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 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.

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:

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 (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 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.

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.