Limnic eruption - AbsoluteAstronomy.com

A limnic eruption, also referred to as a lake overturn, is a rare type of natural disaster

Natural disaster

A natural disaster is the effect of a natural hazard . It leads to financial, environmental or human losses...

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

(CO₂) suddenly erupts from deep lake water, suffocating wildlife, livestock and humans. Such an eruption may also cause tsunami

Tsunami

A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...

s in the lake as the rising CO₂ 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 Monoun

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...

and Lake Nyos

Lake Nyos

Lake Nyos is a crater lake in the Northwest Region 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 observed only twice. The first was in Cameroon

Cameroon

Cameroon, officially the Republic of Cameroon , is a country in west Central 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...

at Lake Monoun

Lake Monoun

in 1984, causing the asphyxia

Asphyxia

Asphyxia or asphyxiation 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. Asphyxia causes generalized hypoxia, which primarily affects the tissues and organs...

tion and death of 37 people living nearby. A second, deadlier eruption happened at neighbouring Lake Nyos

Lake Nyos

in 1986, this time releasing over 80 million cubic meters of CO₂ and killing between 1,700 and 1,800 people, again by asphyxia

Asphyxia

tion.

Due to the nature of the event, it is hard to determine if limnic eruptions have happened elsewhere. However, a third lake—Lake Kivu

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. Lake Kivu empties into the Ruzizi River, which flows southwards into Lake Tanganyika...

—containing massive amounts of dissolved CO₂ exists on the border between the Democratic Republic of the Congo

Democratic Republic of the Congo

The Democratic Republic of the Congo is a state located in Central Africa. It is the second largest country in Africa by area and the eleventh largest in the world...

and Rwanda

Rwanda

Rwanda or , officially the Republic of Rwanda , is a country in central and eastern Africa with a population of approximately 11.4 million . Rwanda is located a few degrees south of the Equator, and is bordered by Uganda, Tanzania, Burundi and the Democratic Republic of the Congo...

. Sample sediments from the lake were taken by Professor Robert Hecky from the University of Michigan

University of Michigan

The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...

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 pit

Messel pit

The Messel Pit is a disused quarry near the village of Messel, about southeast of Frankfurt am Main, Germany. Bituminous shale was mined there. Because of its abundance of fossils, it has significant geological and scientific importance...

fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

deposits of Messel

Messel

Messel 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 property of the lords of Groschlag from ca. 1400 to 1799...

, Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, show evidence of a limnic eruption there in the early Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

. Among the victims are perfectly preserved insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, frog

Frog

Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s, turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, crocodile

Crocodile

A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s, bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, anteater

Anteater

Anteaters, also known as antbear, 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, insectivore

Insectivore

An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....

s, early primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s and paleothere

Propalaeotherium

Propalaeotherium was an early genus of perissodactyl ancestral to the horse endemic to Europe and Asia during the Middle Eocene.Its name means "before Palaeotherium", as it is the ancestor of Palaeotherium, another relative of early horses...

Causes

For a limnic eruption to occur, the lake must be nearly saturated

Saturation (chemistry)

In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...

with gas. In the two known cases, the major component was CO₂

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, however, in Lake Kivu, scientists are concerned about the concentrations of methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

gas as well. This CO₂ may come from volcanic gas

Volcanic gas

|250px|thumb|right|Image of the [[rhyolitic]] [[lava dome]] of [[Chaitén Volcano]] during its 2008-2010 eruption.Volcanic gases include a variety of substances given off by active volcanoes...

emitted from under the lake or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage

Carbonation

Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

(soft drink

Soft drink

A soft drink is a non-alcoholic beverage that typically contains water , a sweetener, and a flavoring agent...

): the CO₂ is dissolved

Solvation

Solvation, 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, CO₂ dissolves much more readily at higher pressure

Pressure

Pressure 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 CO₂ comes out of solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

. 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 CO₂ can be dissolved in large, deep lakes. Also, CO₂ dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in water temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

can lead to the release of a large amount of CO₂.

Once the lake is saturated with CO₂, it is very unstable. 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 earthquake

Earthquake

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

, or even wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

and rain storms

Rain

Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's 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 CO₂ in solution. Bubbles start forming and the water is lifted even higher in the lake (buoyancy

Buoyancy

In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

), where even more of the CO₂ 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 suction

Suction

Suction 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 CO₂ in a runaway process. This eruption pours CO₂ into the air and can also displace water to form a tsunami.

There are several reasons this type of eruption is very rare. First, there must be a source of the CO₂, so only regions with volcanic activity are at risk. Second, temperate

Temperate

In 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 relatively moderate, rather than extreme hot or cold...

lakes, such as North America's Great Lakes

Great Lakes

The Great Lakes are a collection of freshwater lakes located in northeastern 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 by total surface, coming in second by volume...

, turn over each spring and fall as a result of seasonal air temperature changes, mixing water from the bottom and top of the lake, so CO₂ 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 atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. Finally, a lake must be quite deep to have enough pressure to dissolve large volumes of CO₂. So only in deep, stable, tropical, volcanic lakes such as Lake Nyos are limnic eruptions possible.

Consequences

Once an eruption occurs, a large CO₂ cloud forms above the lake and expands to the neighbouring region. Because CO₂ 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 CO₂ cloud reaches them, as there is very little oxygen in the cloud. The CO₂ can make human bodily fluids very acidic, potentially causing CO₂ poisoning. As victims gasp for air they actually hurt themselves more by inhaling the CO₂ 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 (15.5 mi) 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 chloride

Hydrogen chloride

The 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 are likely to have 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 (16.4 ft.) tsunami from the violent eruption.

The carbon dioxide released from the lake is at a very low temperature believed to cause frost bite, which is consistent with blisters found on the skin of survivors of the incident at Lake Minoun. The survivors also reported a smell of rotten eggs and feeling warm before passing out; this is explained by the fact that at high concentrations, carbon dioxide acts as a sensory hallucinogenic. Studies with jet pilots regarding carbon dioxide report similar effects as reported by survivors, i.e. smell of rotten eggs, gunpowder and feeling of warmth.

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 siphon

Siphon

The word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir,...

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 CO₂ 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 CO₂ 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 CO₂ which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There are also fears that the CO₂ 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 CO₂ 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 Kivu

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 CO₂ 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 Nyiragongo

Mount Nyiragongo

Mount Nyiragongo is a stratovolcano in the Virunga Mountains associated with the Great Rift Valley. It is located inside Virunga National Park, 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...

, 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. A scheme initiated in 2010 to utilize methane trapped in the lake as a fuel source to generate electricity in Rwanda has led to a degree of CO₂ degassing. During the procedure for extracting the flammable gas used to fire power stations on the shore, some CO₂ is removed in a process known as scrubbing. It is unclear whether enough of the gas will be removed to eliminate the danger of a limnic eruption at Lake Kivu.

External links

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