A
limnic eruption, also referred to as a
lake overturn, is a rare type of
natural disasterA natural disaster is the effect of a natural hazard that affects the environment, and leads to financial, environmental and/or human losses...
in which
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(CO
2) suddenly erupts from deep lake water, suffocating wildlife, livestock and humans. Such an eruption may also cause
tsunamiA is a series of water waves that is caused by the displacement of a large volume of a body of water, such as an ocean. The original Japanese term literally translates as "harbor wave." Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded...
s in the lake as the rising CO
2 displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption. Lakes in which such activity occurs may be known as
limnically active lakes or
exploding lakes. Some features of limnically active lakes include:
- CO2-saturated incoming water
- A cool lake bottom indicating an absence of direct volcanic interaction with lake waters
- An upper and lower thermal layer with differing CO2 saturations
- Proximity to areas with volcanic activity
Scientists have recently determined, from investigations into the mass casualties in the 1980s at
Lake MonounLake Monoun is a lake in West Province, Cameroon, that lies in the Oku Volcanic Field . On August 15, 1984, the lake exploded in a limnic eruption, which resulted in the release of a large amount of carbon dioxide that killed 37 people. At first, the cause of the deaths was a mystery, and...
and
Lake NyosLake Nyos is a crater lake in the Northwest Province of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...
, that limnic eruptions and volcanic eruptions, although indirectly related, are actually separate types of disaster events.
Historical occurrences
To date, this phenomenon has been knowingly observed only twice. The first was in
CameroonThe Republic of Cameroon is a unitary republic of central and western Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the Bight of...
at
Lake MonounLake Monoun is a lake in West Province, Cameroon, that lies in the Oku Volcanic Field . On August 15, 1984, the lake exploded in a limnic eruption, which resulted in the release of a large amount of carbon dioxide that killed 37 people. At first, the cause of the deaths was a mystery, and...
in 1984, causing the
asphyxiaAsphyxia is a condition of severely deficient supply of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is choking...
tion and death of 37 people living nearby. A second, deadlier eruption happened at neighbouring
Lake NyosLake Nyos is a crater lake in the Northwest Province of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...
in 1986, this time releasing over 80 million cubic meters of CO
2 and killing between 1,700 and 1,800 people, again by
asphyxiaAsphyxia is a condition of severely deficient supply of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is choking...
tion.
Due to the nature of the event, it is hard to determine if limnic eruptions have happened elsewhere. However, a third lake —
Lake KivuLake Kivu is one of the African Great Lakes. It lies on the border between the Democratic Republic of the Congo and Rwanda, and is in the Albertine Rift, a part of the Great Rift Valley...
— containing massive amounts of dissolved CO
2 exists on the border between the
Democratic Republic of the CongoThe Democratic Republic of the Congo is a country located in Central Africa, with a small length of Atlantic coastline. It is the third largest country in Africa...
and
RwandaThe Republic of Rwanda is a small landlocked country in the Great Lakes region of east-central Africa, bordered by Uganda, Burundi, the Democratic Republic of the Congo and Tanzania. Home to approaching 10 million people, Rwanda supports the densest population in continental Africa, most of whom...
. Sample sediments from the lake were taken by Professor Robert Hecky from the
University of MichiganThe University of Michigan, Ann Arbor is a public research university located in the state of Michigan in the United States. It is the state's oldest university, the flagship campus of the University of Michigan, and one of the top public universities in the world...
which showed that an event caused living creatures in the lake to go extinct approximately every thousand years, and caused nearby vegetation to be swept back into the lake.
The
Messel pitThe Messel Pit is a disused quarry near the village of Messel, about 35 km southeast of Frankfurt am Main, Germany. Bituminous shale was mined there. Because of its plethora of fossils, it has significant geological and scientific importance...
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
deposits of
MesselMessel is a municipality in the district of Darmstadt-Dieburg in Hesse near Frankfurt am Main in Germany.The village is first mentioned, as Masilla, in the Lorsch codex.Messel was the proprety of the lords of Groschlag from ca. 1400 to 1799...
,
GermanyGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
, show evidence of a limnic eruption there in the early
EoceneThe Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma , is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the...
. Among the victims are perfectly preserved
insectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s,
frogFrogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...
s,
turtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s,
crocodileA crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all members of the order Crocodilia: i.e...
s,
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s,
anteaterAnteaters are the four mammal species of the suborder Vermilingua commonly known for eating ants and termites. Together with the sloths, they compose the order Pilosa...
s,
insectivoreAn insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures.Although individually small, insects exist in enormous numbers and make up a very large part of the animal biomass in almost all non-marine environments...
s, early
primateA primate is a member of the biological order Primates , the group that contains lemurs, lorisids, galagos, tarsiers, monkeys, and apes, with the last category including great apes. With the exception of humans, who inhabit every continent on Earth, most primates live in tropical or subtropical...
s and
paleotherePropalaeotherium was an early genus of perissodactyl ancestral to the horse. Its name means "before Palaeotherium", as it is the ancestor of Palaeotherium, another relative of early horses...
s.
Causes
For a limnic eruption to occur, the lake must be nearly
saturatedIn chemistry, saturation has five different meanings:#In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance and additional amounts of it will appear as a precipitate...
with a gas. In the two known cases, the major component was
CO2Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
. This CO
2 may come from
volcanic gasVolcanic gases include a variety of substances given off by active volcanoes. These include gases trapped in cavities in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.The sources...
emitted under the lake, or from decomposition of organic material. Before a lake is saturated, it behaves as an unopened
carbonated beverageCarbonation occurs when carbon dioxide is dissolved in water or an aqueous solution. This process yields the "fizz" to carbonated water, sparkling mineral water, and soft drinks; the head to beer; and the cork pop and bubbles to champagne and sparkling wine.-Effervescence:Effervescence is the...
(
soft drinkA soft drink is a drink that does not contain alcohol . Soft drinks are often carbonated and commonly consumed while cold. The most common soft drinks are colas, flavored water, sparkling water, iced tea, sweet tea, lemonade, squash and fruit punch....
): the CO
2 is
dissolvedSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
in the water. In both the lake and the soft drink, CO
2 dissolves much more readily when it is at a higher
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
. This is why bubbles in a can of soda only form after the drink is open; the pressure is released and the CO
2 comes out of
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
. In the case of lakes, the bottom is at a much higher pressure; the deeper it is, the higher the pressure at the bottom. This means that huge amounts of CO
2 can be dissolved in large, deep lakes. Also, CO
2 dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
can lead to the release of a large amount of CO
2.
Once the lake is saturated with CO
2, it is at a critical and very unstable point, and a trigger is all that is needed to set off an eruption. In the case of the 1986 eruption at Lake Nyos, landslides were the suspected triggers, but an actual volcanic eruption, an
earthquakeAn earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph...
, an
explosionAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
, or even
windWind is the flow of air or other gases that compose an atmosphere . On Earth, wind consists of the bulk movement of air...
and
rain stormsRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
are other possible triggers. In any case, the trigger pushes some of the saturated water higher in the lake, where the pressure is insufficient to keep the CO
2 in solution. Then bubbles start forming, and the water is lifted even higher in the lake (
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
), where even more of the CO
2 comes out of solution. This process forms a column of gas. At this point the water at the bottom of this column is pulled up by
suctionSuction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...
, and it too loses its CO
2 in a runaway process. This eruption pours CO
2 into the air, and also displaces water to form a tsunami.
There are several reasons this type of eruption is very rare. First of all, there has to be a source of the CO
2, and so regions with no volcanic activity are not prone to this kind of eruption. Also,
temperateIn geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. But in continental areas, such as central North America the variations between summer...
lakes, such as North America's
Great LakesThe Great Lakes are a collection of freshwater lakes located in eastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth. They are sometimes referred to as the "Third...
region, turn over each spring and fall, mixing water from the bottom and top of the lake, so CO
2 that builds up at the bottom of the lake is brought to the top where the pressure is too low for it to stay in solution and it escapes into the
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
. A lake must be quite deep to have enough pressure to put enough CO
2 into solution too. So only deep, stable, tropical, volcanic lakes such as Lake Nyos are prone to limnic eruptions. As for Lake Kivu, scientists are concerned about the concentrations of
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
gas along with CO
2 and the possibility of a volcanic eruption.
Consequences
Once an eruption occurs, a large CO
2 cloud forms above the lake and expands to the neighbouring region. Because CO
2 is denser than air, it has a tendency to sink to the ground while pushing breathable air up. As a result, life forms that need to breathe oxygen suffocate once the CO
2 cloud reaches them, as there is no breathable air. The CO
2 can make human bodily fluids very acidic, potentially causing CO
2 poisoning. As victims gasp for air they actually hurt themselves more by sucking in the CO
2 gas.
At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. In this eruption, some people as far as 25 km from the lake died. A change in skin color on some bodies led scientists to think that the gas cloud may have contained a dissolved acid such as
hydrogen chlorideThe compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
as well, but that hypothesis is disputed. Many victims were found with blisters on their skin. This is believed to have been caused by pressure ulcers, which likely formed from the low levels of oxygen present in the blood of those asphyxiated by the carbon dioxide. Thousands of cattle and wild animals were also asphyxiated, but no official counts were made. On the other hand, vegetation nearby was mostly unaffected except for that which grew immediately adjacent to the lake. There the vegetation was damaged or destroyed by a 5-meter tsunami from the violent eruption.
A possible solution: Degassing lakes
Efforts have been under way for several years to develop a solution to remove the gas from these lakes and prevent a build-up that could lead to another catastrophe. A team of French scientists began experimenting at Lake Monoun and Lake Nyos in 1990 using
siphonA siphon is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher, or lower, than the reservoir, the flow being driven only by the difference in hydrostatic pressure without any need for pumping...
s to degas the waters of these lakes in a controlled manner. A pipe is positioned vertically in the lake with its upper end above the water's surface. Water saturated with CO
2 enters the bottom of the pipe and rises to the top. The lower pressure at the surface allows the gas to come out of solution. Interestingly, only a small amount of water has to initially be mechanically pumped through the pipe to start the flow. As the saturated water rises, the CO
2 comes out of solution and forms bubbles. The natural buoyancy of the bubbles draws the water up the pipe at high velocity causing a large fountain at the surface. The degassifying water acts as a pump, drawing more water into the bottom of the pipe, and creating a self-sustaining flow. This is the same process that leads to a natural eruption, but in this case it is controlled by the size of the pipe.
Each pipe has a limited pumping capacity and several would be required for both Lake Monoun and Lake Nyos to degas a significant fraction of the deep lake water and render the lakes safe. The deep lake waters are slightly acidic due to the dissolved CO
2 which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There are also fears that the CO
2 from the pipes could settle on the surface of the lake forming a thin layer of unbreathable air and thus causing problems for wildlife.
In January 2001, a single pipe was installed on Lake Nyos. A second pipe was installed at Lake Monoun in late 2002. These two pipes are thought to be sufficient to prevent an increase in CO
2 levels, removing approximately the same amount of gas as that naturally entering at the lake bed. In January 2003, an 18-month project had been given approval to fully degas Lake Monoun. The project appears to have been subsequently cancelled.
Lake Kivu's potential danger
Lake KivuLake Kivu is one of the African Great Lakes. It lies on the border between the Democratic Republic of the Congo and Rwanda, and is in the Albertine Rift, a part of the Great Rift Valley...
is not only 2,000 times larger than Lake Nyos — it is also located in a far more densely populated area, with over two million people living along its shores. Fortunately, it has not reached a high level of CO
2 saturation yet. If the water were to become heavily saturated, it could become an even greater risk to human and animal life, as it is located very close to a potential trigger,
Mount NyiragongoMount Nyiragongo is a stratovolcano in the Virunga Mountains associated with the Great Rift Valley. It is located in the Democratic Republic of the Congo, about 20 km north of the town of Goma and Lake Kivu and just west of the border with Rwanda. The main crater is about two km wide and...
, an active volcano that erupted in January 2002. It is also located in an active earthquake zone and close to other active volcanoes.
While the lake could be degassed in a manner similar to Lake Monoun and Lake Nyos, due to the size of the lake and the volume of gas involved such an operation would be expensive, running into millions of dollars. No plan has been initiated to reduce the risk posed by Lake Kivu.
See also
- Disasters
- Mazuku
In geology, a mazuku is a pocket of oxygen-poor air that can be lethal to any human or animal life inside. The term comes from Swahili and means "evil wind." Mazukus are created when an odorless and invisible gas such as carbon dioxide accumulates in pockets low to the ground...
- Meromictic
A meromictic lake has layers of water that do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters...
- Henry's Law
In chemistry, Henry's law is one of the gas laws, formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...
- Raoult's Law
Established by François-Marie Raoult, Raoult's law states:Once the components in the solution have reached equilibrium, the total vapor pressure p of the solution is:and the individual vapor pressure for each component is...
- Lake Nyos
Lake Nyos is a crater lake in the Northwest Province of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...
- Lake Monoun
Lake Monoun is a lake in West Province, Cameroon, that lies in the Oku Volcanic Field . On August 15, 1984, the lake exploded in a limnic eruption, which resulted in the release of a large amount of carbon dioxide that killed 37 people. At first, the cause of the deaths was a mystery, and...
- Lake Kivu
Lake Kivu is one of the African Great Lakes. It lies on the border between the Democratic Republic of the Congo and Rwanda, and is in the Albertine Rift, a part of the Great Rift Valley...
External links