Mazuku

Mazuku (Swahili for "evil winds") are pockets of dry, cold carbon dioxide-rich gases released from vents or fissures in volcanically and tectonically active areas, and mixed with dispersed atmospheric air and accumulating in typically low-lying areas. Since CO2 is ~1.5 times heavier than air, it tends to flow downhill, hugging the ground like a low fog and gather in enclosed spaces with poor ventilation, such as lava tubes, ditches, depressions, caves, house basements or in the stratified water layers of meromictic lakes if a water column exists. In high concentrations (≥1vol.%), they can pose a deadly risk to both humans and animals in the surrounding area because they are undetectable by olfactory or visual senses in most conditions.
Mazuku primarily occur on northern shores of Lake Kivu on both sides of the twin towns of Goma in the Democratic Republic of the Congo (DRC) and Gisenyi in Rwanda where local communities in these areas use this term in their vernacular (Kinyabwisha language) to describe the dangerous gases. They believe mazuku occur in cursed locations where invisible forces that travel unnoticed often silently kills people during the night when they are sleeping. In many mazuku places, CO2 levels fall during daytime but can rise to significantly dangerous concentration levels of about 90% at night, early mornings or evening hours posing great threat. This is because during nighttime, the atmospheric temperature drops, and wind speeds are significantly reduced. These conditions hinder the rapid dispersal of these heavy gases into the atmosphere, allowing them to accumulate in lower-lying areas, such as valleys and depressions.
The East African Rift System (EARS) is formed by the divergence of three ancient cratonic plates: the Somalian plate, the Nubian plate, and the Arabian plate, which are splitting apart due to the influence of a mantle plume beneath them. The rift extends ~4,000 km, starting from the Afar triple junction in the northern Ethiopian Plateau and running southwards. It is divided into two main segments: the volcanically active Eastern branch, ~45Ma which passes through Djibouti, Eritrea, Kenya, and northeastern Tanzania, and the younger, seismically active Western branch, (~5 and 8Ma), that cuts through the Democratic Republic of the Congo (DRC), Uganda, Rwanda, Burundi, southwestern Tanzania, Zambia, Malawi, Zimbabwe and terminates at the Okavango Delta in Botswana. The rifting process is responsible for the tectonic and volcanic activity in East Africa, leading to the formation of deep rift lake basins e.g. Lake Tanganyika, Lake Malawi, Lake Rukwa, Lake Albert and Lake Kivu as well as frequent natural disasters such as earthquakes, volcanic eruptions, and massive landslides, along with prolonged dry CO2-rich gas emissions like mazuku (toxic gas) releases.
It has been observed that most mazukus are found along the Western branch of the EARS, particularly in areas of active volcanic and tectonic activity. These areas include:
Geologically, mazuku are natural CO2 emissions linked to magmatically and tectonically active regions, such as young and active or dormant volcanic systems, active hydrothermal systems and deep fault structures systems. Isotopic signature from He and C gases analyses has confirmed that the origin of mazuku is mainly magmatic, as opposed to being derived from thermal decomposition of organic matter. These gases are temporarily trapped and stored in subsurface pockets, such as lava tubes formed during previous eruptions and remain isolated from the rest of the surrounding hydrothermal system. Over time, they are released following porous pathways and channeled to the surface through a network of extensional fissures, faults, or fractures. Once at the surface, they accumulate in cavities or in low-lying areas (depressions) due to their densities and the influence of gravity. In meromictic lakes e.g. Lake Kivu, Lake Nyos and Lake Monoun the CO2-rich gases remain trapped in the dense, cold, and anoxic stratified lower layers (monimolimnion), which do not mix with the O2-rich surface layers (mixolimnion) due to density discrepancies.
In the anoxic zones, methanogenic bacteria converts CO2 into CH4 through a process called methanogenesis, whereby over time, both CO2 and CH4 accumulate under extremely high pressure creating a potential future limnic eruption disaster. However, CH4 is currently extracted economically in Lake Kivu through degassing which reduces the risk of a dangerous limnic eruption while providing a valuable energy source for power generation. Mazuku can extend up to 100m in length and cover an area of up to 4,700m e.g. the mazuku of Bulengo Seminaire on the shores of Lake Kivu, DRC and it has been observed that there is a strong correlation between the occurrence and location of mazuku with the regional alignment of tectonic faults and fracture network.
The bulk geochemical composition of the CO2-rich dry gases in mazuku consists of a mixture of variable proportions of other atmospheric components, such as N2, O2, and Ar, with smaller amounts of CH4, H2S and water vapour. These gases contain between 12% and 99% CO2, Ar concentrations range from 0.01% to 0.85%, and CH4 concentrations range from 0.0002% to 0.002%. Helium is also present in low concentrations, ranging between 0.0003% and 0.004%.
The isotopic signature of He-Ar and CO2 systematics identify mazuku's sources as being derived from both mantle (magmatic sources) and/or crustal origins with significant potential secondary modification processes such as magma mixing and solubility-driven degassing fractionation). The dry gases are continuously released very slowly through a passive degassing mechanism from the earth's interior via vents, fractures, cracks, and hot springs, fumaroles, gas plumes without the need/presence for an active volcanic eruption
Areas with mazuku can be readily identified in the field through several distinctive characteristics/features as follows:
CO2 levels in mazuku areas are affected and influenced by a combination of various factors:
The health hazards linked to both short-term and long-term exposure of lethal doses of CO2 in mazuku are outlined in the table below, along with permissible exposure limits (PELs) for CO2 to promote safety in workplaces and for residents near active volcanic areas. These limits specify safe exposure durations at various concentrations to help prevent health risks over time.
Here is a list of "mazuku" case studies from various parts of the world, where volcanic or geologically active regions release CO2-rich gases. These gases accumulate in low-lying areas, valleys, or confined spaces or in the stratified water layers of meromictic lakes, creating hazardous conditions and deadly asphyxiation zones for humans, wildlife, and plants across different continents.
Lake Monoun, volcanic crater lake is situated in the Oku Volcanic Field which is part of the Cameroon Volcanic Line and was formed when a lava flow created a natural barrier. In 1984, the lake experienced a deadly gas exsolution, triggering a violent limnic eruption that claimed the lives of 37 people. The primary source of the gas was volcanic CO2 emissions, confirmed by C-isotope signatures, which had accumulated in the lake's stratified waters over time, leading to increased pressure. Seismic activity and an underwater landslide were responsible for the disturbance of the lake's stratification, releasing the trapped CO2 violently and causing a very dangerous gas outburst.
A similar scenario occurred two years later in 1986 at Lake Nyos, another crater lake in Cameroon, often referred to as a "killer lake". The lake experienced a catastrophic limnic eruption also known as a lake overturn which resulted in the sudden release of a massive amount of CO2, leading to deaths of 1,700 people and 300 cattle.
Geologically, the crater lake sits over a network of active faults and lineaments and is being fed by volatile-rich basaltic dikes underneath. These dikes release magmatic gases/volatiles like CO2 and H2O which upon their release at low pressure, likely contributed to a phreatomagmatic explosive eruption that formed a diatreme beneath the lake and a maar depression on the surface.
Normally, mazuku involves dry CO2 gas seeping through fissures and accumulating in low-lying areas before dispersing into the atmosphere. However, when gas columns are obstructed by rock strata, such as thick pyroclastic deposits or stratified lake water e.g. meromictic lakes, the gases remain trapped or dissolved in the lake waters respectively. In the later case, CO2-rich gas accumulated in the Lake Nyos crater lake waters to significant levels under extreme pressure
It is believed that landslide event was the triggering factor responsible for exsolution of the dissolved gases which caused a limnic eruption. As a result, a massive CO2 cloud (of about (98v.% CO2) rose from the lake's floor at about 208m, spreading over and down the valleys, engulfing the nearby villages and killing everything along the way due to asphyxiation. The event was classified as a lake overturn which is a very rare phenomenon where dissolved volcanic gases are released from the stratified bottom layers of lakes after a mechanical disturbance
Mammoth Mountain, a dormant volcano in the Sierra Nevada region of California, United States, is underlain by a shallow dacitic dome that releases cold and dry CO2-rich gases (98v% CO2) through fumarolic vents and fractures located on the flanks of the mountain. The gas fluxes were estimated at a rate of ~1,200 tonnes/day, comparable to gas fluxes observed at the summit craters of Kīlauea in Hawaii, Mount Etna in Italy, and Mount St. Helens in Washington. The CO2 originates from deeper magmatic sources (evidence from He-CO2 isotopic signature), at about 10 km below the surface, traveling through permeable networks of fractures and faults. The CO2-rich gases accumulates in the soil layers at depths between 0.6-1m, closed subsurface cavities and snow caves, suggesting an ongoing active magmatic activity beneath the mountain.
One visible consequence/manifestation of this toxic degassing is the large-scale mortality of coniferous trees, covering an area of up to 100 hectares on the mountain's flanks. The accumulation of CO2 in closed depressions and subsurface soil layers exposes tree roots to toxic gases, leading to widespread tree death. In addition to CO2 poisoning, the trees are affected by highly altered and acidic soils. The region also experiences frequent earthquakes, often with up to magnitudes of 6 on the Richter scale. These seismic events, combined with the mountain's bulging and exhumation, fracture the surface and allow high-pressure volatiles to escape, further contributing to the release of CO2 in the tree-kill zones.
Mt. Amiata is a dormant volcano located in Tuscany, Central Italy, and it is known for its significant emissions of dry and cold CO2-rich gases, which are primarily magmatic in origin. The gases originate from the deep geothermal system beneath the volcano and pass through a permeable network of faults and fractures by passive mechanism degassing processes. Although the area has not experienced recent volcanic eruptions, it remains geothermally active, with CO2 emissions contributing to environmental risks like soil acidification and potential CO2 build-up in low-lying areas, posing hazards to local wildlife and humans. The region is also notable for its significance in geothermal energy production, and gas emissions are closely monitored to assess both volcanic hazards and energy sustainability.
Mt. Sinila is a volcanic mountain located on the Diëng Plateau in Indonesia. In 1979 it experienced a tragic phreatic eruption disaster when a mixture of steam, lahar and toxic gases were released from the open cracks and fissures located near the crater and gushing down the valley asphyxiating insects, rodents, big animals like goats, dogs and cows as well as claiming lives of 172 people. Before the eruption, the area experienced a series of earthquakes which reactivated ancient fractures over the span of a few hours. After few hours during the main course of eruption, dry gas was emitted from a 1000m long new fissure which had emerged on the western flank of the volcano near Sumur crater. Gas analysis revealed that the dry gas was CO2-rich from magmatic sources, with concentrations reaching up to 99% by volume. Since CO2 is heavier than air, it flowed down the valley, displacing oxygen and hugging the ground like fog. All victims were found dead in a linear path of gas flow, likely caught them off guard as they slept, with the gas suffocating them simultaneously.
Mazukus can cause a variety of effects on flora and fauna in the regions in which they occur depending on the composition and concentration of the gases that they consist of. Massive clouds of CO2, such as those released from lakes in the 1980s, can cause widespread devastation of human and wildlife populations. However, they may have little or no effect on local vegetation. If the concentration of CO2 is high enough and maintained in a prolonged outgassing event, even vegetation can be affected by the mazuku, as is the case on Mammoth Mountain in California, United States, where deforestation has occurred as well as CO2 poisonings, including the deaths of two skiers, one in 1995 and one in 1998.
In some cases, mazuku are large enough to cause a localized flora and fauna extinction events that is documented in the fossil record. For-example, sediment core radiocarbon dating record from Lake Kivu have shown a sequence of repeated and regular massive lake overtuns events circa.0.8kyr that were caused by methane explosions and tsunamis due to accumulation of magmatic CO2.
If mazuku occurs underneath bodies of water e.g. lakes, it can lead to changes in water chemistry, creating meromictic lakes making it dangerous for aquatic life. For example, the buildup of CO2 in Lake Kivu, Nyos and Monoun caused stratification and oxygen depletion, affecting fish and other organisms living in the water.
The areas experiencing mazuku emissions are facing with multiple forms of hazards due to their proximity to active volcanoes. These include:
These are long lasting volcanic hazards that persist for extended periods of time, even without an active volcanic eruption. For instance, in regions near active volcanoes, such as the Virunga Volcanic Province, people, livestock, and wildlife in low-lying areas are silently killed by mazuku gases. These gases flow downhill and accumulate in depressions, displacing oxygen and causing suffocation. The danger from mazuku remains constant, posing a long-term threat to communities living in these volcanic zones
Long-term exposure to mazuku can lead to environmental degradation and loss of biodiversity.
Agricultural lands may be impacted by CO2 accumulation in subsurface layers of soils, creating toxic acidic soil leading to crop failures and economic disruption.
Latent hazards are dormant threats that require an external trigger to become dangerous and deadly under specific conditions e.g. a mechanical disturbance. For-example; dissolved gases in meromictic lakes like Lake Nyos, Lake Kivu and Lake Monoun contains enormous amounts of dissolved carbon dioxide (CO2) and sometimes methane (CH4) in their deep stratified layers (monimolimnion). This presents a latent hazard because, under normal conditions, these gases remain trapped in the lower layers of the lake. However, if triggered by an external mechanical disturbance as volcanic activity, an earthquake, or landslide, a limnic eruption (also known as a lake overturn) could occur, releasing a cloud of these gases explosively. This could lead to widespread asphyxiation and fires across the surrounding regions, putting millions of people at risk.
Also, mazuku may indicate deeper magmatic unrest, posing further natural disasters as earthquakes, volcanic eruptions and massive landslides.
Due to the silent (colorless and odorless) and deadly nature of CO2 in volcanic active areas, authorities must plan for combating this natural hazard and utilize all available resources to mitigate the hazardous effects associated with it. Some of the mitigation measures are;
On ground CO2 detection sensors: Early warning systems should be installed in high-risk areas. For-example at Mt. Amiata in Italy, researchers employ soil CO2 flux sensors to measure diffuse CO2 emissions with a notable flux measurement of about 13,000 tons/day
Volcano Geoengineering technologies: Human-induced degassing technologies should be employed in meromictic lakes to prevent the sudden natural release of gases. For instance, at Lake Nyos, siphons were installed to lower gas pressure by extracting CO2-rich water from the lake's bottom saline layers (monimolimnion). This process enables the dissolved carbon dioxide to escape into the atmosphere as the water rises to the surface. By reducing the concentration of dissolved gases, this method decreases the risk of catastrophic limnic eruptions, like the one happened in 1986. The siphon system effectively promotes controlled gas exsolution, preventing dangerous pressure build-up. Land-use planning: Town planners should indicate buffer zones which are prone to mazuku and prevent settlements in these areas.
Reallocation and closing high CO2 concentrated areas: For essential community safety, there should be an immediate evacuation plans and putting warning signs in hazardous places.
Developing gas hazard and risk maps is essential in volcanic areas prone to mazuku. Key data on CO2, such as soil gas concentrations, carbon isotopes (which help trace CO2 sources), and CO2 flux levels, should be collected. Mapping these areas through gas concentration and flux measurements can be of a great help during construction and settlement allocation decisions
Education and Sensitization campaigns: There should be continued scientific research on CO2 emissions in volcanic active regions that includes creation and improvement of existing CO2 dispersion models on the causes and occurrence of mazuku.