OR/14/005 Volcanic hazards

Vye-Brown, C, Crummy, J, Smith, K, Mruma,A and Kabelwa H. 2014. Volcanic hazards in Tanzania. Nottingham, UK, British geological Survey. (OR/14/005).

The following provides a description of the products of volcanism and the variety of volcanic hazards and their impacts in Tanzania that pose a risk to proximal and distal populations in terms of infrastructure, health, environment, livelihoods and well-being in Tanzania and specifically relating to Mt Meru. Although the volcanoes of Tanzania can generally be characterised in terms of their predominant magma chemistry, size and dominant eruptive style, each has some distinctive characteristics. The following accounts for the principal volcanic hazards in Tanzania and examples of where these are of concern.

Debris avalanches and landslides

Debris avalanches and landslides are common and are not necessarily related to active volcanism. Destabilisation may result from inherent instability or changing state such as regional tectonic earthquakes, extreme weather or triggered by volcanic activity resulting in failure of a volcanic edifice or a flank.

The large size of many central volcanoes in Tanzania means that volumetrically significant debris avalanches and landslides may pose a risk to large populations. Sector collapses resulting in debris avalanches are reported from Mt Meru as well as vents in the Rungwe Volcanic Province. The collapses of Mt Meru are believed to be amongst the largest debris avalanches in the world with deposits reaching the foothills of Kilimanjaro.

Pyroclastic density currents

Pyroclastic density currents (PDCs) are turbulent clouds of hot ash, gases and particles that flow along the ground from the collapse of an eruption column, lava dome or lateral blast of a volcano edifice. PDCs typically flow down valleys away from the vent whilst surges can spread widely irrespective of undulations in the landscape. PDCs flow at speeds of up to 100 metres per second and travel up to 30 km from the vent. PDCs are lethal and responsible for thousands of deaths in individual eruptions, e.g. 29 000 deaths associated with PDCs from the 1902 eruption of Mt Pelée in Martinique.

PDCs are associated with the central volcanoes in northern Tanzania and thick deposits have been found on the southern flanks of Mt Meru. PDCs have been suggested to be absent from the Rungwe Province in the south (Fontjin et al., 2012).

Lahars and floods

Lahars are flows consisting of volcanic debris and water which may be hot or cold depending on their genesis. Lahars can occur as a result of: interaction of an eruption with ice or snow generating large amounts of meltwater moving down river valleys carrying large amounts of debris; heavy rains on loose volcanic deposits cause a mass movement; or failure of the walls of a crater lake resulting in a catastrophic draining of the lake. Lahars have been a major cause of fatalities in historic times e.g. 23 000 deaths from the 1985 Nevado del Ruiz lahar in Colombia. Fatalities and injuries from lahars can be avoided if communities are evacuated in a timely manner to high ground.

Deposits interpreted to have resulted from lahars are often also cited as debris avalanche deposits, although there are clear differences between the characteristics of the emplacement these flows. This includes the interpretation of flows deposited on the north-western and northern flanks of Mt Meru (Wilkinson et al. 1983; Delcamp et al. 2013).

Tephra hazards

Tephra describes ash, rock and magma ejected from an explosive eruption into the atmosphere that can be transported hundreds of kilometres from the vent. Tephra is erupted from the vent to form an eruption column that may rise up to 55 km into the stratosphere. Eruption column heights can be used to measure eruption intensity as it relates to the mass eruption rate. Dispersion of tephra from an eruption column occurs as a combined result of the eruption rate and the meteorological conditions at the time of the eruption. Tephra falls and is deposited from this plume with coarser particles, including pumice, deposited near the vent and finer ash deposited further afield. By capturing data on the dispersal and fragmentation of tephra deposits from past eruptions we can constrain the explosive character of the eruption and height of the eruption column to provide essential data into hazard models. Tephra deposits are hazardous in different ways; it can accumulate on roofs causing collapse, inhalation of ash by animals and humans is a health hazard, and volcanic ash is a major hazard to the aviation industry. Ballistics (also referred to as blocks or bombs) are rocks ejected during volcanic explosions. They are typically several centimetres to a couple of metres in size. In most cases the range of ballistics is a few hundred metres to perhaps a couple kilometres, but they can be thrown to distances of up to 5 kilometres in the most powerful explosions. Fatalities, injuries and structural damage result from direct impacts.

Thick tephra fallout deposits have been found in the Rungwe volcanic province, in southern Tanzania. Fontijn et al. (2010) constructed the stratigraphy of Holocene eruptions, and discovered a major Plinian eruption c. 10 ka, and at least five sub-Plinian to Plinian explosive eruptions occurred in the late Holocene. Ol Doinyo Lengai experienced major ash eruptions in 1917, 1926, 1940, 1966–1967 and 2007–2008 (Dawson, 2008). Local communities reported ashfall in 2007, and three villages were evacuated (Prof Mruma, pers. Comm.).

The western slopes of Mt Meru are blanketed in tephra fallout, and reworked deposits. River gullies expose thick (>2.7 m) pumice-rich fallout deposits, up to 10 km west of the summit of Mt Meru. The age of these eruptions are unknown.

Lava flows

Lava flows usually advance sufficiently slowly to allow people and animals to self-evacuate but anything in the pathway of a lava flow can be damaged or destroyed. including buildings, vegetation and infrastructure.

Lavas rank as a volcanic hazard of lower concern than those already mentioned as the impact is lower. However, the emission of lava flows can accompany significant gas emissions and lava flow emplacement has been prevalent in the East African Rift in the past. Known Holocene activity suggests that volumetrically significant lava flows may be associated with volcanic systems in southern Tanzania, especially the Rungwe Province and are usually small with a relatively limited footprint in northern Tanzania.

Gas emissions

Gases and aerosols are dissolved in magma at depth in the subsurface but escape during reduction in pressure as the magma reaches the surface. Whilst the main component of gaseous release during an eruption is water vapour (60–99%), there are many other volcanic gas species and aerosols released. These may include: carbon dioxide (up to 10%), sulphur dioxide and sulphates (up to 15%), halogens (including fluorine and chlorine, up to 5%) and various metals such as mercury and lead (trace amounts) (Symonds et al, 1994).

The impact of volcanic gases varies widely and depends on the amount of gas emitted, the level it is injected into the troposphere or stratosphere and the meteorological conditions at the time. Emissions are known to have adverse effects on agriculture and health (human and animal) leading to fatalities in high concentrations, cause volcanic fogs and air pollution, result in acid rain, and lead to environmental changes including variations in temperature. Gas emissions are a known hazard in the East African Rift due in part to the 1986 Lake Nyos disaster in Cameroon in which 1,700 people suffocated in a CO₂ cloud. In 2011 the eruption of Nabro on the Eritrea- Ethiopia border produced the most SO₂-rich eruption observed in satellite history.

The peralkaline composition of many central volcanoes in the East African Rift mean that they have the potential to produce eruptions that are rich in sulphur gas species and aerosols, fluorine/fluoride and chlorine/chloride. Fluoride in particular has implications for secondary hazards as it bonds to ash particles in the eruption column and is associated with the deposition of ash. Therefore, high levels of fluoride and fluorosis poisoning are found in areas of thick accumulations of ash over both historic in geologic timescales. Due to the composition of Ol Doinyo Lengai there is clear potential for CO₂ to be a major gas hazard. However, there is no gas monitoring to detect the baseline gas emissions from Holocene volcanoes in Tanzania so any increases, or the potential of any one volcano to emit high levels of gas, are currently unconstrained.