Mount Meru is a 4,562 m stratovolcano 70 km west of Mount Kilimanjaro in Tanzania. It is the second tallest mountain in Tanzania, the fifth highest in Africa.
The volcano suffered a major flank collapse and debris avalanche some 7,800 years ago, creating a 5 km wide horseshoe shaped amphitheater. Though the extent of the debris flow is poorly constrained, it is thought to be at least 28 km3 of material stretching most (perhaps all) of the way to neighboring Kilimanjaro. The volcano may have been taller than Kilimanjaro before the collapse.
The local Waarusha people believe the volcano is sacred. Nearly 830,000 people live within 30 km of the volcano, 2.4 million within 100 km. The volcano sits just 14 km from neighboring Arusha, with a population of around 414,000. Perhaps 75% of all tourists entering Tanzania pass through the Arusha region.
Weather in this region is semi-arid, mostly warm, with temperatures averaging ranging from 28 – 8 C over the course of the year. There are two rainy seasons. Areas north of Arusha around Meru see more precipitation and cooler temperatures.
The surrounding Arusha National Park is mostly savanna with a forest on the lower flanks of the volcano. This supports diverse wildlife including nearly 400 species of birds, colobus monkeys, giraffes, zebra, elephants, leopards and water buffalo.
There is an active climbing operation at Meru. While it is viewed by some as a warm-up for Kilimanjaro, Meru is a challenging climb. The walk around the crater rim is described in one hiking blog as among the most spectacular that you can do in Africa. Climbers are advised to spend 2 – 3 nights on the mountain, ascending around 1,000 m/day. Unlike Kilimanjaro, there is a single climb route up Meru. The Tanzania Expeditions Mt. Meru Climbing Guide notes that climbers must climb with an armed ranger due to the wildlife. Best time to climb is June – Feb yearly, with a wet season in Nov. The Journeys By Design, Tanzania Experience and Adventure Alternative web sites all have good writeups on climbing Meru. These aren’t the only ones out there.
The following video comes out of the climbing world:
Volcano and eruptions
Meru is located on a faulted terrain of flood basalts associated with the Neogene Rift Valley volcanic province. These flood lavas are several hundred meters thick on the underlying basement rocks. First eruptions of these basalts date as far as 2.37 Ma ago.
Mount Meru is the main volcanic center with smaller volcanic centers of Meru West and Little Meru on its flanks. There are small parasitic cones and craters on its lower slopes and surrounding plains. The earliest volcanic activity was the eruption of nephelinite lava flows from the West Meru plateau some 1.5 Ma. The extent of this activity is obscured by follow-on ash and lahar deposits. Only 100 m of the original deposit is exposed. It may be as thick as 700 m.
Alkaline lava flows and breccias from the lower NW slopes of Meru date 380,000 – 280,000 years ago. The most recent date likely represents construction of Little Meru on the NE flank of the main edifice. It is constructed of poorly sorted volcanic breccias overlain by lava flows near the 3,800 m summit, suggesting it was built via multiple strombolian eruptions with a final phase of lava extrusion.
The main cone was built 160,000 – 60,000 years ago, starting after a pause of perhaps 100,000 years after activity stopped at Little Meru. The main cone has a largely explosive history with intermittent effusive lava flows. Main cone deposits are divided into three groups – Lower, Mid-cone and Summit. The Lower group is a series of near horizontal lava flows that could have come from a lava lake. The Mid-cone group is mainly materials from explosive eruptions and date 111,000 – 102,000 years ago. There are numerous cross cutting dikes 3 – 20 m thick.
Summit group activity began 80,000 – 67,000 years ago years ago and continued for another 20,000 years. The deposits are breccias and tuffs, suggesting the final construction of the cone was explosive varying between strombolian and sub-Plinian eruptions.
There are numerous parasitic cones and lava domes on the flanks.
There are four major lahar deposits on the N, S and E flanks of Meru. Two of these on the N flanks were confined mainly to river valleys and spread out on the plains. Debris at the end of these is up to 20 m thick. Estimated volume for these lahars range between 2 – 3 km3 apiece. The volcano was glaciated until the 1960s. Glaciers on top of the volcano appear to have contributed to lahars in the surrounding region. Snow fell on Meru in Feb 2008 for the first time in decades. The likelihood of future lahars may have been mitigated somewhat by the flank collapse which lowered the maximum height of the volcanic cone below the permanent ice / snow level.
Pumice and ash deposits blanket much of the western slopes and some of the northern and southern flanks of the volcano. The main deposits represent fallout from two explosive events that generated over 2.1 km3 from a maximum column height of 23 km.
The east side of the volcano collapsed 7.800 years ago. Creating a collapse scar measuring 5 x 8 km. The resulting debris avalanche and lahars extended to Kilimanjaro to the west. Initial analysis of this collapse suggested it may have been a hot collapse, accompanied by a Plinian eruption that covered much of the volcano. Follow-on analysis of pumice and ash deposits in and around the crater suggest that the explosive events preceded the collapse (no young pumice inside the crater).
Activity following the flank collapse continued building a lava dome and Ash Cone in the caldera. There are post-collapse lava flows covering much of the caldera floor issuing from a fissure between Ash Cone and the crater wall. The most recent eruptions from Meru were relatively minor eruptions in 1886, 1878 and 1910. Four explosions from Ash Cone in Oct and Dec 1910 covered much of the crater floor with ash. Four episodes of fumarolic activity were recorded 1911 – 1953. Although no activity has been recorded since 1953, there were reports starting in 1999 from guides working in the National Park who smelled sulfur while walking along the north crater wall. In March 1999, a 20 cm wide, 100 m long fissure opened on the southern slopes of Meru and emitted steam for about a week.
NASA’s Earth Observatory posted photos of a plume out of Meru in 2015 that was initially mistaken as an eruptive plume. It turns out that it was a massive wildfire on the northern half of the volcano.
The debris avalanche from Meru is one of the largest sub-aerial debris avalanches ever recorded. Total coverage by this event is over 1,500 km2.
The debris avalanche had dry and wet units that originated from the large scarp on the eastern flank. There is a smaller buried scar on the NE flank. The avalanche originated as a dry avalanche and turned into a debris flow while entering a neighboring river. This increased the mobility to the point where it was able to climb or splash over topographic highs in its path. The flow was deviated to the south by neighboring Kilimanjaro and Ngurdoto volcanoes.
The collapse created a 5 x 8 km amphitheater on the eastern flank of the volcano. A 2013 poster by Delcamp et al disputes previous papers that this was a hot collapse, claiming “No evidence of syn-collapse eruption has been observed.” As previously stated, deposits from explosive activity appears to have immediately preceded the collapse.
The Landslide Blog covered this in 2016. Their post was mostly based on the 2016 Delcamp, et al paper Large volcanic landslide and debris avalanche deposit at Meru, Tanzania. There are three flank collapse debris avalanche deposits below the eastern flanks of Meru. At least two of them are from the large eastern amphitheater. The largest of these, Momella landslide had a fall height of 3.4 km and a runout distance of 49 km. It speculates that water played a key role in the exceptional mobility of the landslide.
At least one organization, EcoScience, describes Meru as a dangerous volcano, partly due to its three relatively recent small eruptions, and partly to the massive size of the 7,800 year ago eruption-driven collapse. Worse, if these guys are correct, the volcano is not monitored with seismic stations, meaning the only warning would be by satellite or human observation. EcoScience warns that the million people surrounding the volcano may be at risk, not unlike the people living in the vicinity of El Chichon were when it erupted in 1982 after an extended period of quiet.
The East African Rift is the predominant tectonic activity in the region Meru, Kilimanjaro and Ol Doinyo Lengai grew. It is an active continental rift zone that started 25 – 22 Ma. The rifting motion is rending the Africa Plate in two, creating what is called the Somali Plate to the east and the Nubian Plate to the west. Rifting was either caused or enhanced by the impingement of a mantle plume under the Afar – Ethiopian region. Divergence is on the order of 6 – 7 mm/year.
Rifting led to thinning continental lithosphere and the creation of a series of distinct rift basins starting in the Afar and stretching all the way south to Mozambique. The rift splits in the vicinity of the boundary between Kenya and Tanzania, with the western branch located roughly along the western Tanzanian border called the Western Rift Valley and the other called the Eastern Rift Valley. These branches go on either side of the uplifted Kenya Dome.
As we previously saw with the Walker Lane in North America, branching of continental rifts gets messy. The 200 km wide North Tanzanian Divergence Zone is where the East African Rift splits into three branches, the Eyasi, the Manyara and Pangani Rifts. This zone is much wider than the usual 50 – 60 km wide rift.
Continental thinning associated with these rifts allowed magma an easier pathway to the surface. This is where the largest volcanoes of the divergence zone (Kilimanjaro, Meru, Burko, Essimingor, Kerimasi, Ol Doinyo Lengai and Ngorogoro) are located. Volcanic activity started in the zone started some 8 Ma. There were two significant shifts in magmatic activity starting 2.5 Ma and shifted eastward during the recent period of activity starting 1.5 Ma.
There are ten active volcanoes listed in the Smithsonian GVP database for Tanzania. They form two clusters in the northern and southern parts of the country marking the southern portion of the East African Rift Valley. The northern volcanoes are Mt Kilimanjaro, Meru and Ol Doinyo Lengai. The southern volcanoes are clustered around the Rungwe Volcanic Province. Of these, only Ol Doinyo Lengai is currently active, most recently erupting in 2007. These volcanoes are not actively monitored.
All three of the northern group of volcanoes have suffered flank collapses. Due to their large size, these volcanoes present a substantial hazard to local neighbors.
Mount Meru is a recently active huge stratovolcano located in an active continental rift zone. It suffered one of the largest flank collapses known on the planet nearly 8,000 years ago and three smallish explosive eruptions 110 – 150 years ago. There is also evidence of relatively recent effusive eruptive activity in the crater. The volcano should be considered dangerous for several reasons. First, it is in an active continental rift. Second, it is located in close proximity to millions of local residents. Third, its most recent eruptive activity is mostly explosive in nature. Finally, it is not monitored other than via satellite or by human traffic on and near the mountain.