanches gouge out the land surface (Plate 13.4a). The Momella DAD includes house-sized boulders of phonolite derived from the main cone.
Plate 13.4

a The hummocky ground on the lower eastern slopes of Mount Meru is associated with the Momella DAD; b Greater and Lesser Flamingo fringe Big Momella Lake with hummocky ground typical of the debris avalanche deposit also visible

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

Formation of the caldera and DADs at Meru may be compared with the 1980 eruption of Mount St. Helens, in Washington State, USA. This Plinian-style eruption was triggered by the release of pent-up pressure within the cone associated with the expansion of the magma chamber. The eruption was preceded by seismic shocks including a magnitude 5.1 event. Most of the northern sector of the cone collapsed, or blow out, prior to the extrusion of air-fall ash and pyroclastic flows (Glicken 1996). The eruptive products included fast-travelling pyroclastic flows, comprised of hot ash, pumice and gas (they were restricted to a small fan-shaped area on the upper slopes), as well as a huge ash plume that rose to a height of almost 20 km. The abrupt collapse of part of the Mount St. Helens cone triggered a debris avalanche of rock, ash and hot gases to rush down the northern flanks. This avalanche, together with the near-instantaneously formed mudflows and floods was highly destructive and travelled some 27 km. A feature that was not appreciated prior to the Mount St. Helens event, however, is that pressure release by the rapid disintegration of the cone triggered an instantaneous expansion (explosion) of high temperaturehigh pressure steam within the magma chamber. This expansion created a lateral blast or basal surge which rapidly overtook the avalanche and, travelling at speeds of up to 1078 km/h devastated the surrounding landscapes. The lateral blast and debris avalanche at Mount St. Helens caused far more intensive and widespread damage than the eruptive flows and ashfall (Fig.  13.4 ).
Fig. 13.4

View of Mount St. Helens and surrounding area (width of view approximately 150 km) looking south-east using elevation data produced by the Shuttle Radar Topography Mission in 2000. The explosive eruption of 18 May 1980, caused the upper 400 m of the mountain to collapse, slide and spread northward, covering much of the adjacent terrane (lower left). The distinctive, shortened form of the cone with its summit crater can be compared with the more typical triangular peaks of Mount Adam and Mount Hood (background left and right, respectively). The devastation caused by the avalanche and blast is still apparent 20 years later. The high rainfall has led to the substantial erosion of the poorly consolidated landslide material. The colour coding is related to topographic height (green at lower elevations, rising through yellow and tan, to white at the higher elevations). A similar scene of devastation can be presumed to have resulted from the sector collapse and Momella DAD at Meru more than 7,000 years ago. Source NASA

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.

13.8 Ecosystems

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.

13.9 Tourism

The highlight of a visit to the Arusha National Park is a 4-day trek to the summit with overnights at the Miriakamba and Machame Huts (Fig. 13.1 and Plate 13.5a). Views of Kilimanjaro towards the east are an additional reward of this trek (Plate  12.1). The diversion to Little Meru is an important component of the ascent, as it helps offset altitude sickness and also provides fine views to the north, including that of the Oldoinyo Lengai Volcano. The final part of the ascent is undertaken almost entirely on the rocky summit ridge (Plate 13.2a), and this is both far less strenuous, and yet more exposed, than the crowded treks on Kilimanjaro. Sunrise observed from Socialist Peak reveals a triangular shadow derived from Kilimanjaro (Plate 13.5b). The slopes above approximately 4,000 m are devoid of vegetation, whereas the lower and central slopes and parts of the caldera floor are thickly forested. Hiking in these pristine montane forests with the abundant large game, including African Elephant and Cape Buffalo, a large variety of birds (>400 species), and scenic waterfalls, is an additional experience. The relative youthfulness of the forests, i.e. they would have had to regenerate after the blast associated with the sector collapse is intriguing. The ‘Little Serengeti’ on the lower, eastern slopes is an area of savannah grassland that supports a range of grazers. The Ngurdoto Volcano includes thickly forested slopes with a grassy crater floor which appears as an idyllic ‘Garden of Eden’ (Plate 13.5c).
Plate 13.5

a The sheer western wall of the Meru Caldera glows in the early morning light behind the Miriakamba Huts; b Sunrise on the summit of Meru casts a triangular shadow of Kilimanjaro; c The Ngurdoto Crater provides a glimpse into a verdant ‘lost world’, as access is denied to this park-within-a-park


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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of GeologyRhodes UniversityGrahamstownSouth Africa

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