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 carbon dioxide (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, and house basements—or in the stratified water layers of meromictic lakes if a water column exists. In high concentrations (≥ 1% by volume), 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 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. 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 at night the atmospheric temperature drops and wind speeds significantly reduce. These conditions slow the 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 (~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 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, 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, 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 helium and CO2 gas analyses confirm that the origin of mazuku is mainly magmatic, 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 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 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. However, CH4 is currently extracted economically in Lake Kivu through degassing, which reduces the risk of a dangerous limnic eruption while providing an 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 geochemical composition of mazuku gases consists mainly of CO2 and a variable mixture of other atmospheric components, such as N2, O2, and argon (Ar), with smaller amounts of methane (CH4), hydrogen sulfide (H2S), and water vapour. In these gases, CO2 concentrations range between 12% and 99%, argon 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 sources to be 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, hot springs, fumaroles, and gas plumes, without the need or presence of an active volcanic eruption.
Surface manifestations Areas 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 grasses. 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. This includes tingling and burning sensations in the mouth, lips, eyes, and nose because of the acidic nature of CO2 reacting with surface moisture to form a weak carbonic acid that irritates 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.
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 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 three more gas blowouts along a continuous fault line. Tarmac roads construction: Tarmac roads and concrete surfaces can seal natural gas conduits, blocking the natural flow of gas and leading to its accumulation. As trapped gases build up pressure, they can bulge and swell the ground, and release explosively (gas blowout) when they eventually escape. 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. 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. 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.