What Happened at Lake Nyos?
Lake Nyos is a deep maar lake of explosive origin in northwestern Cameroon. Nyos is infamous, owning to a catastrophic release of carbon dioxide gas on the evening of August 21, 1986 that killed an estimated 1700 people and 6,000 head of livestock in the valley below.
The Lake’s Geological History
Lake Nyos is in the hilly grasslands of western Cameroon, within the so called Cameroon volcanic line: a string of volcanoes that extends 1600 km from the South Atlantic Ocean to the northeast, along the border of Cameroon and Nigeria. Nyos is a classic maar, formed in a coarse-grained biotite-quartz monzonite about 4,000 years ago. Lake Nyos has an area of 1.49 km2, 208 meters deep, and has a water volume of about 132 million cubic meters.
Carbon dioxide is the main gas that escapes from the earth’s mantle, which is also the source of magma, or molten rock, that erupts from volcanoes. The carbon dioxide had chemical characteristics indicating that it could only have come from the earth’s mantle (the layer of the earth that underlies the forty-mile-thick crystal rocks). The gas was gradually seeping into the bottom of the crater lake from fractures and faults in the neck of the old volcano and had accumulated in the lake over several hundred years.
There are two potential sources for the gas released: volcanic and magmatic. Because of undisturbed sediment, clear deeper waters, lack of temperature increase, and the absence of other chemicals (sulfur and chlorine compounds), it appeared that no major volcanic eruptions had occurred in the lake. Scientists then concluded that much if not all of the carbon dioxide released was of magmatic origin, which was dissolved in the cold, deeper lake water prior to the event. Lake Nyos is also strongly stratified – the lower denser water and less dense upper water generally do not mix. This condition led to further buildup an entrapment of carbon dioxide in the lower water.
As the gas saturated lower water rose to the surface, the gases rapidly came out of solution; decreasing pressure allowed bubbles to form, just as when a bottle of soda is opened. The eruption spewed a column of water and gas over 80 meters into the air. Because carbon dioxide is denser than air, it flowed down the mountainside into a populated valley below, causing the residents of the area to lose consciousness and quickly die of asphyxiation.
Although it is not clear what finally triggered these events, possible causes include a heavy rain, strong winds, an earth tremor, a landslide, or a seasonal change in the layers of the lake’s water – any disturbance that may have moved the deeper, gas-rich waters closer to the surface. With the overlying pressure reduced, there was nothing to keep the gas in solution. Bubbles formed and rose rapidly, dragging additional deep water toward the surface and depressurizing it. This process led to a chain reaction, which produced a sudden and violent release of carbon dioxide gas, estimated to have a volume 0.68 km3 – 1.00 km3.
Define the following terms:
Stratified Lake:
A lake that has different densities of water stacked on each other. The lake waters do not mix
Saturated:
Not capable of holding any more. In this case, the water couldn’t hold any more dissolved CO2.
Asphyxiation:
Lack of oxygen. Suffocation
Part A Properties of CO2
Materials:1000 ml beaker, vinegar, baking soda, and bubble solution with wand.
Procedures:
- Produce CO2 gas by placing approximately 2 tsp. of baking soda in the 1000 ml beaker. Add 200 ml of vinegar and observe rapid bubbling.
- After the bubbling has stopped, use a bubble wand to blow bubbles into the air.
- Pick up the beaker and gently try to catch a bubble within the container. Do not blow into the container, as this will disturb the carbon dioxide present.
What did you observe?
Bubble will “float” above the bottom of the beaker.
Describe some of the physical properties of CO2.
Odorless, colorless, tasteless, more dense than air.
How did the carbon dioxide gas kill the people in the valley below Lake Nyos? Relate your response to the experiment?
The gas hugged the ground and displaced the air near the ground, creating a zone of no oxygen. Thus, the people lying down/sleeping (the event happened at night) died of asphyxiation.
Part BVelocity of the CO2 Cloud
Materials:Ring stand with clamp,four teacandles, 3.8 liter container, 1.5 m rain gutter,
stop watch, 2 tsp baking soda and 200 ml vinegar.
Procedures:
- Secure one end of the rain gutter 30 cm high on the ring clamp.
- Place one candle at the top edge of the ramp and one at the bottom. Distribute the remaining two candles between the upper and lower candles.
- Place 2 tsp. of baking soda in the 3.8 liter plastic container. Add 200 ml of vinegar, cover with paper.
- Light all four candles. Slowly pour the CO2 into the elevated part of the gutter. Don’t pour any liquid into the rain gutter.
- Record the time it took the CO2 to travel down the 1.5 meter ramp.
- Repeat steps 3 – 5 using the remaining CO2.
What did you observe?
The candles went out as the CO2 traveled down the gutter
How do you know CO2 was present? Explain.
Candles need oxygen in the air to burn and if the CO2 displaced that oxygen
Explain how the flow of the carbon dioxide in the experiment relates to the topography surrounding Lake Nyos.
The flow of carbon dioxide was mainly in the valley – the low area and traveled down stream to the villages that were affected.
In relation to the Lake Nyos Disaster, explain why there were no casualties 100 meters above the surface of Lake Nyos (elev. 1101 m).
Carbon dioxide is more dense than air so it sinks, it doesn’t rise with respect to the air around the lake.
Calculate the two velocities (m/s) of the CO2. Show calculations.
V1=V2=
Explain why V1 was faster than V2.
More volume of carbon dioxide.
The estimated velocity of the Lake Nyos CO2 cloud flowing into the valley was 20 m/s. How did your calculated velocity compare to the velocity of the Lake Nyos CO2 cloud?
Much lower velocity.
Explain why there is a difference between the estimated velocity of the Lake Nyos CO2 cloud and your calculated velocity.
More volume at Lake Nyos = more mass so it traveled down hill faster.
Analysis (Use complete sentences for full credit)
- Carbonated beverages contain dissolved carbon dioxide. When you open a bottle, bubbles begin to appear as the gas comes out of solution. Explain why this happens and relate it to the Lake Nyos disaster.
Same principle. The gas came out of solution when the pressure of the overlying water was released. It is the same in a bottle of soda/carbonated water when the cap is removed. The pressure is removed and the gasses can then escape.
- Was the source of CO2 that was stored in the depths of Lake Nyos magmatic or volcanic? Explain why.
More than likely magmatic. It seeped into the water over hundreds of year.
- Air is composed of 78% nitrogen (N2) and 21% oxygen (O2). Calculate molecular weights of these two gases and carbon dioxide (CO2) and suggest a reason why the carbon dioxide might have a greater density than air. (Use the periodic table to calculate the molecular weights.)
N = 14, O = 16, C = 12, O = 16
Air (nitrogen and oxygen) = ((2 X 14) x 0.78) + ((2 x 16) x 0.21) = 21.84 + 6.72 = 28.56
CO2 = (12 + (16 x 2)) = 44 = greater density
- Explain why the CO2 stayed in solution and was confined in the lower depths of Lake Nyos over a period of several hundred years.
Held in place by the pressure of the overlying water…it’s a deep lake. And the lake doesn’t overturn like lakes in higher latitudes do.
- List three possible events (a – c) that may have triggered the upwelling of the lake’s waters.
a. Heavy rain
Supporting evidence: rain gauges and person accounts
Arguments against: few rain gauges and few eye witnesses to the event, sulfur smell
b. Volcanic eruption
Supporting evidence: seems logical, volcanic history, sulfur smell
Arguments against: undisturbed sediment, clear deeper waters, lack of temperature increase, and the absence of other chemicals (sulfur and chlorine compounds), it appeared that no major volcanic eruptions had occurred in the lake
c. Landslide
Supporting evidence: some scars along the shorelines
Arguments against: don’t seem large enough to have caused such a large lake to overturn
- Choose one of the above hypotheses to be evaluated. Which hypothesis is being evaluated? Are there elements in this hypothesis that are untestable? Explain.
Hedeenlake_nyos_key.doc10/13/18