Volcanoes in Tanzania

Tanzania has 5 Holocene volcanoes spread across two segments of the East African Rift system, making it one of the most volcanologically significant countries in Africa. The country's volcanic crown jewel is [[volcano:lengai-ol-doinyo|Ol Doinyo Lengai]], the only volcano on Earth known to currently erupt carbonatite lava — a bizarre, sodium-rich magma that erupts at temperatures below 600°C and appears black rather than red, turning white within hours of exposure to moisture. This remarkable stratovolcano, whose name means "Mountain of God" in the Maasai language, rises to 2,962 m (9,718 ft) above the Gregory Rift Valley south of Lake Natron and has been in a state of near-continuous activity since 1983.

Nearby, Mount Meru — the fifth-highest peak in Africa at 4,550 m (14,928 ft) — dominates the skyline near the city of Arusha and last erupted in 1910. In southwestern Tanzania, the Rungwe Volcanic Province hosts three additional Holocene centres: Rungwe, Ngozi, and Kyejo, clustered at the junction of the Rukwa and Malawi rift segments. Tanzania's volcanoes are products of the same continental rifting process that is slowly splitting Africa apart, a tectonic drama that also created the iconic landscapes of [[country:ethiopia|Ethiopia's]] Afar Triangle and [[country:kenya|Kenya's]] Great Rift Valley.

Together, these five volcanoes record eruptions spanning from approximately 8250 BCE to the present day, with VEI values reaching 5 in the geological past. The country's volcanic features are monitored by the Geological Survey of Tanzania and the University of Dar es Salaam, with international support from the [[ext:https://www.usgs.gov/volcanoes|USGS Volcano Hazards Program]].

Tanzania's volcanoes exist because the African continent is being pulled apart. The East African Rift System (EARS) — a 3,000-km-long network of rift valleys stretching from the Afar Triple Junction in [[country:eritrea|Eritrea]] to Mozambique — represents the early stages of continental break-up, where the Somali Plate is separating from the Nubian Plate at a rate of approximately 6–7 mm per year. This extension thins the continental crust, reduces pressure on the underlying mantle, and allows hot, partially molten rock to rise toward the surface.

In Tanzania, the rift splits into two branches: the Eastern (Gregory) Rift and the Western (Albertine) Rift. The Gregory Rift, which hosts Ol Doinyo Lengai and Mount Meru, is the more volcanically active branch, benefiting from additional heat supplied by a deep mantle plume beneath the East African Plateau. The southwestern Rungwe Volcanic Province sits at the intersection of the Rukwa and Malawi rift segments, a structurally complex junction where deep faulting provides pathways for magma ascent.

Unlike the subduction zone volcanism of the [[special:ring-of-fire|Ring of Fire]], Tanzania's rift volcanism produces predominantly alkaline magmas — including the extraordinary natrocarbonatite lavas of Ol Doinyo Lengai, phonolites at Mount Meru, and trachytes in the Rungwe province. These unusual compositions reflect the partial melting of metasomatised (chemically altered) continental lithospheric mantle, a process unique to intracontinental rift settings.

**Ol Doinyo Lengai** — [[volcano:lengai-ol-doinyo|Ol Doinyo Lengai]] is arguably the most scientifically unique volcano on Earth. It is the only volcano known to erupt natrocarbonatite lava — a magma composed primarily of sodium carbonate and calcium carbonate rather than silicate minerals. This extraordinary lava erupts at temperatures of just 500–600°C (compared to 1,000–1,200°C for basalt), appears jet black when molten rather than red, and turns white within hours as it absorbs atmospheric moisture.

The 2,962 m (9,718 ft) stratovolcano rises abruptly from the flat floor of the Gregory Rift Valley, south of Lake Natron in northern Tanzania. It has been nearly continuously active since 1983, with carbonatite lava effusion in the summit crater punctuated by periodic explosive events. A major explosive eruption in 2007–2008 (VEI 3) sent ash columns 15 km high and forced the evacuation of Maasai communities.

The current eruption phase began in April 2017 and continues to the present. Ol Doinyo Lengai has 27 recorded Holocene eruptions — by far the most prolific volcano in Tanzania. The Smithsonian GVP profile is available at [[ext:https://volcano.si.edu/volcano.cfm?vn=222120|Smithsonian GVP: Ol Doinyo Lengai]].

**Mount Meru** — Tanzania's second-highest mountain and the fifth-highest peak in Africa, [[volcano:meru|Mount Meru]] is a massive stratovolcano rising to 4,550 m (14,928 ft) near the city of Arusha. The volcano has a dramatic asymmetric profile: from the west it appears as a perfect cone, but from the east a massive 4-km-wide horseshoe-shaped collapse crater is visible, formed by a catastrophic debris avalanche during the early Holocene (the Momella event). Lahars from this collapse reached as far as the western flanks of Kilimanjaro, 70 km to the east.

The most recent eruption in 1910 was a minor event from the Ash Cone, a subsidiary vent inside the collapse crater. Despite its proximity to the rapidly growing city of Arusha (population ~600,000), Mount Meru receives far less monitoring attention than it deserves. The peak is popular with trekkers as an acclimatisation climb before Kilimanjaro.

**Ngozi** — The [[volcano:ngozi|Ngozi]] caldera, at 2,614 m (8,576 ft), is the most prominent volcanic feature of the Poroto Ridge in southwestern Tanzania. A 3-km-wide caldera with 150–300 m high steep walls contains a beautiful 1.5 × 2.5 km crater lake. The caldera formed approximately 10,200 years ago during eruption of the Kitulo Pumice, a VEI 5 event that deposited pyroclastic flows over a wide area.

The youngest known eruption, around 1450 CE, produced a pyroclastic flow that swept at least 10 km to the south. Ngozi is considered potentially hazardous given the large populations in southwestern Tanzania, but it receives minimal monitoring.

**Rungwe** — [[volcano:rungwe|Rungwe]] is the largest volcano in the Rungwe Volcanic Province, a 2,953 m (9,688 ft) trachytic stratovolcano with a 4-km-wide caldera breached to the southwest. A catastrophic debris avalanche from the summit produced hummocky terrain extending at least 20 km southwest. The caldera contains youthful-looking pumice cones, lava domes, and explosion craters.

The most recent dated eruption occurred around 1250 CE. The volcano's most powerful known event, a VEI 5 eruption approximately 2050 BCE, deposited thick pyroclastic sequences across the region.

**Kyejo** — The southernmost volcano in Tanzania, [[volcano:kyejo|Kyejo]] is a trachytic stratovolcano rising to 2,176 m (7,139 ft) northwest of Lake Malawi. Its most recent eruption around 1800 CE was the last confirmed volcanic event in the Rungwe Province: two tephritic cones (Sarabwe and Fiteko) constructed along a northwest-trending fissure produced lava flows that travelled 8 km and overran several villages. Numerous maars — some containing lakes — surround the volcano, and young basaltic cinder cones extend to the south.

Tanzania's volcanic eruption record encompasses at least 37 confirmed events across its five Holocene centres, spanning from approximately 8250 BCE to the ongoing eruption at Ol Doinyo Lengai. The oldest recorded eruptions are the VEI 5 events at Ngozi (~8250 BCE) and Rungwe (~2050 BCE), both powerful explosive eruptions that deposited pyroclastic flows across wide areas of southwestern Tanzania. These represent the largest known eruptions in the country's geological record.

In the Gregory Rift, Ol Doinyo Lengai dominates the eruption record with 27 confirmed events since approximately 1550 BCE. The volcano's historical activity has alternated between long periods of quiet carbonatite lava effusion within the summit crater and periodic violent explosive eruptions that send ash columns kilometres into the atmosphere. The most significant modern eruptions include the explosive events of 1917 (VEI 3), 1940–1941 (VEI 3), 1960–1966 (VEI 3), 1967 (VEI 3), and 2007–2010 (VEI 3).

Mount Meru last erupted in 1910, a relatively minor event from the Ash Cone inside the collapse crater, though the volcano's history includes a VEI 4 eruption approximately 5850 BCE. The Rungwe Province's most recent eruption was at Kyejo around 1800 CE, when lava flows from fissure vents overran villages. Tanzania's eruption frequency is modest compared to [[country:indonesia|Indonesia]] or [[country:japan|Japan]], but the country's volcanoes have demonstrated the capacity for large explosive events (VEI 5), and the long repose periods at several centres mean future eruptions could be difficult to forecast.

Tanzania's volcanic hazards are concentrated around two main areas: the Gregory Rift near Arusha and the Rungwe Volcanic Province in the southwest. Ol Doinyo Lengai poses the most immediate threat, particularly during its explosive phases. The 2007–2008 eruption forced the evacuation of Maasai pastoralist communities living on the volcano's flanks and deposited ash on settlements near Lake Natron.

Lahars triggered by explosive eruptions mixing with seasonal rains have historically affected communities downstream. Mount Meru presents a more significant long-term hazard due to its proximity to Arusha, a city of approximately 600,000 people at the volcano's southern base. The Momella debris avalanche demonstrated that catastrophic flank collapse is possible, and lahars channelled down valleys toward the city represent the most credible near-term threat.

Ashfall from a moderate eruption could also disrupt Kilimanjaro International Airport, located between Meru and Kilimanjaro. In the Rungwe Province, Ngozi caldera is the primary concern. The caldera lake could be expelled during an eruption, generating lahars, and the VEI 5 potential demonstrated 10,200 years ago indicates that large explosive eruptions are within the volcano's capability.

CO₂ degassing from the Rungwe volcanic system has been measured at significant levels, and lethal gas accumulations in low-lying areas have killed livestock. Monitoring across Tanzania's volcanic centres is limited, with Ol Doinyo Lengai receiving the most attention from international research teams.

Tanzania's five Holocene volcanoes are distributed across two distinct volcanic zones separated by approximately 700 km. In the north, the Gregory Rift hosts Ol Doinyo Lengai (2.76°S, 35.91°E) near Lake Natron and Mount Meru (3.24°S, 36.75°E) near Arusha. These two volcanoes lie approximately 150 km apart along the eastern branch of the East African Rift.

Notably, Kilimanjaro — Africa's highest peak at 5,895 m — is not classified as a Holocene volcano by the Smithsonian despite its young appearance, as its most recent eruptions are dated to approximately 200 years ago from fumarolic activity only. In the southwest, the Rungwe Volcanic Province clusters three centres along the Rukwa-Malawi rift junction: Ngozi (9.00°S, 33.55°E), Rungwe (9.14°S, 33.67°E), and Kyejo (9.23°S, 33.79°E), all within a 30-km radius near the northern end of Lake Malawi. This tight clustering reflects the structural complexity of the rift junction, where intersecting fault systems provide multiple magma pathways.

Ol Doinyo Lengai holds deep spiritual significance for the Maasai people, who regard it as the home of their god Engai. The volcano's name translates as "Mountain of God," and the Maasai traditionally make pilgrimages to the summit to pray for fertility and rain. The mountain's periodic explosive eruptions, which shower the surrounding plains with white ash, are interpreted within Maasai cosmology as divine activity.

Beyond cultural significance, Tanzania's volcanic landscapes are major economic assets. Mount Meru is a popular trekking destination within Arusha National Park, drawing thousands of climbers annually as an acclimatisation route before Kilimanjaro. The fertile volcanic soils around Mount Meru support some of Tanzania's most productive coffee and banana cultivation.

The Rungwe Volcanic Province's rich volcanic soils support intensive agriculture in the Mbeya region, including tea plantations on the slopes of Rungwe itself. Geothermal exploration in both the Gregory Rift and the Rungwe Province is ongoing, with the Ngozi geothermal prospect identified as having significant energy potential — a resource that could be transformative for a country where less than 40% of the population has access to electricity.

Tanzania's volcanoes offer extraordinary experiences for adventurous travellers. Mount Meru is climbed via the Momella Route within Arusha National Park, a 3–4 day trek to the summit at 4,550 m that passes through montane forest, alpine meadows, and the dramatic rim of the collapse crater. The trek is often used as acclimatisation for Kilimanjaro and requires park fees and an armed ranger escort (due to wildlife).

Ol Doinyo Lengai is a far more challenging proposition — the standard ascent is a gruelling overnight climb (typically beginning at midnight) up steep, loose scree slopes, reaching the summit crater at dawn. During periods of carbonatite lava activity, climbers may witness the extraordinary sight of black lava flows turning white. Access is from the Maasai village of Engare Sero, and conditions can be dangerous during explosive phases.

Lake Natron, at the base of Lengai, is a stunning destination in its own right, famous for vast flocks of flamingos breeding on its caustic soda-encrusted surface. The Rungwe volcanic region near Mbeya is less touristically developed but offers hiking opportunities through montane forest to Ngozi's crater lake — one of the most scenic volcanic lakes in East Africa. The best time for volcanic trekking in Tanzania is during the dry seasons: June–October and January–February.