Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Events of High Consequence but Low Probability
Sudden Gas Release From Crater Lake
The August 12, 1986, sudden release of ~1 km3 of CO2 gas from Lake Nyos in Cameroon resulted in death of at least 1,700 people by asphyxiation (Kling and others, 1987). The source of CO2 at Lake Nyos was prolonged degassing of subsurface magma. This event drew attention to the potential for dissolved lethal gas to accumulate in the deepest parts of volcanic crater lakes. Depressurization of this water if the lake overturns suddenly results in rapid exsolution of the gas and its liberation to the atmosphere. The high density of cold CO2 relative to ambient air allows the gas cloud to flow down topographic depressions and accumulate locally.
Crater Lake does not appear capable of producing a disastrous release of CO2. The input of thermal fluid through the floor of Crater Lake carries with it dissolved CO2 in the form of carbonic acid, but most of the added carbon is in bicarbonate (Collier and others, 1991). Carbon and helium isotope studies indicate a magmatic source for the CO2 input. The added carbonic acid is mixed with lake water in the deeper part of lake, reducing concentrations to fairly low levels. The upper 200 m of Crater Lake overturns and equilibrates with atmospheric gases twice a year. Any excess dissolved CO2 (as carbonic acid) comes out of solution but does not accumulate in lethal concentration. Between 200 m and maximum depth, lake water has been shown to mix completely with the upper portion of the lake over a period of 2.5 to 3.5 years (McManus, 1992). In this process, the bottom part of the lake becomes re-oxygenated by equilibrating with oxygenated surface water. At the same time, higher concentrations of dissolved CO2 are lowered by mixing with water containing only atmospheric concentrations of dissolved CO2. Thus, Crater Lake has a source of elevated CO2, but the possibility of buildup of large amounts of dissolved CO2 is prevented by the mixing processes in the lake. The mixing process in the deep lake is driven by heating of lake water through input of the thermal fluid that carries the dissolved CO2. Any change in the thermal fluid input could affect the mixing processes in the deep lake. The state of the deep lake should continue to be monitored to detect if the mixing process changes.