Mazuku (Swahili language 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.
Mazuku primarily occur on the northern shores of Lake Kivu to either side of the twin towns of Goma (Democratic Republic of the Congo) and Gisenyi (Rwanda), where local communities use this term in their vernacular (Kinyabwisha language) to describe the dangerous gases.
Geological setting and occurrence
The East African Rift System (EARS) is formed by the divergence of three ancient
plates: the
Somali 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 (~45 Ma), which passes through Djibouti, Eritrea, Kenya, and northeastern Tanzania; and the younger, seismically active Western branch (~5 and 8 Ma), 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 CO
2-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 Nyamuragira and Mount Nyiragongo, north of Lake Kivu, and particularly in the busy city centers of Goma and Sake (DRC) and Gisenyi (Rwanda).
-
Rungwe Volcanic Province (RVP) in southwestern Tanzania, at the triple junction of the Tanganyika–Malawi–Usangu rifts,
Formation
Geologically, mazuku are natural CO
2 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 helium and CO
2 gas analyses confirm that the origin of mazuku is mainly
Magma, as opposed to 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
such as
Lake Kivu,
Lake Nyos, and
Lake Monoun, the CO
2-rich gases remain trapped in the dense, cold, and anoxic stratified lower layers (
monimolimnion), which do not mix with the O
2-rich surface layers (
mixolimnion) due to density discrepancies.
In anoxic zones, methanogenic bacteria convert carbon dioxide (CO2) into methane () through a process called methanogenesis, whereby over time, both CO2 and CH4 accumulate under extremely high pressure, creating a potential future limnic eruption disaster.
Geochemical composition and origin
The geochemical composition of mazuku gases consists mainly of CO
2 and a variable mixture of other
Atmosphere components, such as N
2, O
2, and
argon (Ar), with smaller amounts of methane (CH
4),
hydrogen sulfide (H
2S), and water vapour.
In these gases, CO
2 concentrations range between 12% and 99%, argon concentrations range from 0.01% to 0.85%, and CH
4 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 sources to be derived from both the Mantle plume (magmatic sources) and the crust, with significant potential secondary modification processes such as magma mixing and solubility-driven degassing fractionation.
Surface manifestations
with mazuku can be readily identified in the field through several distinctive characteristics and features, such as:
-
Peculiar types and species of vegetation that thrive in CO2-rich waters and gases, such as cyperus papyrus, fern, reeds, and Poaceae.
-
Burnt-out vegetation and altered rocks due to high acidity levels associated with normally elevated CO2 concentrations (70–90v.%), resulting in patches of weathered bareland.
-
Regions with ultrahigh CO2 concentrations, where the high CO2/O2 ratio can be perceived as a sensation of heat on human skin, a condition related to hypercapnia.
-
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
CO
2 levels in mazuku areas are affected and influenced by a combination of various factors.
Geological factors
-
Increased volcanic and seismic activities: CO2 concentration levels may increase proportionally with volcanic and seismic activities (e.g. earthquakes), which can produce more permeable fractures in the earth's crust, allowing more CO2 to escape from underground and forming new degassing zones.
Anthropogenic activities
-
Unauthorized well drilling: For instance, at Colli Albani 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 three more gas blowouts along a continuous fault line.
-
Tarmac roads construction: Tarmacadam roads and concrete surfaces can seal natural gas conduits, blocking the natural flow of gas and leading to its accumulation.
-
Drilling of pit latrines: A man died from asphyxiation near the foothills of Lake Ngozi, in the Rungwe Volcanic Province in Tanzania, while digging a 6m deep pit latrine. This was likely caused by the accumulation of hazardous gases in the hole after the gas pockets were mechanically disturbed. CO2 continuously degasses in the area to date, leading to more deaths of birds, cows, and rodents due to the toxic gas buildup.
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 allows heavy gases to accumulate in low-lying areas like valleys and depressions.
-
Soil moisture content and season: During heavy winter rains, subsurface soil voids are totally filled with water, into which dissolve a significant amount of CO2. In contrast, when the soil is dry during summer, these voids remain empty and can accumulate large amounts of degassed CO2, which 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.
CO2 exposure health effects and international guideline limits
The health hazards linked to both short-term and long-term exposure of lethal doses of CO
2 in mazuku are outlined in the table below, along with permissible exposure limits (PELs) for CO
2 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.
|
| 0–1.5% | Mostly unnoticed by olfactory or visual senses | More noticeable conditions such as shortness of breath, lightheadedness, and dizziness | 8 hours maximum exposure |
| 1.5–6% | Difficulty in breathing, increased heart rate, dyspnoea (shortness of breath) | Tingling sensations in lips, eyes, and nose because of the acidic nature of CO2, which reacts with surface moisture to form a weak carbonic acid that irritates these soft body parts | 15 minutes maximum exposure |
| 6–10% | Dizziness, buzzing sound in ears, lightheadedness, muscular and joint weakness, drowsiness, headaches, sweating, shortness of breath, low mood and mental distress, fainting, increased heart rate | Dizziness and loss of consciousness | Tolerable within a span of several minutes |
| 11–15% | Severe abrupt muscle contractions, caused because body cells lack enough oxygen for respiration and subsequently become unconscious within few seconds. | Severe muscle cramps and loss of consciousness | Death in less than a minute |
| ˃ 25% | Intolerable amount of CO2 for full functioning of the human body. Generally a victim suffers convulsions, coma, and finally death. | Convulsions, coma, and death | Death in less than a minute |
Case studies
Mazuku occur in various parts of the world where volcanic or geologically active regions release CO
2-rich gases. These gases accumulate in low-lying areas, valleys, or confined spaces or in the stratified water layers of
, creating hazardous conditions and deadly
asphyxiation zones for humans, wildlife, and plants. The following are case studies.
Lake Monoun
Lake Monoun is a volcanic crater lake situated in the Oku Volcanic Field, which is part of the
Cameroon 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 killed 37 people.
The primary source of the gas was volcanic CO
2 emissions, confirmed by carbon 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 CO
2 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"; a rare phenomenon where dissolved volcanic gases are released from the stratified bottom layers of lakes after a mechanical disturbance)
that resulted in the sudden release of a massive amount of CO
2, 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 underneath by volatile-rich basaltic dikes.
Normally, a mazuku involves dry CO2 gas seeping through fissures and accumulating in low-lying areas before dispersing into the atmosphere.
It is believed that a landslide triggered the exsolution of the dissolved gases, which caused a limnic eruption.
Mammoth Mountain
Mammoth Mountain, a
Volcano in the
Sierra Nevada region of California, United States, is underlaid by a shallow
dacite dome that releases cold and dry CO
2-rich gases (98v% CO
2) through
fumarole vents and fractures located on the flanks of the mountain.
The gas fluxes were estimated at a rate of ~1,200 tonnes per 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 CO
2 originates from deeper magmatic sources (as evidenced from He-CO
2 isotopic signature) at about 10 km below the surface, and travels through permeable networks of fractures and faults.
The CO
2-rich gases accumulate in the soil layers at depths between 0.6–1m, in closed subsurface cavities, and in snow caves, suggesting an ongoing active magmatic activity beneath the mountain.
One visible manifestation of this toxic degassing is the large-scale mortality of Conifer, covering an area of up to 100 hectares on the mountain's flanks.
Mount Amiata
Monte Amiata is a dormant volcano located in
Tuscany, central Italy, known for its significant emissions of dry and cold CO
2-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 CO
2 emissions contributing to environmental risks like soil acidification and potential CO
2 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
Dieng Plateau in Indonesia. In 1979 it experienced a tragic phreatic eruption disaster when a mixture of steam, lahar, and toxic gases was released from the open cracks and fissures located near the crater and gushed down the valley, asphyxiating insects, rodents, goats, dogs, and cows, as well as claiming the lives of 172 people. Before the eruption, the area experienced a series of earthquakes that reactivated ancient fractures over the span of a few hours.
After a few hours during the main course of eruption, dry gas was emitted from a new 1,000m-long fissure that had emerged on the western flank of the volcano near
Sumur crater.
Gas analysis revealed that the dry gas was CO
2-rich from magmatic sources, with concentrations reaching up to 99% by volume.
Since CO
2 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
Depending on their gas composition and concentration, mazuku can cause various effects on flora and fauna.
Massive clouds of CO
2, 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 CO
2 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, where deforestation has occurred, as well as CO
2 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.
If mazuku occurs underneath lakes, it can lead to changes in water chemistry, creating that are dangerous for aquatic life.
Summary
|
| 1 | DRC and Rwanda | Virunga Volcanic Province
-
Mt. Nyiragongo
-
Mt. Nyamulagira
| 1900s to present | Active | Dry CO2 degassing | 90% | Acidic soil, death of animals due to ˃ 500,000 ppm of CO2 in the soil | ~13 deaths per year |
| 2 | DRC | Virunga Volcanic Province
| 1900s to present | Active | Diffuse outgassing of CO2 into the lake water | ˃ 25% | Water chemistry alteration, habitat disruption, loss of biodiversity | Dizziness and convulsions lead to 2 swimmers drowning; potential future catastrophic limnic eruption |
| 3 | Cameroon | Cameroon Volcanic Line
| 1984 | Active | Limnic eruption | 96.73% | Water chemistry alteration, habitat disruption, loss of biodiversity | 37 people died |
| 4 | Cameroon | Cameroon Volcanic Line
| 1986 | Active | Limnic eruption | 98% | Water chemistry alteration, habitat disruption, loss of biodiversity | 1,700 deaths |
| 5 | United States | Mammoth Mountain | 1998 | Dormant | Dry diffuse CO2 through the soil | 98% | Acidic soil, barren land (~100 hectares in a tree-kill area and dead animals) | A skier died from acute pulmonary edema in a snow cave with ~98% CO2 |
| 6 | Indonesia | Diëng Plateau
| 1979 | Dormant | Phreatic eruption | 99% | Tree kill zones due to acidic soils; dead reptiles and rodents | 172 people died |
| 7 | Tanzania | Rungwe Volcanic Province
| 2001, 2004 and 2022 | Dormant | Dry CO2 degassing | 95% | Tree kill zones due to acidic soils; dead reptiles and rodents | 1 man died while digging a pit latrine; 2 men died when they fell into a ditch |
| 8 | Italy | Vulcano Island | 1980 | Active | Diffuse CO2 on mountain flanks | 50% | Tree kill zones due to acidic soils; dead reptiles and rodents | 2 children died from asphyxiation |
| 9 | Italy | Lazio and Alban Hills | 2000 | Dormant | Dry diffuse CO2 through the soil | 92.7% | Tree kill zones due to acidic soils; dead reptiles and rodents | 1 man died when he fell into an abandoned well |
| 10 | Italy | Alban Hills
| 2011 | Dormant | Dry diffuse CO2 through the soil | 99% | Cows and pets died from Asphixiation; gas blowouts, ground swells, and roads collapses | 3 people died in an open spa |
| 11 | Japan | Hakkoda | 1997 | Dormant | Dry diffuse CO2 through the soil into depressions | 15–20% | Bare land and a pattern of dead animals were observed | 3 soldiers died after falling into a depression |
| 12 | Portugal | Furnas, São Miguel, Azores | 1999 | Active | Dry diffuse CO2 through the soil | 99% | Tree kill zones due to acidic soils; dead reptiles and rodents | 3 people died from asphyxiation in house cellars and a well |
Hazard assessment and mitigation
Hazard assessment
Areas experiencing mazuku emissions face 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 asphyxiated by mazuku gases.
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.
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,
Meromictic lake lakes like
Lake Nyos,
Lake Kivu, and
Lake Monoun can contain enormous amounts of dissolved carbon dioxide (and sometimes methane) in their deep stratified layers (
monimolimnion).
Under normal conditions, these gases remain trapped in the lower layers of the lake.
However, if triggered by an external mechanical disturbance such as volcanic activity, an earthquake, or a landslide, these gases can release explosively in a
limnic eruption. This could lead to widespread asphyxiation and fires across the surrounding regions.
Mazuku may also indicate deeper magmatic unrest, warning of further natural disasters such as earthquakes, volcanic eruptions, and landslides.
Mitigation measures
Due to the silent (colorless and odorless) and deadly nature of CO
2 in volcanic active areas, authorities must be proactive and prepared to combat this natural hazard and reduce its hazardous effects. Some of the mitigation measures are:
On-ground CO2 detection sensors: Early warning systems can 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 can be employed in 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.
Land-use planning: Town planners can indicate buffer zones which are prone to mazuku and prevent settlements in these areas.
Relocation 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 CO2 data—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.
Education and sensitization campaigns: There should be continued scientific research on CO2 emissions in volcanic active regions, including the 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 CO
2 both during eruptions and through continuous magma upwelling (passive degassing) in non-eruptive states through fumaroles, hot springs, and gas plumes.
The cycling of CO
2 and other gases—such as sulfur dioxide, hydrogen sulfide, water vapor, and hydrogen chloride—is driven by magma convection, where degassed magma sinks, recharges with CO
2 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.
See also
