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, 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 gathering 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 roam, silently killing people in the night while they sleep. In many places where mazuku occur, CO₂ levels fall during daytime but can rise to significantly dangerous concentration levels of about 90% at night, early mornings or evening hours, posing a 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.

Geological setting and occurrence

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 and 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, such as 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:

* '' Virunga Volcanic Province (VVP)'' at the foothills of the volcanic mountains of Nyamulagira and Nyiragongo in the Virunga Mountains on the northern shoreline of Lake Kivu, particularly in the busy city centers of Goma and Sake on the Democratic Republic of Congo (DRC) and Gisenyi in Rwanda.
* '' Rungwe Volcanic Province (RVP)'' in southwestern Tanzania, at the intersection of three rift segments (Tanganyika-Malawi-Usangu rifts, forming a triple junction), where CO₂ is mined commercially by TOL Company Limited for supply to the beverage industry.

Formation

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 signatures from He and CO₂ gas 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 such as 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 convert 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 4,700m², as seen in the mazuku of Bulengo Seminaire on the shores of Lake Kivu, DRC. 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.

Geochemical composition and origin

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 the mantle (magmatic sources) and the crust, 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

Surface manifestations

Areas with mazuku can be readily identified in the field through several distinctive characteristics/features as follows:

* The peculiar types and species of vegetation that thrive in CO₂-rich waters and gases, such as cyperus papyrus, fern, reeds and grasses serve as indicators of the mazuku environments
* Burnt-out vegetation and altered rocks due to high acidity levels associated with elevated CO₂ concentrations normally (70-90v.%) results in patches of weathered bareland, which is a typical feature for identifying mazuku areas.
* Regions with ultrahigh CO₂ concentrations, the high CO₂/O₂ ratio can be perceived as a sensation of heat on human skin, a condition related to hypercapnia. This includes tingling and burning sensations in the mouth lips, eyes and nose because of the acidic nature of CO₂ which reacts with moisture to form a weak carbonic acid that causes irritation and a burning sensation in these soft body parts
* Systematic occurrences of dead animals, such as insects, rodents and reptiles alongside larger animals like cattle, dogs, and goats indicate areas of high CO₂ concentration.
* Bulging and swelling of the ground due to pressure caused by CO₂ accumulation.
These characteristics collectively aid in the identification of mazuku regions in the field.

Factors affecting CO₂ levels in mazuku

CO₂ levels in mazuku areas are affected and influenced by a combination of various factors:

* Increased volcanic and seismic activities: Increasing CO₂ concentration levels in mazuku areas may be influenced by an increasing amount of volcanic and seismic activities (e.g. earthquakes), which can result in the creation of more permeable fractures in the earth's crust, allowing more CO₂ to escape from underground, leading to the formation of new degassing zones with higher CO₂ levels.
* Anthropogenic activities
** Unauthorized well drilling: For instance, at Colli Albali volcano in Italy, a well was dug through a pressurized gas pocket, causing it to explode. This created a low-pressure zone leading to more CO₂ gas dispersion in the area and resulted in 3 more gas blowouts along a continuous fault line.
** Tarmac roads construction: Tarmac roads and other concrete surfaces can seal natural gas conduits, blocking the natural flow of gas and leading to its accumulation. As pressure builds up due to the trapped gases, it can cause bulging and swelling and subsequently explosive release (gas blowout) when the gas eventually escapes. This can cause a road collapse or other infrastructure damage
** Drilling of pit latrines: A man died from asphyxiation near the foothills of Ngozi volcanic crater, in the Rungwe Volcanic Province in Tanzania while digging a 6m deep pit latrine. The cause was likely due to the accumulation of hazardous gases in the hole after the gas pockets was mechanically disturbed. However, CO₂ continuously degasses in the area to date, leading to more deaths of birds, cows, and rodents due to the toxic gas buildup.
* Weather conditions and atmospheric influences (meteorological parameters)
** Pressure: Pressure variations in the atmosphere have an inverse relationship with CO₂ emissions from the soil, i.e. when the atmospheric pressure drops the CO₂ emissions are high, but when pressure rises the CO₂ emissions are lower
** Wind speed: Low wind speed decreases the chance of CO₂ dispersion to the atmosphere and, allowing the heavy gases to accumulate in low-lying areas like valleys and depressions.
** Soil moisture content and season of the year: During heavy winter rains, subsurface soil voids are totally filled with water, causing a significant amount of CO₂ to dissolve in them. In contrast, during summer, when the soil is dry, these voids remain empty and can accumulate large amounts of degassed CO₂ that can escape and fill in low-lying areas, posing great health risks.
** Time of day: At night, with no solar radiation and reduced solar intensity, atmospheric temperatures drop, and wind speeds decrease significantly. These conditions slow the dispersal of heavy gases, causing them to accumulate in low-lying areas. During the day, sunlight heats the air, creating low pressure that allows CO₂ emissions to rise and disperse, reducing the risk of dangerous concentration levels.

CO₂ exposure health effects and international guideline limits

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.

Mazuku hazard case studies else where around the world

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

Lake Monoun, a volcanic crater lake, is situated in the Oku Volcanic Field, which is part of the Cameroon Volcanic Line. It 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.

Lake Nyos

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 a 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

Mammoth Mountain, a dormant volcano in the Sierra Nevada region of California, United States, is underlaid by a shallow dacitic dome that releases cold and dry CO₂-rich gases (98v% CO₂) through fumarole 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.

Mount Amiata

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.

Mount Sinila

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 the 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.

Effects

Mazuku 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. Local vegetation is typically not very strongly affected. If the concentration of CO₂ is high enough and maintained in a prolonged outgassing event, however, 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 localized flora and fauna extinction events that are 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 approximately every 800-1000 years that were caused by methane explosions and tsunamis due to accumulation of magmatic CO₂.

If mazuku occurs underneath lakes, it can lead to changes in water chemistry, creating meromictic lakes that are 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.

Summary

Hazard assessment and mitigation

Hazard assessment

The areas experiencing mazuku emissions are faced with multiple forms of hazards due to their proximity to active volcanoes.

Continuous hazards

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

Latent hazards are dormant threats that require an external trigger, such as a mechanical disturbance, to become dangerous and deadly under specific conditions. 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, warning of further natural disasters such as earthquakes, volcanic eruptions and massive landslides.

Mitigation measures

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 immediate evacuation plans and warning signs in hazardous places.

Developing gas hazard and risk maps: Key data on CO₂, such as soil gas concentrations, carbon isotopes (which help trace CO₂ sources), and CO₂ flux levels, should be collected in volcanic areas prone to mazuku. 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.

Mazuku's influence on climate

Volcanic mountains such as Mount Etna in Italy, Kilauea in Hawaii, Nyiragongo and Nyamulagira in Congo, and their adjoining areas are significant sources of magma-derived gases, releasing massive amounts of CO₂ both during eruptions and through continuous magma upwelling (passive degassing) in non-eruptive states through fumaroles, hot springs and gas plumes. The cycling of CO₂ and other gases, such as SO₂, H₂S, water vapor and HCl is driven by magma convection, where degassed magma sinks, recharges with CO₂ at depth, and rises again, ensuring a constant supply of volatile-rich magma, a process likely fueled by a mantle source beneath.

This process contributes to global warming primarily through their large continuous emissions of the greenhouse gas CO₂. During both quiescence and high eruptive activity periods the volcanoes releases significant amounts of CO₂ into the atmosphere. The continuous release and accumulation of CO₂ (accounting for up to 10% of the global total budget) leads to an increase in the concentration of greenhouse gases in the atmosphere. This gas acts like a blanket, trapping heat that would otherwise be re-radiated into space, causing the heat to accumulate and, as a result, warming the Earth's surface. Although volcanic CO₂ emissions are relatively small compared to human-caused emissions from burning fossil fuels, the persistent degassing of volcanoes like Mt. Etna still plays a role in the overall carbon cycle, indirectly contributing to climate change by increasing the amount of CO₂ in the atmosphere.

See also

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References

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Geological hazards
Swahili words and phrases
Volcanic degassing
Carbon dioxide